mod.rs

//! Boa's implementation of ECMAScript's global `String` object.
//!
//! The `String` global object is a constructor for strings or a sequence of characters.
//!
//! More information:
//!  - [ECMAScript reference][spec]
//!  - [MDN documentation][mdn]
//!
//! [spec]: https://tc39.es/ecma262/#sec-string-object
//! [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String

use crate::{
    builtins::{Array, BuiltInObject, Number, RegExp},
    context::intrinsics::{Intrinsics, StandardConstructor, StandardConstructors},
    error::JsNativeError,
    js_string,
    object::{internal_methods::get_prototype_from_constructor, JsObject},
    property::{Attribute, PropertyDescriptor},
    realm::Realm,
    string::{CodePoint, StaticJsStrings},
    symbol::JsSymbol,
    value::IntegerOrInfinity,
    Context, JsArgs, JsResult, JsString, JsValue,
};
use boa_macros::utf16;

use boa_profiler::Profiler;
use icu_normalizer::{ComposingNormalizer, DecomposingNormalizer};
use std::cmp::{max, min};

use super::{BuiltInBuilder, BuiltInConstructor, IntrinsicObject};

mod string_iterator;
pub(crate) use string_iterator::StringIterator;

#[cfg(feature = "annex-b")]
pub use crate::{js_str, JsStr};

/// The set of normalizers required for the `String.prototype.normalize` function.
#[derive(Debug)]
pub(crate) struct StringNormalizers {
    pub(crate) nfc: ComposingNormalizer,
    pub(crate) nfkc: ComposingNormalizer,
    pub(crate) nfd: DecomposingNormalizer,
    pub(crate) nfkd: DecomposingNormalizer,
}

#[cfg(test)]
mod tests;

#[derive(Clone, Copy, Eq, PartialEq)]
pub(crate) enum Placement {
    Start,
    End,
}

/// Helper function to check if a `char` is trimmable.
pub(crate) const fn is_trimmable_whitespace(c: char) -> bool {
    // The rust implementation of `trim` does not regard the same characters whitespace as ecma standard does
    //
    // Rust uses \p{White_Space} by default, which also includes:
    // `\u{0085}' (next line)
    // And does not include:
    // '\u{FEFF}' (zero width non-breaking space)
    // Explicit whitespace: https://tc39.es/ecma262/#sec-white-space
    matches!(
        c,
        '\u{0009}' | '\u{000B}' | '\u{000C}' | '\u{0020}' | '\u{00A0}' | '\u{FEFF}' |
    // Unicode Space_Separator category
    '\u{1680}' | '\u{2000}'
            ..='\u{200A}' | '\u{202F}' | '\u{205F}' | '\u{3000}' |
    // Line terminators: https://tc39.es/ecma262/#sec-line-terminators
    '\u{000A}' | '\u{000D}' | '\u{2028}' | '\u{2029}'
    )
}

/// JavaScript `String` implementation.
#[derive(Debug, Clone, Copy)]
pub(crate) struct String;

impl IntrinsicObject for String {
    fn init(realm: &Realm) {
        let _timer = Profiler::global().start_event(std::any::type_name::<Self>(), "init");

        let trim_start = BuiltInBuilder::callable(realm, Self::trim_start)
            .length(0)
            .name(js_string!("trimStart"))
            .build();

        let trim_end = BuiltInBuilder::callable(realm, Self::trim_end)
            .length(0)
            .name(js_string!("trimEnd"))
            .build();

        #[cfg(feature = "annex-b")]
        let trim_left = trim_start.clone();

        #[cfg(feature = "annex-b")]
        let trim_right = trim_end.clone();

        let attribute = Attribute::READONLY | Attribute::NON_ENUMERABLE | Attribute::PERMANENT;
        let builder = BuiltInBuilder::from_standard_constructor::<Self>(realm)
            .property(js_string!("length"), 0, attribute)
            .property(
                js_string!("trimStart"),
                trim_start,
                Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
            )
            .property(
                js_string!("trimEnd"),
                trim_end,
                Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
            )
            .static_method(Self::raw, js_string!("raw"), 1)
            .static_method(Self::from_char_code, js_string!("fromCharCode"), 1)
            .static_method(Self::from_code_point, js_string!("fromCodePoint"), 1)
            .method(Self::char_at, js_string!("charAt"), 1)
            .method(Self::char_code_at, js_string!("charCodeAt"), 1)
            .method(Self::code_point_at, js_string!("codePointAt"), 1)
            .method(Self::to_string, js_string!("toString"), 0)
            .method(Self::concat, js_string!("concat"), 1)
            .method(Self::repeat, js_string!("repeat"), 1)
            .method(Self::slice, js_string!("slice"), 2)
            .method(Self::starts_with, js_string!("startsWith"), 1)
            .method(Self::ends_with, js_string!("endsWith"), 1)
            .method(Self::includes, js_string!("includes"), 1)
            .method(Self::index_of, js_string!("indexOf"), 1)
            .method(Self::is_well_formed, js_string!("isWellFormed"), 0)
            .method(Self::last_index_of, js_string!("lastIndexOf"), 1)
            .method(Self::locale_compare, js_string!("localeCompare"), 1)
            .method(Self::r#match, js_string!("match"), 1)
            .method(Self::normalize, js_string!("normalize"), 0)
            .method(Self::pad_end, js_string!("padEnd"), 1)
            .method(Self::pad_start, js_string!("padStart"), 1)
            .method(Self::trim, js_string!("trim"), 0)
            .method(Self::to_case::<false>, js_string!("toLowerCase"), 0)
            .method(Self::to_case::<true>, js_string!("toUpperCase"), 0)
            .method(Self::to_well_formed, js_string!("toWellFormed"), 0)
            .method(
                Self::to_locale_case::<false>,
                js_string!("toLocaleLowerCase"),
                0,
            )
            .method(
                Self::to_locale_case::<true>,
                js_string!("toLocaleUpperCase"),
                0,
            )
            .method(Self::substring, js_string!("substring"), 2)
            .method(Self::split, js_string!("split"), 2)
            .method(Self::value_of, js_string!("valueOf"), 0)
            .method(Self::match_all, js_string!("matchAll"), 1)
            .method(Self::replace, js_string!("replace"), 2)
            .method(Self::replace_all, js_string!("replaceAll"), 2)
            .method(Self::iterator, JsSymbol::iterator(), 0)
            .method(Self::search, js_string!("search"), 1)
            .method(Self::at, js_string!("at"), 1);

        #[cfg(feature = "annex-b")]
        let builder = {
            builder
                .property(
                    js_string!("trimLeft"),
                    trim_left,
                    Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
                )
                .property(
                    js_string!("trimRight"),
                    trim_right,
                    Attribute::WRITABLE | Attribute::NON_ENUMERABLE | Attribute::CONFIGURABLE,
                )
                .method(Self::substr, js_string!("substr"), 2)
                .method(Self::anchor, js_string!("anchor"), 1)
                .method(Self::big, js_string!("big"), 0)
                .method(Self::blink, js_string!("blink"), 0)
                .method(Self::bold, js_string!("bold"), 0)
                .method(Self::fixed, js_string!("fixed"), 0)
                .method(Self::fontcolor, js_string!("fontcolor"), 1)
                .method(Self::fontsize, js_string!("fontsize"), 1)
                .method(Self::italics, js_string!("italics"), 0)
                .method(Self::link, js_string!("link"), 1)
                .method(Self::small, js_string!("small"), 0)
                .method(Self::strike, js_string!("strike"), 0)
                .method(Self::sub, js_string!("sub"), 0)
                .method(Self::sup, js_string!("sup"), 0)
        };

        builder.build();
    }

    fn get(intrinsics: &Intrinsics) -> JsObject {
        Self::STANDARD_CONSTRUCTOR(intrinsics.constructors()).constructor()
    }
}

impl BuiltInObject for String {
    const NAME: JsString = StaticJsStrings::STRING;
}

impl BuiltInConstructor for String {
    const LENGTH: usize = 1;
    const P: usize = 36;
    const SP: usize = 3;

    const STANDARD_CONSTRUCTOR: fn(&StandardConstructors) -> &StandardConstructor =
        StandardConstructors::string;

    /// Constructor `String( value )`
    ///
    /// <https://tc39.es/ecma262/#sec-string-constructor-string-value>
    fn constructor(
        new_target: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // This value is used by console.log and other routines to match Object type
        // to its Javascript Identifier (global constructor method name)
        let string = match args.first() {
            // 2. Else,
            // a. If NewTarget is undefined and Type(value) is Symbol, return SymbolDescriptiveString(value).
            Some(JsValue::Symbol(ref sym)) if new_target.is_undefined() => {
                return Ok(sym.descriptive_string().into())
            }
            // b. Let s be ? ToString(value).
            Some(value) => value.to_string(context)?,
            // 1. If value is not present, let s be the empty String.
            None => js_string!(),
        };

        // 3. If NewTarget is undefined, return s.
        if new_target.is_undefined() {
            return Ok(string.into());
        }

        let prototype =
            get_prototype_from_constructor(new_target, StandardConstructors::string, context)?;
        // 4. Return ! StringCreate(s, ? GetPrototypeFromConstructor(NewTarget, "%String.prototype%")).
        Ok(Self::string_create(string, prototype, context).into())
    }
}

impl String {
    /// JavaScript strings must be between `0` and less than positive `Infinity` and cannot be a negative number.
    /// The range of allowed values can be described like this: `[0, +∞)`.
    ///
    /// The resulting string can also not be larger than the maximum string size,
    /// which can differ in JavaScript engines. In Boa it is `2^32 - 1`
    pub(crate) const MAX_STRING_LENGTH: usize = u32::MAX as usize;

    /// Abstract function `StringCreate( value, prototype )`.
    ///
    /// Call this function if you want to create a `String` exotic object.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-stringcreate
    fn string_create(value: JsString, prototype: JsObject, context: &mut Context) -> JsObject {
        // 7. Let length be the number of code unit elements in value.
        let len = value.len();

