From 16d2c089bc48e1652c9effb3229ee362dc207283 Mon Sep 17 00:00:00 2001
From: nd419 <5161147+neeldug@users.noreply.github.com>
Date: Tue, 29 Jun 2021 22:43:51 +0100
Subject: [PATCH 1/3] feat(boa): implements `at` method for string

Closes #13
---
 boa/src/builtins/string/mod.rs | 34 ++++++++++++++++++++++++++++++++++
 1 file changed, 34 insertions(+)

diff --git a/boa/src/builtins/string/mod.rs b/boa/src/builtins/string/mod.rs
index b2ee8972e0f..a37e4008247 100644
--- a/boa/src/builtins/string/mod.rs
+++ b/boa/src/builtins/string/mod.rs
@@ -134,6 +134,7 @@ impl BuiltIn for String {
         .method(Self::replace, "replace", 2)
         .method(Self::iterator, (symbol_iterator, "[Symbol.iterator]"), 0)
         .method(Self::search, "search", 1)
+        .method(Self::at, "at", 1)
         .build();
 
         (Self::NAME, string_object.into(), Self::attribute())
@@ -266,6 +267,39 @@ impl String {
         }
     }
 
+    /// `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-additions
+    /// [mdn]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/String/at
+    pub(crate) fn at(this: &Value, args: &[Value], context: &mut Context) -> Result<Value> {
+        let this = this.require_object_coercible(context)?;
+        let s = this.to_string(context)?;
+        let len = s.len();
+        let relative_index = args
+            .get(0)
+            .cloned()
+            .expect("No index provided")
+            .to_integer(context)?;
+        let k = if relative_index < 0 as f64 {
+            len - (-relative_index as usize)
+        } else {
+            relative_index as usize
+        };
+
+        if let Some(utf16_val) = s.encode_utf16().nth(k) {
+            Ok(Value::from(from_u32(utf16_val as u32).unwrap()))
+        } else {
+            Ok(Value::undefined())
+        }
+    }
+
     /// `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.

From ea985a5c96dc6581ff1719ea66b6e57bf8455eca Mon Sep 17 00:00:00 2001
From: nd419 <5161147+neeldug@users.noreply.github.com>
Date: Wed, 30 Jun 2021 19:01:56 +0100
Subject: [PATCH 2/3] cleanup

---
 boa/src/builtins/string/mod.rs | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/boa/src/builtins/string/mod.rs b/boa/src/builtins/string/mod.rs
index a37e4008247..434e7dfedd7 100644
--- a/boa/src/builtins/string/mod.rs
+++ b/boa/src/builtins/string/mod.rs
@@ -276,7 +276,7 @@ impl String {
     ///  - [ECMAScript reference][spec]
     ///  - [MDN documentation][mdn]
     ///
-    /// [spec]: https://tc39.es/proposal-relative-indexing-method/#sec-string-prototype-additions
+    /// [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: &Value, args: &[Value], context: &mut Context) -> Result<Value> {
         let this = this.require_object_coercible(context)?;
@@ -285,7 +285,7 @@ impl String {
         let relative_index = args
             .get(0)
             .cloned()
-            .expect("No index provided")
+            .unwrap_or_default()
             .to_integer(context)?;
         let k = if relative_index < 0 as f64 {
             len - (-relative_index as usize)
@@ -294,7 +294,9 @@ impl String {
         };
 
         if let Some(utf16_val) = s.encode_utf16().nth(k) {
-            Ok(Value::from(from_u32(utf16_val as u32).unwrap()))
+            Ok(Value::from(
+                from_u32(u32::from(utf16_val)).expect("invalid utf-16 character"),
+            ))
         } else {
             Ok(Value::undefined())
         }

From 3b9806e20ec3447e77edcf3af327d9c2a531b569 Mon Sep 17 00:00:00 2001
From: nd419 <5161147+neeldug@users.noreply.github.com>
Date: Wed, 30 Jun 2021 19:41:07 +0100
Subject: [PATCH 3/3] fix len

---
 boa/src/builtins/string/mod.rs | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/boa/src/builtins/string/mod.rs b/boa/src/builtins/string/mod.rs
index 434e7dfedd7..bb4bd9f5587 100644
--- a/boa/src/builtins/string/mod.rs
+++ b/boa/src/builtins/string/mod.rs
@@ -281,7 +281,7 @@ impl String {
     pub(crate) fn at(this: &Value, args: &[Value], context: &mut Context) -> Result<Value> {
         let this = this.require_object_coercible(context)?;
         let s = this.to_string(context)?;
-        let len = s.len();
+        let len = s.encode_utf16().count();
         let relative_index = args
             .get(0)
             .cloned()