Replace more instances and document function

boa/src/builtins/bigint/mod.rs

@@ -135,7 +135,7 @@ impl BigInt {
         // 1. Let x be ? thisBigIntValue(this value).
         let x = Self::this_bigint_value(this, context)?;
 
-        let radix = args.get(0).cloned().unwrap_or_default();
+        let radix = args.get_or_undefined(0);
 
         // 2. If radix is undefined, let radixMV be 10.
         let radix_mv = if radix.is_undefined() {

boa/src/builtins/date/mod.rs

@@ -13,6 +13,8 @@ use crate::{
 use chrono::{prelude::*, Duration, LocalResult};
 use std::fmt::Display;
 
+use super::JsArgs;
+
 /// The number of nanoseconds in a millisecond.
 const NANOS_PER_MS: i64 = 1_000_000;
 /// The number of milliseconds in an hour.
@@ -523,7 +525,7 @@ impl Date {
             return context.throw_type_error("Date.prototype[@@toPrimitive] called on non object");
         };
 
-        let hint = args.get(0).cloned().unwrap_or_default();
+        let hint = args.get_or_undefined(0);
 
         let try_first = match hint.as_string().map(|s| s.as_str()) {
             // 3. If hint is "string" or "default", then

boa/src/builtins/json/mod.rs

@@ -28,6 +28,8 @@ use crate::{
 };
 use serde_json::{self, Value as JSONValue};
 
+use super::JsArgs;
+
 #[cfg(test)]
 mod tests;
 
@@ -158,7 +160,7 @@ impl Json {
         let mut property_list = None;
         let mut replacer_function = None;
 
-        let replacer = args.get(1).cloned().unwrap_or_default();
+        let replacer = args.get_or_undefined(1);
 
         // 4. If Type(replacer) is Object, then
         if let Some(replacer_obj) = replacer.as_object() {
@@ -214,7 +216,7 @@ impl Json {
             }
         }
 
-        let mut space = args.get(2).cloned().unwrap_or_default();
+        let mut space = args.get_or_undefined(2).clone();
 
         // 5. If Type(space) is Object, then
         if let Some(space_obj) = space.as_object() {
@@ -266,7 +268,7 @@ impl Json {
 
         // 10. Perform ! CreateDataPropertyOrThrow(wrapper, the empty String, value).
         wrapper
-            .create_data_property_or_throw("", args.get(0).cloned().unwrap_or_default(), context)
+            .create_data_property_or_throw("", args.get_or_undefined(0).clone(), context)
             .expect("CreateDataPropertyOrThrow should never fail here");
 
         // 11. Let state be the Record { [[ReplacerFunction]]: ReplacerFunction, [[Stack]]: stack, [[Indent]]: indent, [[Gap]]: gap, [[PropertyList]]: PropertyList }.

boa/src/builtins/mod.rs

@@ -114,6 +114,17 @@ pub fn init(context: &mut Context) {
 }
 
 pub trait JsArgs {
+    /// Utility function to `get` a parameter from
+    /// a `[JsValue]` or default to `JsValue::Undefined`
+    /// if `get` returns `None`.
+    ///
+    /// Call this if you are thinking of calling something similar to
+    /// `args.get(n).cloned().unwrap_or_default()` or
+    /// `args.get(n).unwrap_or(&undefined)`.
+    ///
+    /// This returns a reference for efficiency, in case
+    /// you only need to call methods of `JsValue`, so
+    /// try to minimize calling `clone`.
     fn get_or_undefined(&self, index: usize) -> &JsValue;
 }