        // 1. Let S be ! MakeBasicObject(« [[Prototype]], [[Extensible]], [[StringData]] »).
        // 2. Set S.[[Prototype]] to prototype.
        // 3. Set S.[[StringData]] to value.
        // 4. Set S.[[GetOwnProperty]] as specified in 10.4.3.1.
        // 5. Set S.[[DefineOwnProperty]] as specified in 10.4.3.2.
        // 6. Set S.[[OwnPropertyKeys]] as specified in 10.4.3.3.
        let s =
            JsObject::from_proto_and_data_with_shared_shape(context.root_shape(), prototype, value);

        // 8. Perform ! DefinePropertyOrThrow(S, "length", PropertyDescriptor { [[Value]]: 𝔽(length),
        // [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false }).
        s.define_property_or_throw(
            StaticJsStrings::LENGTH,
            PropertyDescriptor::builder()
                .value(len)
                .writable(false)
                .enumerable(false)
                .configurable(false),
            context,
        )
        .expect("length definition for a new string must not fail");

        // 9. Return S.
        s
    }

    /// Abstract operation `thisStringValue( value )`
    ///
    /// More information:
    /// - [ECMAScript reference][spec]
    ///
    /// [spec]: https://tc39.es/ecma262/#thisstringvalue
    fn this_string_value(this: &JsValue) -> JsResult<JsString> {
        // 1. If Type(value) is String, return value.
        this.as_string()
            .cloned()
            // 2. If Type(value) is Object and value has a [[StringData]] internal slot, then
            //     a. Let s be value.[[StringData]].
            //     b. Assert: Type(s) is String.
            //     c. Return s.
            .or_else(|| {
                this.as_object()
                    .and_then(|obj| obj.downcast_ref::<JsString>().as_deref().cloned())
            })
            // 3. Throw a TypeError exception.
            .ok_or_else(|| {
                JsNativeError::typ()
                    .with_message("'this' is not a string")
                    .into()
            })
    }

    /// `String.fromCodePoint(num1[, ...[, numN]])`
    ///
    /// The static `String.fromCodePoint()` method returns a string created by using the specified sequence of code points.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.fromcodepoint
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/fromCodePoint
    pub(crate) fn from_code_point(
        _: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let result be the empty String.
        let mut result = Vec::with_capacity(args.len());

        let mut buf = [0; 2];

        // 2. For each element next of codePoints, do
        for arg in args {
            // a. Let nextCP be ? ToNumber(next).
            let nextcp = arg.to_number(context)?;

            // b. If ! IsIntegralNumber(nextCP) is false, throw a RangeError exception.
            if !Number::is_float_integer(nextcp) {
                return Err(JsNativeError::range()
                    .with_message(format!("codepoint `{nextcp}` is not an integer"))
                    .into());
            }

            // c. If ℝ(nextCP) < 0 or ℝ(nextCP) > 0x10FFFF, throw a RangeError exception.
            if nextcp < 0.0 || nextcp > f64::from(0x0010_FFFF) {
                return Err(JsNativeError::range()
                    .with_message(format!("codepoint `{nextcp}` outside of Unicode range"))
                    .into());
            }

            // SAFETY:
            // - `nextcp` is not NaN (by the call to `is_float_integer`).
            // - `nextcp` is not infinite (by the call to `is_float_integer`).
            // - `nextcp` is in the u32 range (by the check above).
            let nextcp = unsafe { nextcp.to_int_unchecked::<u32>() };

            // d. Set result to the string-concatenation of result and ! UTF16EncodeCodePoint(ℝ(nextCP)).
            result.extend_from_slice(match u16::try_from(nextcp) {
                Ok(ref cp) => std::slice::from_ref(cp),
                Err(_) => char::from_u32(nextcp)
                    .expect("u32 is in range and cannot be a surrogate by the conversion above")
                    .encode_utf16(&mut buf),
            });
        }

        // 3. Assert: If codePoints is empty, then result is the empty String.
        // 4. Return result.
        Ok(js_string!(&result[..]).into())
    }

    /// `String.raw( template, ...substitutions )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.raw
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/raw
    pub(crate) fn raw(_: &JsValue, args: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        let substitutions = args.get(1..).unwrap_or_default();

        // 1. Let numberOfSubstitutions be the number of elements in substitutions.
        let number_of_substitutions = substitutions.len() as u64;

        // 2. Let cooked be ? ToObject(template).
        let cooked = args.get_or_undefined(0).to_object(context)?;

        // 3. Let raw be ? ToObject(? Get(cooked, "raw")).
        let raw = cooked.get(js_string!("raw"), context)?.to_object(context)?;

        // 4. Let literalSegments be ? LengthOfArrayLike(raw).
        let literal_segments = raw.length_of_array_like(context)?;

        // 5. If literalSegments ≤ 0, return the empty String.
        // This is not <= because a `usize` is always positive.
        if literal_segments == 0 {
            return Ok(js_string!().into());
        }

        // 6. Let stringElements be a new empty List.
        let mut string_elements = vec![];

        // 7. Let nextIndex be 0.
        let mut next_index = 0;
        // 8. Repeat,
        loop {
            // a. Let nextKey be ! ToString(𝔽(nextIndex)).
            let next_key = next_index;

            // b. Let nextSeg be ? ToString(? Get(raw, nextKey)).
            let next_seg = raw.get(next_key, context)?.to_string(context)?;

            // c. Append the code unit elements of nextSeg to the end of stringElements.
            string_elements.extend(next_seg.iter());

            // d. If nextIndex + 1 = literalSegments, then
            if next_index + 1 == literal_segments {
                // i. Return the String value whose code units are the elements in the List stringElements.
                //    If stringElements has no elements, the empty String is returned.
                return Ok(js_string!(&string_elements[..]).into());
            }

            // e. If nextIndex < numberOfSubstitutions, let next be substitutions[nextIndex].
            let next = if next_index < number_of_substitutions {
                substitutions.get_or_undefined(next_index as usize).clone()

            // f. Else, let next be the empty String.
            } else {
                js_string!().into()
            };

            // g. Let nextSub be ? ToString(next).
            let next_sub = next.to_string(context)?;

            // h. Append the code unit elements of nextSub to the end of stringElements.
            string_elements.extend(next_sub.iter());

            // i. Set nextIndex to nextIndex + 1.
            next_index += 1;
        }
    }

    /// `String.fromCharCode(...codeUnits)`
    ///
    /// Construct a `String` from one or more code points (as numbers).
    /// More information:
    /// - [ECMAScript reference][spec]
    ///
    /// [spec]: https://tc39.es/ecma262/multipage/text-processing.html#sec-string.fromcharcode
    pub(crate) fn from_char_code(
        _: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let result be the empty String.
        let mut result = Vec::new();

        // 2. For each element next of codeUnits, do
        for next in args {
            // a. Let nextCU be the code unit whose numeric value is ℝ(? ToUint16(next)).
            let next_cu = next.to_uint16(context)?;

            // b. Set result to the string-concatenation of result and nextCU.
            result.push(next_cu);
        }

        // 3. Return result.
        Ok(js_string!(&result[..]).into())
    }

    /// `String.prototype.toString ( )`
    ///
    /// More information:
    /// - [ECMAScript reference][spec]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.tostring
    #[allow(clippy::wrong_self_convention)]
    pub(crate) fn to_string(this: &JsValue, _: &[JsValue], _: &mut Context) -> JsResult<JsValue> {
        // 1. Return ? thisStringValue(this value).
        Ok(Self::this_string_value(this)?.into())
    }

    /// `String.prototype.charAt( index )`
    ///
    /// The `String` object's `charAt()` method returns a new string consisting of the single UTF-16 code unit located at the specified offset into the string.
    ///
    /// Characters in a string are indexed from left to right. The index of the first character is `0`,
    /// and the index of the last character—in a string called `stringName`—is `stringName.length - 1`.
    /// If the `index` you supply is out of this range, JavaScript returns an empty string.
    ///
    /// If no index is provided to `charAt()`, the default is `0`.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.charat
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/charAt
    pub(crate) fn char_at(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        // 3. Let position be ? ToIntegerOrInfinity(pos).
        let position = args.get_or_undefined(0).to_integer_or_infinity(context)?;

        match position {
            // 4. Let size be the length of S.
            // 6. Return the substring of S from position to position + 1.
            IntegerOrInfinity::Integer(i) if i >= 0 && i < string.len() as i64 => {
                let i = i as usize;
                Ok(js_string!(string.get_expect(i..=i)).into())
            }
            // 5. If position < 0 or position ≥ size, return the empty String.
            _ => Ok(js_string!().into()),
        }
    }

    /// `String.prototype.at ( index )`
    ///
    /// This String object's `at()` method returns a String consisting of the single UTF-16 code unit located at the specified position.
    /// Returns undefined if the given index cannot be found.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/proposal-relative-indexing-method/#sec-string.prototype.at
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/at
    pub(crate) fn at(this: &JsValue, args: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let s = this.to_string(context)?;

        // 3. Let len be the length of S.
        let len = s.len() as i64;

        // 4. Let relativeIndex be ? ToIntegerOrInfinity(index).
        let relative_index = args.get_or_undefined(0).to_integer_or_infinity(context)?;
        let k = match relative_index {
            // 5. If relativeIndex ≥ 0, then
            // a. Let k be relativeIndex.
            IntegerOrInfinity::Integer(i) if i >= 0 && i < len => i as usize,
            // 6. Else,
            // a. Let k be len + relativeIndex.
            IntegerOrInfinity::Integer(i) if i < 0 && (-i) <= len => (len + i) as usize,
            // 7. If k < 0 or k ≥ len, return undefined.
            _ => return Ok(JsValue::undefined()),
        };

        // 8. Return the substring of S from k to k + 1.
        Ok(js_string!(s.get_expect(k..=k)).into())
    }

