Issue #1996 - Implement at() method on built-in indexables.

Resolves #1996

js/src/builtin/Array.js

@@ -833,7 +833,7 @@ function ArrayFrom(items, mapfn=undefined, thisArg=undefined) {
     }
 
     // Step 7.
-    assert(usingIterator === undefined, "`items` can't be an Iterable after step 6.g.iv");
+    assert(usingIterator === undefined, "`items` cannot be an Iterable after step 6.g.iv");
 
     // Steps 8-9.
     var arrayLike = ToObject(items);
@@ -1181,6 +1181,35 @@ function FlattenIntoArray(target, source, sourceLen, start, depth, mapperFunctio
     return targetIndex;
 }
 
+// ES2022 at() method on the built-in indexables
+// Array.prototype.at(index)
+function ArrayAt(index) {
+     // Step 1.
+    var O = ToObject(this);
+
+    // Step 2.
+    var len = ToLength(O.length);
+
+    // Step 3.
+    var relativeIndex = ToInteger(index);
+
+    // Steps 4-5.
+    var k;
+    if (relativeIndex >= 0) {
+        k = relativeIndex;
+    } else {
+        k = len + relativeIndex;
+    }
+
+    // Step 6.
+    if (k < 0 || k >= len) {
+        return undefined;
+    }
+
+    // Step 7.
+    return O[k];
+}
+
 function ArrayStaticConcat(arr, arg1) {
     if (arguments.length < 1)
         ThrowTypeError(JSMSG_MISSING_FUN_ARG, 0, 'Array.concat');

js/src/builtin/String.js

@@ -784,6 +784,39 @@ function String_static_raw(callSite, ...substitutions) {
     }
 }
 
+// ES2022 at() method on the built-in indexables
+// String.prototype.at(index)
+function String_at(index) {
+    // Step 1.
+    if (this === undefined || this === null)
+        ThrowIncompatibleMethod("at", this);
+
+    // Step 2.
+    var string = ToString(this);
+
+    // Step 3.
+    var len = string.length;
+
+    // Step 4.
+    var relativeIndex = ToInteger(index);
+
+    // Steps 5-6.
+    var k;
+    if (relativeIndex >= 0) {
+        k = relativeIndex;
+    } else {
+        k = len + relativeIndex;
+    }
+
+    // Step 7.
+    if (k < 0 || k >= len) {
+        return undefined;
+    }
+
+    // Step 8.
+    return string[k];
+}
+
 /**
  * Compare String str1 against String str2, using the locale and collation
  * options provided.

js/src/builtin/TypedArray.js

@@ -1308,6 +1308,45 @@ function TypedArraySubarray(begin, end) {
     return TypedArraySpeciesCreateWithBuffer(obj, buffer, beginByteOffset, newLength);
 }
 
+// ES2022 at() method on the built-in indexables
+// %TypedArray%.prototype.at(index)
+function TypedArrayAt(index) {
+    // Step 1.
+    var obj = this;
+
+    // Step 2.
+    // This function is not generic.
+    if (!IsObject(obj) || !IsTypedArray(obj)) {
+        return callFunction(CallTypedArrayMethodIfWrapped, obj, index,
+                            "TypedArrayAt");
+    }
+    GetAttachedArrayBuffer(obj);
+
+    // If we got here, `this` is either a typed array or a wrapper for one.
+
+    // Step 3.
+    var len = TypedArrayLength(obj);
+
+    // Step 4.
+    var relativeIndex = ToInteger(index);
+
+    // Steps 5-6.
+    var k;
+    if (relativeIndex >= 0) {
+        k = relativeIndex;
+    } else {
+        k = len + relativeIndex;
+    }
+
+    // Step 7.
+    if (k < 0 || k >= len) {
+        return undefined;
+    }
+
+    // Step 8.
+    return obj[k];
+}
+
 // ES6 draft rev30 (2014/12/24) 22.2.3.30 %TypedArray%.prototype.values()
 function TypedArrayValues() {
     // Step 1.

js/src/jsarray.cpp

@@ -3189,6 +3189,9 @@ static const JSFunctionSpec array_methods[] = {
     JS_SELF_HOSTED_FN("flat",        "ArrayFlat",        0,0),
     JS_SELF_HOSTED_FN("flatMap",     "ArrayFlatMap",     1,0),
     
+    /* ES2022 additions */
+    JS_SELF_HOSTED_FN("at",          "ArrayAt",          1,0),
+    
     JS_FS_END
 };
 

js/src/jsstr.cpp

@@ -2847,6 +2847,9 @@ static const JSFunctionSpec string_methods[] = {
     /* Python-esque sequence methods. */
     JS_FN("concat",            str_concat,            1,0),
     JS_SELF_HOSTED_FN("slice", "String_slice",        2,0),
+    
+    /* ES2022 additions */
+    JS_SELF_HOSTED_FN("at",    "String_at",           1,0),
 
     /* HTML string methods. */
     JS_SELF_HOSTED_FN("bold",     "String_bold",       0,0),

js/src/vm/TypedArrayObject.cpp

@@ -1538,6 +1538,10 @@ TypedArrayObject::protoFunctions[] = {
     JS_SELF_HOSTED_FN("includes", "TypedArrayIncludes", 2, 0),
     JS_SELF_HOSTED_FN("toString", "ArrayToString", 0, 0),
     JS_SELF_HOSTED_FN("toLocaleString", "TypedArrayToLocaleString", 2, 0),
+    
+    /* ES2022 additions */
+    JS_SELF_HOSTED_FN("at", "TypedArrayAt", 1, 0),
+    
     JS_FS_END
 };