    /// `String.prototype.codePointAt( index )`
    ///
    /// The `codePointAt()` method returns an integer between `0` to `1114111` (`0x10FFFF`) representing the UTF-16 code unit at the given index.
    ///
    /// If no UTF-16 surrogate pair begins at the index, the code point at the index is returned.
    ///
    /// `codePointAt()` returns `undefined` if the given index is less than `0`, or if it is equal to or greater than the `length` of the string.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.codepointat
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/codePointAt
    pub(crate) fn code_point_at(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        // 3. Let position be ? ToIntegerOrInfinity(pos).
        let position = args.get_or_undefined(0).to_integer_or_infinity(context)?;

        match position {
            // 4. Let size be the length of S.
            IntegerOrInfinity::Integer(i) if i >= 0 && i < string.len() as i64 => {
                // 6. Let cp be ! CodePointAt(S, position).
                // 7. Return 𝔽(cp.[[CodePoint]]).
                Ok(string
                    .code_point_at(usize::try_from(i).expect("already checked that i >= 0"))
                    .as_u32()
                    .into())
            }
            // 5. If position < 0 or position ≥ size, return undefined.
            _ => Ok(JsValue::undefined()),
        }
    }

    /// `String.prototype.charCodeAt( index )`
    ///
    /// The `charCodeAt()` method returns an integer between `0` and `65535` representing the UTF-16 code unit at the given index.
    ///
    /// Unicode code points range from `0` to `1114111` (`0x10FFFF`). The first 128 Unicode code points are a direct match of the ASCII character encoding.
    ///
    /// `charCodeAt()` returns `NaN` if the given index is less than `0`, or if it is equal to or greater than the `length` of the string.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.charcodeat
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/charCodeAt
    pub(crate) fn char_code_at(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        // 3. Let position be ? ToIntegerOrInfinity(pos).
        let position = args.get_or_undefined(0).to_integer_or_infinity(context)?;

        match position {
            // 4. Let size be the length of S.
            IntegerOrInfinity::Integer(i) if i >= 0 => {
                // 6. Return the Number value for the numeric value of the code unit at index position within the String S.
                Ok(string
                    .get(i as usize)
                    .map_or_else(JsValue::nan, JsValue::from))
            }
            // 5. If position < 0 or position ≥ size, return NaN.
            _ => Ok(JsValue::nan()),
        }
    }

    /// `String.prototype.concat( str1[, ...strN] )`
    ///
    /// The `concat()` method concatenates the string arguments to the calling string and returns a new string.
    ///
    /// Changes to the original string or the returned string don't affect the other.
    ///
    /// If the arguments are not of the type string, they are converted to string values before concatenating.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.concat
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/concat
    pub(crate) fn concat(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let mut string = this.to_string(context)?;

        // 3. Let R be S.
        // 4. For each element next of args, do
        for arg in args {
            // a. Let nextString be ? ToString(next).
            // b. Set R to the string-concatenation of R and nextString.
            string = js_string!(&string, &arg.to_string(context)?);
        }

        // 5. Return R.
        Ok(JsValue::new(string))
    }

    /// `String.prototype.repeat( count )`
    ///
    /// The `repeat()` method constructs and returns a new string which contains the specified number of
    /// copies of the string on which it was called, concatenated together.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.repeat
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/repeat
    pub(crate) fn repeat(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        let len = string.len();

        // 3. Let n be ? ToIntegerOrInfinity(count).
        match args.get_or_undefined(0).to_integer_or_infinity(context)? {
            IntegerOrInfinity::Integer(n)
                if n > 0 && (n as usize) * len <= Self::MAX_STRING_LENGTH =>
            {
                if string.is_empty() {
                    return Ok(js_string!().into());
                }
                let n = n as usize;
                let mut result = Vec::with_capacity(n);

                std::iter::repeat(string.as_str())
                    .take(n)
                    .for_each(|s| result.push(s));

                // 6. Return the String value that is made from n copies of S appended together.
                Ok(JsString::concat_array(&result).into())
            }
            // 5. If n is 0, return the empty String.
            IntegerOrInfinity::Integer(0) => Ok(js_string!().into()),
            // 4. If n < 0 or n is +∞, throw a RangeError exception.
            _ => Err(JsNativeError::range()
                .with_message(
                    "repeat count must be a positive finite number \
                        that doesn't overflow the maximum string length (2^32 - 1)",
                )
                .into()),
        }
    }

    /// `String.prototype.slice( beginIndex [, endIndex] )`
    ///
    /// The `slice()` method extracts a section of a string and returns it as a new string, without modifying the original string.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.slice
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/slice
    pub(crate) fn slice(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        // 3. Let len be the length of S.
        let len = string.len() as i64;

        // 4. Let intStart be ? ToIntegerOrInfinity(start).
        let from = match args.get_or_undefined(0).to_integer_or_infinity(context)? {
            // 6. Else if intStart < 0, let from be max(len + intStart, 0).
            IntegerOrInfinity::Integer(i) if i < 0 => max(len + i, 0),

            // 7. Else, let from be min(intStart, len).
            IntegerOrInfinity::Integer(i) => min(i, len),
            IntegerOrInfinity::PositiveInfinity => len,

            // 5. If intStart is -∞, let from be 0.
            IntegerOrInfinity::NegativeInfinity => 0,
        } as usize;

        // 8. If end is undefined, let intEnd be len; else let intEnd be ? ToIntegerOrInfinity(end).
        let to = match args
            .get(1)
            .filter(|end| !end.is_undefined())
            .map(|end| end.to_integer_or_infinity(context))
            .transpose()?
            .unwrap_or(IntegerOrInfinity::Integer(len))
        {
            // 10. Else if intEnd < 0, let to be max(len + intEnd, 0).
            IntegerOrInfinity::Integer(i) if i < 0 => max(len + i, 0),

            // 11. Else, let to be min(intEnd, len).
            IntegerOrInfinity::Integer(i) => min(i, len),
            IntegerOrInfinity::PositiveInfinity => len,

            // 9. If intEnd is -∞, let to be 0.
            IntegerOrInfinity::NegativeInfinity => 0,
        } as usize;

        // 12. If from ≥ to, return the empty String.
        if from >= to {
            Ok(js_string!().into())
        } else {
            // 13. Return the substring of S from from to to.
            Ok(js_string!(string.get_expect(from..to)).into())
        }
    }

    /// `String.prototype.startWith( searchString[, position] )`
    ///
    /// The `startsWith()` method determines whether a string begins with the characters of a specified string, returning `true` or `false` as appropriate.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.startswith
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/startsWith
    pub(crate) fn starts_with(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        let search_string = args.get_or_undefined(0);

        // 3. Let isRegExp be ? IsRegExp(searchString).
        // 4. If isRegExp is true, throw a TypeError exception.
        if RegExp::is_reg_exp(search_string, context)?.is_some() {
            return Err(JsNativeError::typ().with_message(
                "First argument to String.prototype.startsWith must not be a regular expression",
            ).into());
        }

        // 5. Let searchStr be ? ToString(searchString).
        let search_string = search_string.to_string(context)?;

        // 6. Let len be the length of S.
        let len = string.len() as i64;

        // 7. If position is undefined, let pos be 0; else let pos be ? ToIntegerOrInfinity(position).
        let pos = match args.get_or_undefined(1) {
            &JsValue::Undefined => IntegerOrInfinity::Integer(0),
            position => position.to_integer_or_infinity(context)?,
        };

        // 8. Let start be the result of clamping pos between 0 and len.
        let start = pos.clamp_finite(0, len) as usize;

        // 9. Let searchLength be the length of searchStr.
        let search_length = search_string.len();

        // 10. If searchLength = 0, return true.
        if search_length == 0 {
            return Ok(JsValue::new(true));
        }

        // 11. Let end be start + searchLength.
        let end = start + search_length;

        // 12. If end > len, return false.
        if end > len as usize {
            Ok(JsValue::new(false))
        } else {
            // 13. Let substring be the substring of S from start to end.
            // 14. Return ! SameValueNonNumeric(substring, searchStr).
            // `SameValueNonNumeric` forwards to `==`, so directly check
            // equality to avoid converting to `JsValue`
            Ok(JsValue::new(search_string == string.get_expect(start..end)))
        }
    }

    /// `String.prototype.endsWith( searchString[, length] )`
    ///
    /// The `endsWith()` method determines whether a string ends with the characters of a specified string, returning `true` or `false` as appropriate.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.endswith
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/endsWith
    pub(crate) fn ends_with(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        let search_str = match args.get_or_undefined(0) {
            // 3. Let isRegExp be ? IsRegExp(searchString).
            // 4. If isRegExp is true, throw a TypeError exception.
            search_string if RegExp::is_reg_exp(search_string, context)?.is_some() => {
                return Err(JsNativeError::typ().with_message(
                    "First argument to String.prototype.endsWith must not be a regular expression",
                ).into());
            }
            // 5. Let searchStr be ? ToString(searchString).
            search_string => search_string.to_string(context)?,
        };

        // 6. Let len be the length of S.
        let len = string.len() as i64;

        // 7. If endPosition is undefined, let pos be len; else let pos be ? ToIntegerOrInfinity(endPosition).
        let end = match args.get_or_undefined(1) {
            end_position if end_position.is_undefined() => IntegerOrInfinity::Integer(len),
            end_position => end_position.to_integer_or_infinity(context)?,
        };

        // 8. Let end be the result of clamping pos between 0 and len.
        let end = end.clamp_finite(0, len) as usize;

        // 9. Let searchLength be the length of searchStr.
        let search_length = search_str.len();

        // 10. If searchLength = 0, return true.
        if search_length == 0 {
            return Ok(true.into());
        }

        // 11. Let start be end - searchLength.
        if let Some(start) = end.checked_sub(search_length) {
            // 13. Let substring be the substring of S from start to end.
            // 14. Return ! SameValueNonNumeric(substring, searchStr).
            // `SameValueNonNumeric` forwards to `==`, so directly check
            // equality to avoid converting to `JsValue`
            Ok(JsValue::new(search_str == string.get_expect(start..end)))
        } else {
            // 12. If start < 0, return false.
            Ok(false.into())
        }
    }

    /// `String.prototype.includes( searchString[, position] )`
    ///
    /// The `includes()` method determines whether one string may be found within another string, returning `true` or `false` as appropriate.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.includes
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/includes
    pub(crate) fn includes(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        let search_str = match args.get_or_undefined(0) {
            // 3. Let isRegExp be ? IsRegExp(searchString).
            search_string if RegExp::is_reg_exp(search_string, context)?.is_some() => {
                return Err(JsNativeError::typ().with_message(
                    // 4. If isRegExp is true, throw a TypeError exception.
                    "First argument to String.prototype.includes must not be a regular expression",
                ).into());
            }
            // 5. Let searchStr be ? ToString(searchString).
            search_string => search_string.to_string(context)?,
        };

        // 6. Let pos be ? ToIntegerOrInfinity(position).
        // 7. Assert: If position is undefined, then pos is 0.
        let pos = args.get_or_undefined(1).to_integer_or_infinity(context)?;

        // 8. Let len be the length of S.
        // 9. Let start be the result of clamping pos between 0 and len.
        let start = pos.clamp_finite(0, string.len() as i64) as usize;

        // 10. Let index be ! StringIndexOf(S, searchStr, start).
        // 11. If index is not -1, return true.
        // 12. Return false.
        Ok(string.index_of(search_str.as_str(), start).is_some().into())
    }

    /// `String.prototype.replace( regexp|substr, newSubstr|function )`
    ///
    /// The `replace()` method returns a new string with some or all matches of a `pattern` replaced by a `replacement`.
    ///
    /// The `pattern` can be a string or a `RegExp`, and the `replacement` can be a string or a function to be called for each match.
    /// If `pattern` is a string, only the first occurrence will be replaced.
    ///
    /// The original string is left unchanged.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.replace
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace
    pub(crate) fn replace(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // Helper enum.
        enum CallableOrString<'a> {
            FunctionalReplace(&'a JsObject),
            ReplaceValue(JsString),
        }

        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        let search_value = args.get_or_undefined(0);
        let replace_value = args.get_or_undefined(1);

        // 2. If searchValue is neither undefined nor null, then
        if !search_value.is_null_or_undefined() {
            // a. Let replacer be ? GetMethod(searchValue, @@replace).
            let replacer = search_value.get_method(JsSymbol::replace(), context)?;

            // b. If replacer is not undefined, then
            if let Some(replacer) = replacer {
                // i. Return ? Call(replacer, searchValue, « O, replaceValue »).
                return replacer.call(search_value, &[o.clone(), replace_value.clone()], context);
            }
        }

        // 3. Let string be ? ToString(O).
        let string = o.to_string(context)?;

        // 4. Let searchString be ? ToString(searchValue).
        let search_string = search_value.to_string(context)?;

        // 5. Let functionalReplace be IsCallable(replaceValue).
        let functional_replace = replace_value.as_callable();

        // 6. If functionalReplace is false, then
        let replace_value = if let Some(callable) = functional_replace {
            CallableOrString::FunctionalReplace(callable)
        } else {
            // a. Set replaceValue to ? ToString(replaceValue).
            CallableOrString::ReplaceValue(replace_value.to_string(context)?)
        };

        // 7. Let searchLength be the length of searchString.
        let search_length = search_string.len();

        // 8. Let position be ! StringIndexOf(string, searchString, 0).
        // 9. If position is -1, return string.
        let Some(position) = string.index_of(search_string.as_str(), 0) else {
            return Ok(string.into());
        };

        // 10. Let preserved be the substring of string from 0 to position.
        let preserved = JsString::from(string.get_expect(..position));

        let replacement = match replace_value {
            // 11. If functionalReplace is true, then
            CallableOrString::FunctionalReplace(replace_fn) => {
                // a. Let replacement be ? ToString(? Call(replaceValue, undefined, « searchString, 𝔽(position), string »)).
                replace_fn
                    .call(
                        &JsValue::undefined(),
                        &[search_string.into(), position.into(), string.clone().into()],
                        context,
                    )?
                    .to_string(context)?
            }
            // 12. Else,
            CallableOrString::ReplaceValue(replace_value) => {
                // a. Assert: Type(replaceValue) is String.
                // b. Let captures be a new empty List.
                let captures = Vec::new();

                // c. Let replacement be ! GetSubstitution(searchString, string, position, captures, undefined, replaceValue).
                get_substitution(
                    &search_string,
                    &string,
                    position,
                    &captures,
                    &JsValue::undefined(),
                    &replace_value,
                    context,
                )?
            }
        };

        // 13. Return the string-concatenation of preserved, replacement, and the substring of string from position + searchLength.
        Ok(js_string!(
            &preserved,
            &replacement,
            &JsString::from(string.get_expect(position + search_length..))
        )
        .into())
    }

    /// `22.1.3.18 String.prototype.replaceAll ( searchValue, replaceValue )`
    ///
    /// The `replaceAll()` method returns a new string with all matches of a pattern replaced by a
    /// replacement.
    ///
    /// The pattern can be a string or a `RegExp`, and the replacement can be a string or a
    /// function to be called for each match.
    ///
    /// The original string is left unchanged.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.replaceall
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/replace
    pub(crate) fn replace_all(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        let search_value = args.get_or_undefined(0);
        let replace_value = args.get_or_undefined(1);

        // 2. If searchValue is neither undefined nor null, then
        if !search_value.is_null_or_undefined() {
            // a. Let isRegExp be ? IsRegExp(searchValue).
            // b. If isRegExp is true, then
            if let Some(obj) = RegExp::is_reg_exp(search_value, context)? {
                // i. Let flags be ? Get(searchValue, "flags").
                let flags = obj.get(js_string!("flags"), context)?;

                // ii. Perform ? RequireObjectCoercible(flags).
                flags.require_object_coercible()?;

                // iii. If ? ToString(flags) does not contain "g", throw a TypeError exception.
                if !flags.to_string(context)?.contains(b'g') {
                    return Err(JsNativeError::typ()
                        .with_message(
                            "String.prototype.replaceAll called with a non-global RegExp argument",
                        )
                        .into());
                }
            }

            // c. Let replacer be ? GetMethod(searchValue, @@replace).
            let replacer = search_value.get_method(JsSymbol::replace(), context)?;

            // d. If replacer is not undefined, then
            if let Some(replacer) = replacer {
                // i. Return ? Call(replacer, searchValue, « O, replaceValue »).
                return replacer.call(search_value, &[o.clone(), replace_value.clone()], context);
            }
        }

        // 3. Let string be ? ToString(O).
        let string = o.to_string(context)?;

        // 4. Let searchString be ? ToString(searchValue).
        let search_string = search_value.to_string(context)?;

        // 5. Let functionalReplace be IsCallable(replaceValue).
        let replace = if let Some(f) = replace_value.as_callable() {
            Ok(f)
        } else {
            // 6. If functionalReplace is false, then
            // a. Set replaceValue to ? ToString(replaceValue).
            Err(replace_value.to_string(context)?)
        };

        // 7. Let searchLength be the length of searchString.
        let search_length = search_string.len();

        // 8. Let advanceBy be max(1, searchLength).
        let advance_by = max(1, search_length);

        // 9. Let matchPositions be a new empty List.
        let mut match_positions = Vec::new();

        // 10. Let position be ! StringIndexOf(string, searchString, 0).
        let mut position = string.index_of(search_string.as_str(), 0);

        // 11. Repeat, while position is not -1,
        while let Some(p) = position {
            // a. Append position to the end of matchPositions.
            match_positions.push(p);

            // b. Set position to ! StringIndexOf(string, searchString, position + advanceBy).
            position = string.index_of(search_string.as_str(), p + advance_by);
        }

        // 12. Let endOfLastMatch be 0.
        let mut end_of_last_match = 0;

        // 13. Let result be the empty String.
        let mut result = Vec::with_capacity(string.len());

        // 14. For each element p of matchPositions, do
        for p in match_positions {
            // a. Let preserved be the substring of string from endOfLastMatch to p.
            let preserved = string.get_expect(end_of_last_match..p);

            // c. Else,
            let replacement = match replace {
                // b. If functionalReplace is true, then
                Ok(replace_fn) => {
                    // i. Let replacement be ? ToString(? Call(replaceValue, undefined, « searchString, 𝔽(p), string »)).
                    replace_fn
                        .call(
                            &JsValue::undefined(),
                            &[
                                search_string.clone().into(),
                                p.into(),
                                string.clone().into(),
                            ],
                            context,
                        )?
                        .to_string(context)?
                }
                // i. Assert: Type(replaceValue) is String.
                // ii. Let captures be a new empty List.
                // iii. Let replacement be ! GetSubstitution(searchString, string, p, captures, undefined, replaceValue).
                Err(ref replace_str) => get_substitution(
                    &search_string,
                    &string,
                    p,
                    &[],
                    &JsValue::undefined(),
                    replace_str,
                    context,
                )
                .expect("GetSubstitution should never fail here."),
            };

            // d. Set result to the string-concatenation of result, preserved, and replacement.
            result.extend(preserved.iter());
            result.extend(replacement.iter());

            // e. Set endOfLastMatch to p + searchLength.
            end_of_last_match = p + search_length;
        }

        // 15. If endOfLastMatch < the length of string, then
        if end_of_last_match < string.len() {
            // a. Set result to the string-concatenation of result and the substring of string from endOfLastMatch.
            result.extend(string.get_expect(end_of_last_match..).iter());
        }

        // 16. Return result.
        Ok(js_string!(&result[..]).into())
    }

    /// `String.prototype.indexOf( searchValue[, fromIndex] )`
    ///
    /// The `indexOf()` method returns the index within the calling `String` object of the first occurrence
    /// of the specified value, starting the search at `fromIndex`.
    ///
    /// Returns `-1` if the value is not found.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.indexof
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/indexOf
    pub(crate) fn index_of(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        // 3. Let searchStr be ? ToString(searchString).
        let search_str = args.get_or_undefined(0).to_string(context)?;

        // 4. Let pos be ? ToIntegerOrInfinity(position).
        // 5. Assert: If position is undefined, then pos is 0.
        let pos = args.get_or_undefined(1).to_integer_or_infinity(context)?;

        // 6. Let len be the length of S.
        let len = string.len() as i64;

        // 7. Let start be the result of clamping pos between 0 and len.
        let start = pos.clamp_finite(0, len) as usize;

        // 8. Return 𝔽(! StringIndexOf(S, searchStr, start)).
        Ok(string
            .index_of(search_str.as_str(), start)
            .map_or(-1, |i| i as i64)
            .into())
    }

    /// `String.prototype.isWellFormed ( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.iswellformed
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/isWellFormed
    pub(crate) fn is_well_formed(
        this: &JsValue,
        _: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let s = o.to_string(context)?;

        // 3. Return IsStringWellFormedUnicode(S).
        for code_point in s.code_points() {
            if let CodePoint::UnpairedSurrogate(_) = code_point {
                return Ok(false.into());
            }
        }
        Ok(true.into())
    }

    /// `String.prototype.lastIndexOf( searchValue[, fromIndex] )`
    ///
    /// The `lastIndexOf()` method returns the index within the calling `String` object of the last occurrence
    /// of the specified value, searching backwards from `fromIndex`.
    ///
    /// Returns `-1` if the value is not found.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.lastindexof
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/lastIndexOf
    pub(crate) fn last_index_of(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        // 3. Let searchStr be ? ToString(searchString).
        let search_str = args.get_or_undefined(0).to_string(context)?;

        // 4. Let numPos be ? ToNumber(position).
        // 5. Assert: If position is undefined, then numPos is NaN.
        let num_pos = args.get_or_undefined(1).to_number(context)?;

        // 6. If numPos is NaN, let pos be +∞; otherwise, let pos be ! ToIntegerOrInfinity(numPos).
        let pos = if num_pos.is_nan() {
            IntegerOrInfinity::PositiveInfinity
        } else {
            JsValue::new(num_pos)
                .to_integer_or_infinity(context)
                .expect("Already called `to_number so this must not fail.")
        };

        // 7. Let len be the length of S.
        let len = string.len();
        // 8. Let start be the result of clamping pos between 0 and len.
        let start = pos.clamp_finite(0, len as i64) as usize;

        // 9. If searchStr is the empty String, return 𝔽(start).
        if search_str.is_empty() {
            return Ok(JsValue::new(start));
        }

        // 10. Let searchLen be the length of searchStr.
        let search_len = search_str.len();

        if let Some(end) = len.checked_sub(search_len) {
            // 11. For each non-negative integer i starting with start such that i ≤ len - searchLen, in descending order, do
            for i in (0..=min(start, end)).rev() {
                // a. Let candidate be the substring of S from i to i + searchLen.
                let candidate = string.get_expect(i..i + search_len);

                // b. If candidate is the same sequence of code units as searchStr, return 𝔽(i).
                if candidate == search_str {
                    return Ok(i.into());
                }
            }
        }

        // 12. Return -1𝔽.
        Ok(JsValue::new(-1))
    }

    /// [`String.prototype.localeCompare ( that [ , locales [ , options ] ] )`][spec]
    ///
    /// Returns a number indicating whether a reference string comes before, or after, or is the
    /// same as the given string in sort order.
    ///
    /// More information:
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma402/#sup-String.prototype.localeCompare
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/localeCompare
    pub(crate) fn locale_compare(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let s = o.to_string(context)?;

        // 3. Let thatValue be ? ToString(that).
        let that_value = args.get_or_undefined(0).to_string(context)?;

        let cmp = {
            #[cfg(feature = "intl")]
            {
                use crate::builtins::intl::Collator;

                // 4. Let collator be ? Construct(%Collator%, « locales, options »).
                let collator = Collator::constructor(
                    &context
                        .intrinsics()
                        .constructors()
                        .collator()
                        .constructor()
                        .into(),
                    args.get(1..).unwrap_or_default(),
                    context,
                )?;

                let collator = collator
                    .as_object()
                    .map(JsObject::borrow)
                    .expect("constructor must return a JsObject");
                let collator = collator
                    .downcast_ref::<Collator>()
                    .expect("constructor must return a `Collator` object")
                    .collator();

                let s = s.iter().collect::<Vec<_>>();
                let that_value = that_value.iter().collect::<Vec<_>>();

                collator.compare_utf16(&s, &that_value) as i8
            }

            // Default to common comparison if the user doesn't have `Intl` enabled.
            #[cfg(not(feature = "intl"))]
            {
                s.cmp(&that_value) as i8
            }
        };

        // 5. Return CompareStrings(collator, S, thatValue).
        Ok(cmp.into())
    }

    /// `String.prototype.match( regexp )`
    ///
    /// The `match()` method retrieves the result of matching a **string** against a [`regular expression`][regex].
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.match
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/match
    /// [regex]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
    pub(crate) fn r#match(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        // 2. If regexp is neither undefined nor null, then
        let regexp = args.get_or_undefined(0);
        if !regexp.is_null_or_undefined() {
            // a. Let matcher be ? GetMethod(regexp, @@match).
            let matcher = regexp.get_method(JsSymbol::r#match(), context)?;
            // b. If matcher is not undefined, then
            if let Some(matcher) = matcher {
                // i. Return ? Call(matcher, regexp, « O »).
                return matcher.call(regexp, &[o.clone()], context);
            }
        }

        // 3. Let S be ? ToString(O).
        let s = o.to_string(context)?;

        // 4. Let rx be ? RegExpCreate(regexp, undefined).
        let rx = RegExp::create(regexp, &JsValue::Undefined, context)?;

        // 5. Return ? Invoke(rx, @@match, « S »).
        rx.invoke(JsSymbol::r#match(), &[JsValue::new(s)], context)
    }

    /// Abstract operation `StringPad ( O, maxLength, fillString, placement )`.
    ///
    /// Performs the actual string padding for padStart/End.
    ///
    /// More information:
    /// - [ECMAScript reference][spec]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-stringpad
    pub(crate) fn string_pad(
        object: &JsValue,
        max_length: &JsValue,
        fill_string: &JsValue,
        placement: Placement,
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let S be ? ToString(O).
        let string = object.to_string(context)?;

        // 2. Let intMaxLength be ℝ(? ToLength(maxLength)).
        let int_max_length = max_length.to_length(context)?;

        // 3. Let stringLength be the length of S.
        let string_length = string.len() as u64;

        // 4. If intMaxLength ≤ stringLength, return S.
        if int_max_length <= string_length {
            return Ok(string.into());
        }

        // 5. If fillString is undefined, let filler be the String value consisting solely of the code unit 0x0020 (SPACE).
        let filler = if fill_string.is_undefined() {
            js_string!("\u{0020}")
        } else {
            // 6. Else, let filler be ? ToString(fillString).
            fill_string.to_string(context)?
        };

        // 7. If filler is the empty String, return S.
        if filler.is_empty() {
            return Ok(string.into());
        }

        // 8. Let fillLen be intMaxLength - stringLength.
        let fill_len = int_max_length - string_length;
        let filler_len = filler.len() as u64;

        // 9. Let truncatedStringFiller be the String value consisting of repeated
        // concatenations of filler truncated to length fillLen.
        let repetitions = {
            let q = fill_len / filler_len;
            let r = fill_len % filler_len;
            if r == 0 {
                q
            } else {
                q + 1
            }
        };

        let truncated_string_filler = filler.to_vec().repeat(repetitions as usize);
        let truncated_string_filler = JsString::from(&truncated_string_filler[..fill_len as usize]);

        // 10. If placement is start, return the string-concatenation of truncatedStringFiller and S.
        if placement == Placement::Start {
            Ok(js_string!(&truncated_string_filler, &string).into())
        } else {
            // 11. Else, return the string-concatenation of S and truncatedStringFiller.
            Ok(js_string!(&string, &truncated_string_filler).into())
        }
    }

    /// `String.prototype.padEnd( targetLength[, padString] )`
    ///
    /// The `padEnd()` method pads the current string with a given string (repeated, if needed) so that the resulting string reaches a given length.
    ///
    /// The padding is applied from the end of the current string.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.padend
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padEnd
    pub(crate) fn pad_end(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        let max_length = args.get_or_undefined(0);
        let fill_string = args.get_or_undefined(1);

        // 2. Return ? StringPad(O, maxLength, fillString, end).
        Self::string_pad(this, max_length, fill_string, Placement::End, context)
    }

    /// `String.prototype.padStart( targetLength [, padString] )`
    ///
    /// The `padStart()` method pads the current string with another string (multiple times, if needed) until the resulting string reaches the given length.
    ///
    /// The padding is applied from the start of the current string.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.padstart
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/padStart
    pub(crate) fn pad_start(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        let max_length = args.get_or_undefined(0);
        let fill_string = args.get_or_undefined(1);

        // 2. Return ? StringPad(O, maxLength, fillString, end).
        Self::string_pad(this, max_length, fill_string, Placement::Start, context)
    }

    /// `String.prototype.trim()`
    ///
    /// The `trim()` method removes whitespace from both ends of a string.
    ///
    /// Whitespace in this context is all the whitespace characters (space, tab, no-break space, etc.) and all the line terminator characters (LF, CR, etc.).
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.trim
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/trim
    pub(crate) fn trim(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        // 2. Return ? TrimString(S, start+end).
        let object = this.require_object_coercible()?;
        let string = object.to_string(context)?;
        Ok(js_string!(string.trim()).into())
    }

    /// `String.prototype.trimStart()`
    ///
    /// The `trimStart()` method removes whitespace from the beginning of a string.
    ///
    /// Whitespace in this context is all the whitespace characters (space, tab, no-break space, etc.) and all the line terminator characters (LF, CR, etc.).
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.trimstart
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/trimStart
    pub(crate) fn trim_start(
        this: &JsValue,
        _: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        // 2. Return ? TrimString(S, start).
        let object = this.require_object_coercible()?;
        let string = object.to_string(context)?;
        Ok(js_string!(string.trim_start()).into())
    }

    /// `String.prototype.trimEnd()`
    ///
    /// The `trimEnd()` method removes whitespace from the end of a string.
    ///
    /// Whitespace in this context is all the whitespace characters (space, tab, no-break space, etc.) and all the line terminator characters (LF, CR, etc.).
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.trimend
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/trimEnd
    pub(crate) fn trim_end(
        this: &JsValue,
        _: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        // 2. Return ? TrimString(S, end).
        let object = this.require_object_coercible()?;
        let string = object.to_string(context)?;
        Ok(js_string!(string.trim_end()).into())
    }

    /// [`String.prototype.toUpperCase()`][upper] and [`String.prototype.toLowerCase()`][lower]
    ///
    /// The case methods return the calling string value converted to uppercase or lowercase.
    ///
    /// The value will be **converted** to a string if it isn't one.
    ///
    /// More information:
    ///  - [MDN documentation][mdn]
    ///
    /// [upper]: https://tc39.es/ecma262/#sec-string.prototype.toUppercase
    /// [lower]: https://tc39.es/ecma262/#sec-string.prototype.toLowercase
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/toUpperCase
    pub(crate) fn to_case<const UPPER: bool>(
        this: &JsValue,
        _: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        // 3. Let sText be ! StringToCodePoints(S).
        // 4. Let upperText be the result of toUppercase(sText), according to
        // the Unicode Default Case Conversion algorithm.
        let text = string.map_valid_segments(|s| {
            if UPPER {
                s.to_uppercase()
            } else {
                s.to_lowercase()
            }
        });

        // 5. Let L be ! CodePointsToString(upperText).
        // 6. Return L.
        Ok(js_string!(text).into())
    }

    /// [`String.prototype.toLocaleLowerCase ( [ locales ] )`][lower] and
    /// [`String.prototype.toLocaleUpperCase ( [ locales ] )`][upper]
    ///
    /// [lower]: https://tc39.es/ecma402/#sup-string.prototype.tolocalelowercase
    /// [upper]: https://tc39.es/ecma402/#sup-string.prototype.tolocaleuppercase
    pub(crate) fn to_locale_case<const UPPER: bool>(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        #[cfg(feature = "intl")]
        {
            use super::intl::locale::{
                canonicalize_locale_list, default_locale, lookup_matching_locale_by_prefix,
            };
            // TODO: Small hack to make lookups behave.
            // We would really like to be able to use `icu_casemap::provider::CaseMapV1Marker`
            use icu_locid::Locale;
            use icu_plurals::provider::OrdinalV1Marker;

            // 1. Let O be ? RequireObjectCoercible(this value).
            let this = this.require_object_coercible()?;

            // 2. Let S be ? ToString(O).
            let string = this.to_string(context)?;

            // 3. Return ? TransformCase(S, locales, lower).

            //  TransformCase ( S, locales, targetCase )
            // https://tc39.es/ecma402/#sec-transform-case

            // 1. Let requestedLocales be ? CanonicalizeLocaleList(locales).
            // 2. If requestedLocales is not an empty List, then
            let mut requested_locale = if let Some(locale) =
                canonicalize_locale_list(args.get_or_undefined(0), context)?
                    .into_iter()
                    .next()
            {
                // a. Let requestedLocale be requestedLocales[0].
                locale
            } else {
                // 3. Else,
                //     a. Let requestedLocale be ! DefaultLocale().
                default_locale(context.intl_provider().locale_canonicalizer()?)
            };
            // 4. Let noExtensionsLocale be the String value that is requestedLocale with any Unicode locale extension sequences (6.2.1) removed.
            requested_locale.extensions.unicode.clear();

            // 5. Let availableLocales be a List with language tags that includes the languages for which the Unicode
            //    Character Database contains language sensitive case mappings. Implementations may add additional
            //    language tags if they support case mapping for additional locales.
            // 6. Let match be LookupMatchingLocaleByPrefix(availableLocales, noExtensionsLocale).
            // 7. If match is not undefined, let locale be match.[[locale]]; else let locale be "und".
            let locale = lookup_matching_locale_by_prefix::<OrdinalV1Marker>(
                [requested_locale],
                context.intl_provider(),
            )
            .unwrap_or(Locale::UND);

            let casemapper = context.intl_provider().case_mapper()?;

            // 8. Let codePoints be StringToCodePoints(S).
            let result = string.map_valid_segments(|segment| {
                if UPPER {
                    // 10. Else,
                    //     a. Assert: targetCase is upper.
                    //     b. Let newCodePoints be a List whose elements are the result of an uppercase transformation of codePoints according to an implementation-derived algorithm using locale or the Unicode Default Case Conversion algorithm.
                    casemapper.uppercase_to_string(&segment, &locale.id)
                } else {
                    // 9. If targetCase is lower, then
                    //     a. Let newCodePoints be a List whose elements are the result of a lowercase transformation of codePoints according to an implementation-derived algorithm using locale or the Unicode Default Case Conversion algorithm.
                    casemapper.lowercase_to_string(&segment, &locale.id)
                }
            });

            // 11. Return CodePointsToString(newCodePoints).
            Ok(result.into())
        }

        #[cfg(not(feature = "intl"))]
        {
            Self::to_case::<UPPER>(this, args, context)
        }
    }

    /// `String.prototype.toWellFormed ( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.towellformed
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/toWellFormed
    pub(crate) fn to_well_formed(
        this: &JsValue,
        _: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let s = o.to_string(context)?;

        // 3. Let strLen be the length of S.
        // 4. Let k be 0.
        // 5. Let result be the empty String.
        // 6. Repeat, while k < strLen,
        // a. Let cp be CodePointAt(S, k).
        // b. If cp.[[IsUnpairedSurrogate]] is true, then
        // i. Set result to the string-concatenation of result and 0xFFFD (REPLACEMENT CHARACTER).
        // c. Else,
        // i. Set result to the string-concatenation of result and UTF16EncodeCodePoint(cp.[[CodePoint]]).
        // d. Set k to k + cp.[[CodeUnitCount]].
        let result = s
            .code_points()
            .map(|code_point| match code_point {
                CodePoint::UnpairedSurrogate(_) => '\u{FFFD}',
                CodePoint::Unicode(char) => char,
            })
            .collect::<std::string::String>();

        // 7. Return result.
        Ok(js_string!(result).into())
    }

    /// `String.prototype.substring( indexStart[, indexEnd] )`
    ///
    /// The `substring()` method returns the part of the `string` between the start and end indexes, or to the end of the string.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.substring
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substring
    pub(crate) fn substring(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let string = this.to_string(context)?;

        // 3. Let len be the length of S.
        let len = string.len() as i64;

        // 4. Let intStart be ? ToIntegerOrInfinity(start).
        let int_start = args.get_or_undefined(0).to_integer_or_infinity(context)?;

        // 5. If end is undefined, let intEnd be len; else let intEnd be ? ToIntegerOrInfinity(end).
        let int_end = match args.get_or_undefined(1) {
            &JsValue::Undefined => IntegerOrInfinity::Integer(len),
            end => end.to_integer_or_infinity(context)?,
        };

        // 6. Let finalStart be the result of clamping intStart between 0 and len.
        let final_start = int_start.clamp_finite(0, len) as usize;

        // 7. Let finalEnd be the result of clamping intEnd between 0 and len.
        let final_end = int_end.clamp_finite(0, len) as usize;

        // 8. Let from be min(finalStart, finalEnd).
        let from = min(final_start, final_end);

        // 9. Let to be max(finalStart, finalEnd).
        let to = max(final_start, final_end);

        // 10. Return the substring of S from from to to.
        Ok(js_string!(string.get_expect(from..to)).into())
    }

    /// `String.prototype.split ( separator, limit )`
    ///
    /// The `split()` method divides a String into an ordered list of substrings, puts these substrings into an array, and returns the array.
    /// The division is done by searching for a pattern; where the pattern is provided as the first parameter in the method's call.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.split
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/split
    pub(crate) fn split(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        let separator = args.get_or_undefined(0);
        let limit = args.get_or_undefined(1);

        // 2. If separator is neither undefined nor null, then
        if !separator.is_null_or_undefined() {
            // a. Let splitter be ? GetMethod(separator, @@split).
            let splitter = separator.get_method(JsSymbol::split(), context)?;
            // b. If splitter is not undefined, then
            if let Some(splitter) = splitter {
                // i. Return ? Call(splitter, separator, « O, limit »).
                return splitter.call(separator, &[this.clone(), limit.clone()], context);
            }
        }

        // 3. Let S be ? ToString(O).
        let this_str = this.to_string(context)?;

        // 4.  If limit is undefined, let lim be 2^32 - 1; else let lim be ℝ(? ToUint32(limit)).
        let lim = if limit.is_undefined() {
            u32::MAX
        } else {
            limit.to_u32(context)?
        } as usize;

        // 5. Let R be ? ToString(separator).
        let separator_str = separator.to_string(context)?;

        // 6. If lim = 0, return A.
        if lim == 0 {
            // a. Return ! CreateArrayFromList(« »).
            return Ok(Array::create_array_from_list([], context).into());
        }

        // 7. If separator is undefined, then
        if separator.is_undefined() {
            // a. Return ! CreateArrayFromList(« S »).
            return Ok(Array::create_array_from_list([this_str.into()], context).into());
        }

        // 8. Let separatorLength be the length of R.
        let separator_length = separator_str.len();

        // 9. If separatorLength is 0, then
        if separator_length == 0 {
            // a. Let head be the substring of S from 0 to lim.
            // b. Let codeUnits be a List consisting of the sequence of code units that are the elements of head.
            let head = this_str
                .get(..lim)
                .unwrap_or(this_str.as_str())
                .iter()
                .map(|code| js_string!(std::slice::from_ref(&code)).into());

            // c. Return ! CreateArrayFromList(codeUnits).
            return Ok(Array::create_array_from_list(head, context).into());
        }

        // 10. If S is the empty String, return ! CreateArrayFromList(« S »).
        if this_str.is_empty() {
            return Ok(Array::create_array_from_list([this_str.into()], context).into());
        }

        // 11. Let substrings be a new empty List.
        let mut substrings = vec![];

        // 12. Let i be 0.
        let mut i = 0;

        // 13. Let j be ! StringIndexOf(S, R, 0).
        let mut j = this_str.index_of(separator_str.as_str(), 0);

        // 14. Repeat, while j is not -1
        while let Some(index) = j {
            // a. Let T be the substring of S from i to j.
            // b. Append T as the last element of substrings.
            substrings.push(this_str.get_expect(i..index).into());

            // c. If the number of elements of substrings is lim, return ! CreateArrayFromList(substrings).
            if substrings.len() == lim {
                return Ok(Array::create_array_from_list(
                    substrings.into_iter().map(JsValue::from),
                    context,
                )
                .into());
            }
            // d. Set i to j + separatorLength.
            i = index + separator_length;

            // e. Set j to ! StringIndexOf(S, R, i).
            j = this_str.index_of(separator_str.as_str(), i);
        }

        // 15. Let T be the substring of S from i.
        // 16. Append T to substrings.
        substrings.push(JsString::from(this_str.get_expect(i..)));

        // 17. Return ! CreateArrayFromList(substrings).
        Ok(
            Array::create_array_from_list(substrings.into_iter().map(JsValue::from), context)
                .into(),
        )
    }

    /// `String.prototype.valueOf()`
    ///
    /// The `valueOf()` method returns the primitive value of a `String` object.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.value_of
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/valueOf
    pub(crate) fn value_of(
        this: &JsValue,
        _args: &[JsValue],
        _context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Return ? thisStringValue(this value).
        Self::this_string_value(this).map(JsValue::from)
    }

    /// `String.prototype.matchAll( regexp )`
    ///
    /// The `matchAll()` method returns an iterator of all results matching a string against a [`regular expression`][regex], including [capturing groups][cg].
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.matchall
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/matchAll
    /// [regex]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions
    /// [cg]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Guide/Regular_Expressions/Groups_and_Ranges
    pub(crate) fn match_all(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        // 2. If regexp is neither undefined nor null, then
        let regexp = args.get_or_undefined(0);
        if !regexp.is_null_or_undefined() {
            // a. Let isRegExp be ? IsRegExp(regexp).
            // b. If isRegExp is true, then
            if let Some(regexp) = RegExp::is_reg_exp(regexp, context)? {
                // i. Let flags be ? Get(regexp, "flags").
                let flags = regexp.get(js_string!("flags"), context)?;

                // ii. Perform ? RequireObjectCoercible(flags).
                flags.require_object_coercible()?;

                // iii. If ? ToString(flags) does not contain "g", throw a TypeError exception.
                if !flags.to_string(context)?.contains(b'g') {
                    return Err(JsNativeError::typ()
                        .with_message(
                            "String.prototype.matchAll called with a non-global RegExp argument",
                        )
                        .into());
                }
            }
            // c. Let matcher be ? GetMethod(regexp, @@matchAll).
            let matcher = regexp.get_method(JsSymbol::match_all(), context)?;
            // d. If matcher is not undefined, then
            if let Some(matcher) = matcher {
                return matcher.call(regexp, &[o.clone()], context);
            }
        }

        // 3. Let S be ? ToString(O).
        let s = o.to_string(context)?;

        // 4. Let rx be ? RegExpCreate(regexp, "g").
        let rx = RegExp::create(regexp, &JsValue::new(js_string!("g")), context)?;

        // 5. Return ? Invoke(rx, @@matchAll, « S »).
        rx.invoke(JsSymbol::match_all(), &[JsValue::new(s)], context)
    }

    /// `String.prototype.normalize( [ form ] )`
    ///
    /// The `normalize()` method normalizes a string into a form specified in the Unicode® Standard Annex #15
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.normalize
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/normalize
    pub(crate) fn normalize(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        /// Represents the type of normalization applied to a [`JsString`]
        #[derive(Clone, Copy)]
        pub(crate) enum Normalization {
            Nfc,
            Nfd,
            Nfkc,
            Nfkd,
        }

        // 1. Let O be ? RequireObjectCoercible(this value).
        let this = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let s = this.to_string(context)?;

        // 6. Let ns be the String value that is the result of normalizing S
        // into the normalization form named by f as specified in
        // https://unicode.org/reports/tr15/.
        let normalization = match args.get_or_undefined(0) {
            // 3. If form is undefined, let f be "NFC".
            &JsValue::Undefined => Normalization::Nfc,
            // 4. Else, let f be ? ToString(form).
            f => match f.to_string(context)? {
                ntype if &ntype == "NFC" => Normalization::Nfc,
                ntype if &ntype == "NFD" => Normalization::Nfd,
                ntype if &ntype == "NFKC" => Normalization::Nfkc,
                ntype if &ntype == "NFKD" => Normalization::Nfkd,
                // 5. If f is not one of "NFC", "NFD", "NFKC", or "NFKD", throw a RangeError exception.
                _ => {
                    return Err(JsNativeError::range()
                        .with_message(
                            "The normalization form should be one of NFC, NFD, NFKC, NFKD.",
                        )
                        .into());
                }
            },
        };

        let normalizers = {
            #[cfg(not(feature = "intl"))]
            {
                const NORMALIZERS: StringNormalizers = StringNormalizers {
                    nfc: ComposingNormalizer::new_nfc(),
                    nfkc: ComposingNormalizer::new_nfkc(),
                    nfd: DecomposingNormalizer::new_nfd(),
                    nfkd: DecomposingNormalizer::new_nfkd(),
                };
                &NORMALIZERS
            }
            #[cfg(feature = "intl")]
            {
                context.intl_provider().string_normalizers()?
            }
        };

        let s = s.iter().collect::<Vec<_>>();

        let result = match normalization {
            Normalization::Nfc => normalizers.nfc.normalize_utf16(&s),
            Normalization::Nfd => normalizers.nfd.normalize_utf16(&s),
            Normalization::Nfkc => normalizers.nfkc.normalize_utf16(&s),
            Normalization::Nfkd => normalizers.nfkd.normalize_utf16(&s),
        };

        // 7. Return ns.
        Ok(js_string!(&result[..]).into())
    }

    /// `String.prototype.search( regexp )`
    ///
    /// The `search()` method executes a search for a match between a regular expression and this String object.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.search
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/search
    pub(crate) fn search(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        // 2. If regexp is neither undefined nor null, then
        let regexp = args.get_or_undefined(0);
        if !regexp.is_null_or_undefined() {
            // a. Let searcher be ? GetMethod(regexp, @@search).
            let searcher = regexp.get_method(JsSymbol::search(), context)?;
            // b. If searcher is not undefined, then
            if let Some(searcher) = searcher {
                // i. Return ? Call(searcher, regexp, « O »).
                return searcher.call(regexp, &[o.clone()], context);
            }
        }

        // 3. Let string be ? ToString(O).
        let string = o.to_string(context)?;

        // 4. Let rx be ? RegExpCreate(regexp, undefined).
        let rx = RegExp::create(regexp, &JsValue::Undefined, context)?;

        // 5. Return ? Invoke(rx, @@search, « string »).
        rx.invoke(JsSymbol::search(), &[JsValue::new(string)], context)
    }

    pub(crate) fn iterator(
        this: &JsValue,
        _: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;
        // 2. Let s be ? ToString(O).
        let s = o.to_string(context)?;

        Ok(StringIterator::create_string_iterator(s, context).into())
    }
}

#[cfg(feature = "annex-b")]
impl String {
    /// `String.prototype.substr( start[, length] )`
    ///
    /// The `substr()` method returns a portion of the string, starting at the specified index and extending for a given number of characters afterward.
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.substr
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/substr
    /// <https://tc39.es/ecma262/#sec-string.prototype.substr>
    pub(crate) fn substr(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let O be ? RequireObjectCoercible(this value).
        let o = this.require_object_coercible()?;

        // 2. Let S be ? ToString(O).
        let s = o.to_string(context)?;

        // 3. Let size be the length of S.
        let size = s.len() as i64;

        // 4. Let intStart be ? ToIntegerOrInfinity(start).
        let start = args.get_or_undefined(0);
        let int_start = start.to_integer_or_infinity(context)?;

        let int_start = match int_start {
            // 5. If intStart is -∞, set intStart to 0.
            IntegerOrInfinity::NegativeInfinity => 0,
            // 6. Else if intStart < 0, set intStart to max(size + intStart, 0).
            IntegerOrInfinity::Integer(int_start) if int_start < 0 => max(size + int_start, 0),
            IntegerOrInfinity::Integer(int_start) => int_start,
            // 7. Else, set intStart to min(intStart, size).
            //
            // NOTE: size will always we smaller than +∞
            IntegerOrInfinity::PositiveInfinity => size,
        } as usize;

        // 8. If length is undefined, let intLength be size;
        //    otherwise let intLength be ? ToIntegerOrInfinity(length).
        let length = args.get_or_undefined(1);
        let int_length = if length.is_undefined() {
            IntegerOrInfinity::Integer(size)
        } else {
            length.to_integer_or_infinity(context)?
        };

        // 9. Set intLength to the result of clamping intLength between 0 and size.
        let int_length = match int_length {
            IntegerOrInfinity::NegativeInfinity => 0,
            IntegerOrInfinity::PositiveInfinity => size,
            IntegerOrInfinity::Integer(i) => i.clamp(0, size),
        } as usize;

        // 10. Let intEnd be min(intStart + intLength, size).
        let int_end = min(int_start + int_length, size as usize);

        // 11. Return the substring of S from intStart to intEnd.
        if let Some(substr) = s.get(int_start..int_end) {
            Ok(substr.into())
        } else {
            Ok(js_string!().into())
        }
    }

    /// `CreateHTML(string, tag, attribute, value)`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-createhtml
    pub(crate) fn create_html(
        string: &JsValue,
        tag: JsStr<'_>,
        attribute_and_value: Option<(JsStr<'_>, &JsValue)>,
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let str be ? RequireObjectCoercible(string).
        let str = string.require_object_coercible()?;

        // 2. Let S be ? ToString(str).
        let s = str.to_string(context)?;

        // 3. Let p1 be the string-concatenation of "<" and tag.
        let mut p1 = js_string!(js_str!("<"), tag);

        // 4. If attribute is not the empty String, then
        if let Some((attribute, value)) = attribute_and_value {
            // a. Let V be ? ToString(value).
            let v = value.to_string(context)?;

            // b. Let escapedV be the String value that is the same as V except that each occurrence
            //    of the code unit 0x0022 (QUOTATION MARK) in V has been replaced with the six
            //    code unit sequence "&quot;".
            let mut escaped_v = Vec::with_capacity(v.len());
            for c in &v {
                if c == 0x0022 {
                    escaped_v.extend(utf16!("&quot;"));
                    continue;
                }
                escaped_v.push(c);
            }

            // c. Set p1 to the string-concatenation of:
            //    p1
            //    the code unit 0x0020 (SPACE)
            //    attribute
            //    the code unit 0x003D (EQUALS SIGN)
            //    the code unit 0x0022 (QUOTATION MARK)
            //    escapedV
            //    the code unit 0x0022 (QUOTATION MARK)
            p1 = js_string!(
                &p1,
                js_str!(" "),
                attribute,
                js_str!("=\""),
                &JsString::from(&escaped_v[..]),
                js_str!("\"")
            );
        }

        // 5. Let p2 be the string-concatenation of p1 and ">".
        // 6. Let p3 be the string-concatenation of p2 and S.
        // 7. Let p4 be the string-concatenation of p3, "</", tag, and ">".
        let p4 = js_string!(&p1, js_str!(">"), &s, js_str!("</"), tag, js_str!(">"));

        // 8. Return p4.
        Ok(p4.into())
    }

    /// `String.prototype.anchor( name )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.anchor
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/anchor
    pub(crate) fn anchor(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        let name = args.get_or_undefined(0);

        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "a", "name", name).
        Self::create_html(s, js_str!("a"), Some((js_str!("name"), name)), context)
    }

    /// `String.prototype.big( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.big
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/big
    pub(crate) fn big(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "big", "", "").
        Self::create_html(s, js_str!("big"), None, context)
    }

    /// `String.prototype.blink( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.blink
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/blink
    pub(crate) fn blink(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "blink", "", "").
        Self::create_html(s, js_str!("blink"), None, context)
    }

    /// `String.prototype.bold( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.bold
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/bold
    pub(crate) fn bold(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "b", "", "").
        Self::create_html(s, js_str!("b"), None, context)
    }

    /// `String.prototype.fixed( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.fixed
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/fixed
    pub(crate) fn fixed(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "big", "", "").
        Self::create_html(s, js_str!("tt"), None, context)
    }

    /// `String.prototype.fontcolor( color )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.fontcolor
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/fontcolor
    pub(crate) fn fontcolor(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        let color = args.get_or_undefined(0);

        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "font", "color", color).
        Self::create_html(s, js_str!("font"), Some((js_str!("color"), color)), context)
    }

    /// `String.prototype.fontsize( size )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.fontsize
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/fontsize
    pub(crate) fn fontsize(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        let size = args.get_or_undefined(0);

        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "font", "size", size).
        Self::create_html(s, js_str!("font"), Some((js_str!("size"), size)), context)
    }

    /// `String.prototype.italics( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.italics
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/italics
    pub(crate) fn italics(
        this: &JsValue,
        _: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "i", "", "").
        Self::create_html(s, js_str!("i"), None, context)
    }

    /// `String.prototype.link( url )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.link
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/link
    pub(crate) fn link(
        this: &JsValue,
        args: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        let url = args.get_or_undefined(0);

        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "a", "href", url).
        Self::create_html(s, js_str!("a"), Some((js_str!("href"), url)), context)
    }

    /// `String.prototype.small( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.small
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/small
    pub(crate) fn small(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "small", "", "").
        Self::create_html(s, js_str!("small"), None, context)
    }

    /// `String.prototype.strike( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.strike
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/strike
    pub(crate) fn strike(
        this: &JsValue,
        _: &[JsValue],
        context: &mut Context,
    ) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "strike", "", "").
        Self::create_html(s, js_str!("strike"), None, context)
    }

    /// `String.prototype.sub( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.sub
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/sub
    pub(crate) fn sub(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "sub", "", "").
        Self::create_html(s, js_str!("sub"), None, context)
    }

    /// `String.prototype.sup( )`
    ///
    /// More information:
    ///  - [ECMAScript reference][spec]
    ///  - [MDN documentation][mdn]
    ///
    /// [spec]: https://tc39.es/ecma262/#sec-string.prototype.sup
    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/sup
    pub(crate) fn sup(this: &JsValue, _: &[JsValue], context: &mut Context) -> JsResult<JsValue> {
        // 1. Let S be the this value.
        let s = this;
        // 2. Return ? CreateHTML(S, "sup", "", "").
        Self::create_html(s, js_str!("sup"), None, context)
    }
}

/// Abstract operation `GetSubstitution ( matched, str, position, captures, namedCaptures, replacement )`
///
/// More information:
///  - [ECMAScript reference][spec]
///
/// [spec]: https://tc39.es/ecma262/#sec-getsubstitution
pub(crate) fn get_substitution(
    matched: &JsString,
    str: &JsString,
    position: usize,
    captures: &[JsValue],
    named_captures: &JsValue,
    replacement: &JsString,
    context: &mut Context,
) -> JsResult<JsString> {
    let mut buf = [0; 2];
    // 1. Assert: Type(matched) is String.

    // 2. Let matchLength be the number of code units in matched.
    let match_length = matched.len();

    // 3. Assert: Type(str) is String.

    // 4. Let stringLength be the number of code units in str.
    let str_length = str.len();

    // 5. Assert: position ≤ stringLength.
    // 6. Assert: captures is a possibly empty List of Strings.
    // 7. Assert: Type(replacement) is String.

    // 8. Let tailPos be position + matchLength.
    let tail_pos = position + match_length;

    // 10. Let result be the String value derived from replacement by copying code unit elements
    //     from replacement to result while performing replacements as specified in Table 58.
    //     These $ replacements are done left-to-right, and, once such a replacement is performed,
    //     the new replacement text is not subject to further replacements.
    let mut result = vec![];
    let mut chars = replacement.code_points().peekable();

    while let Some(first) = chars.next() {
        if first == CodePoint::Unicode('$') {
            let second = chars.next();
            let second_is_digit = second
                .and_then(CodePoint::as_char)
                .as_ref()
                .map_or(false, char::is_ascii_digit);

            match second {
                // $$
                Some(CodePoint::Unicode('$')) => {
                    // $
                    result.push('$' as u16);
                }
                // $&
                Some(CodePoint::Unicode('&')) => {
                    // matched
                    result.extend(matched.iter());
                }
                // $`
                Some(CodePoint::Unicode('`')) => {
                    // The replacement is the substring of str from 0 to position.
                    result.extend(str.get_expect(..position).iter());
                }
                // $'
                Some(CodePoint::Unicode('\'')) => {
                    // If tailPos ≥ stringLength, the replacement is the empty String.
                    // Otherwise the replacement is the substring of str from tailPos.
                    if tail_pos < str_length {
                        result.extend(str.get_expect(tail_pos..).iter());
                    }
                }
                // $nn
                // f. Else if templateRemainder starts with "$" followed by 1 or more decimal digits, then
                Some(CodePoint::Unicode(second)) if second_is_digit => {
                    // i. If templateRemainder starts with "$" followed by 2 or more decimal digits, let digitCount be 2. Otherwise, let digitCount be 1.
                    // ii. Let ref be the substring of templateRemainder from 0 to 1 + digitCount.
                    // iii. Let digits be the substring of templateRemainder from 1 to 1 + digitCount.
                    // iv. Let index be ℝ(StringToNumber(digits)).
                    let mut index = second
                        .to_digit(10)
                        .expect("could not convert character to digit after checking it")
                        as usize;

                    // vi. Let captureLen be the number of elements in captures.
                    let capture_len = captures.len();

                    // NOTE(HalidOdat): We deviate from the spec, because of a bug in GetSubstitutions
                    //
                    // See: https://github.com/tc39/ecma262/issues/1426
                    if let Some(digit) = chars
                        .peek()
                        .copied()
                        .and_then(CodePoint::as_char)
                        .and_then(|n| n.to_digit(10))
                    {
                        // If there is two digits, and it's not in range fallback to one digit.
                        let two_digit_index = index * 10 + digit as usize;
                        if (1..=capture_len).contains(&two_digit_index) {
                            index = two_digit_index;
                            chars.next();
                        }
                    }

                    // v. Assert: 0 ≤ index ≤ 99.
                    debug_assert!((0..=99).contains(&index));

                    // vii. If 1 ≤ index ≤ captureLen, then
                    if (1..=capture_len).contains(&index) {
                        // 1. Let capture be captures[index - 1].
                        // 2. If capture is undefined, then
                        //     a. Let refReplacement be the empty String.
                        // 3. Else,
                        //     a. Let refReplacement be capture.
                        if let Some(capture) = captures.get(index - 1) {
                            if let Some(s) = capture.as_string() {
                                result.extend(s.iter());
                            }
                        }

                    // viii. Else,
                    } else {
                        // 1. Let refReplacement be ref.
                        result.extend_from_slice(&['$' as u16, second as u16]);
                    }
                }
                // $<
                Some(CodePoint::Unicode('<')) => {
                    // 1. If namedCaptures is undefined, the replacement text is the String "$<".
                    // 2. Else,
                    if named_captures.is_undefined() {
                        result.extend_from_slice(utf16!("$<"));
                    } else {
                        // a. Assert: Type(namedCaptures) is Object.
                        let named_captures = named_captures
                            .as_object()
                            .expect("should be an object according to spec");

                        // b. Scan until the next > U+003E (GREATER-THAN SIGN).
                        let mut group_name = vec![];
                        let mut found = false;
                        loop {
                            match chars.next() {
                                Some(CodePoint::Unicode('>')) => {
                                    found = true;
                                    break;
                                }
                                Some(c) => group_name.extend_from_slice(c.encode_utf16(&mut buf)),
                                None => break,
                            }
                        }

                        // c. If none is found, the replacement text is the String "$<".
                        #[allow(clippy::if_not_else)]
                        if !found {
                            result.extend_from_slice(utf16!("$<"));
                            result.extend_from_slice(&group_name);
                        // d. Else,
                        } else {
                            // i. Let groupName be the enclosed substring.
                            let group_name = js_string!(&group_name[..]);
                            // ii. Let capture be ? Get(namedCaptures, groupName).
                            let capture = named_captures.get(group_name, context)?;

                            // iii. If capture is undefined, replace the text through > with the empty String.
                            // iv. Otherwise, replace the text through > with ? ToString(capture).
                            if !capture.is_undefined() {
                                result.extend(capture.to_string(context)?.iter());
                            }
                        }
                    }
                }
                // $?, ? is none of the above
                _ => {
                    result.push('$' as u16);
                    if let Some(second) = second {
                        result.extend_from_slice(second.encode_utf16(&mut buf));
                    }
                }
            }
        } else {
            result.extend_from_slice(first.encode_utf16(&mut buf));
        }
    }

    // 11. Return result.
    Ok(js_string!(&result[..]))
}