[JSC] Reinstate String#at

https://bugs.webkit.org/show_bug.cgi?id=219124

Reviewed by Yusuke Suzuki.

JSTests:

* stress/at-method.js: Re-add tests.
* test262/config.yaml: Re-enable feature.
* test262/expectations.yaml:
"at/prop-desc.js" failures are due to a typo; will be fixed in tc39/test262#2908.

Source/JavaScriptCore:

At this week's TC39 meeting, consensus was achieved on renaming item() *and* keeping it for strings too.
Accordingly, this patch reinstates String.prototype.at behind the existing useAtMethod runtime option.

* builtins/StringPrototype.js:
(at):
* runtime/StringPrototype.cpp:
(JSC::StringPrototype::finishCreation):


Canonical link: https://commits.webkit.org/231748@main
git-svn-id: https://svn.webkit.org/repository/webkit/trunk@270005 268f45cc-cd09-0410-ab3c-d52691b4dbfc

JSTests/ChangeLog

@@ -1,3 +1,15 @@
+2020-11-18  Ross Kirsling  <ross.kirsling@sony.com>
+
+        [JSC] Reinstate String#at
+        https://bugs.webkit.org/show_bug.cgi?id=219124
+
+        Reviewed by Yusuke Suzuki.
+
+        * stress/at-method.js: Re-add tests.
+        * test262/config.yaml: Re-enable feature.
+        * test262/expectations.yaml:
+        "at/prop-desc.js" failures are due to a typo; will be fixed in https://github.com/tc39/test262/pull/2908.
+
 2020-11-17  Yusuke Suzuki  <ysuzuki@apple.com>
 
         [JSC] Improve Wasm binary test coverage

JSTests/stress/at-method.js

@@ -49,3 +49,21 @@ for (const TA of [Int8Array, Uint8Array, Uint8ClampedArray, Int16Array, Uint16Ar
   shouldBe(ta.at(null), ta[0]);
   shouldBe(ta.at({ valueOf: () => -1 }), ta[ta.length - 1]);
 }
+
+shouldBe(String.prototype.at.length, 1);  
+shouldThrowTypeError(() => String.prototype.at.call(undefined));  
+shouldThrowTypeError(() => String.prototype.at.call(null)); 
+
+const string = 'abc'; 
+// intentionally go one too far to ensure that we get undefined instead of wrapping 
+for (let i = 0; i <= string.length; i++) {  
+  shouldBe(string.at(i), string[i]);  
+  shouldBe(string.at(-i - 1), string[string.length - i - 1]); 
+} 
+shouldBe(string.at(), string[0]); 
+shouldBe(string.at(null), string[0]); 
+shouldBe(string.at({ valueOf: () => -1 }), string[string.length - 1]);  
+
+const emojiPseudoString = { toString: () => '😅' };  
+shouldBe(String.prototype.at.call(emojiPseudoString, 0), '\u{d83d}'); 
+shouldBe(String.prototype.at.call(emojiPseudoString, -1), '\u{de05}');

JSTests/test262/config.yaml

@@ -10,6 +10,7 @@ flags:
   Atomics: useSharedArrayBuffer
   Array.prototype.at: useAtMethod
   TypedArray.prototype.at: useAtMethod
+  String.prototype.at: useAtMethod
 skip:
   features:
     - Atomics.waitAsync
@@ -27,8 +28,6 @@ skip:
     - regexp-match-indices
     - top-level-await
     - Intl.ListFormat
-
-    - String.prototype.at
   paths:
     - test/built-ins/DataView/prototype/getBigInt64
     - test/built-ins/DataView/prototype/getBigUint64

JSTests/test262/expectations.yaml

@@ -823,6 +823,9 @@ test/built-ins/RegExp/prototype/exec/u-lastindex-adv.js:
 test/built-ins/RegExp/quantifier-integer-limit.js:
   default: 'SyntaxError: Invalid regular expression: number too large in {} quantifier'
   strict mode: 'SyntaxError: Invalid regular expression: number too large in {} quantifier'
+test/built-ins/String/prototype/at/prop-desc.js:
+  default: 'Test262Error: descriptor should be writable'
+  strict mode: 'Test262Error: descriptor should be writable'
 test/built-ins/TypedArray/prototype/at/prop-desc.js:
   default: 'Test262Error: descriptor should be writable'
   strict mode: 'Test262Error: descriptor should be writable'

Source/JavaScriptCore/ChangeLog

@@ -1,3 +1,18 @@
+2020-11-18  Ross Kirsling  <ross.kirsling@sony.com>
+
+        [JSC] Reinstate String#at
+        https://bugs.webkit.org/show_bug.cgi?id=219124
+
+        Reviewed by Yusuke Suzuki.
+
+        At this week's TC39 meeting, consensus was achieved on renaming item() *and* keeping it for strings too.
+        Accordingly, this patch reinstates String.prototype.at behind the existing useAtMethod runtime option.
+
+        * builtins/StringPrototype.js:
+        (at):
+        * runtime/StringPrototype.cpp:
+        (JSC::StringPrototype::finishCreation):
+
 2020-11-17  Yusuke Suzuki  <ysuzuki@apple.com>
 
         [JSC] Improve Wasm binary test coverage

Source/JavaScriptCore/builtins/StringPrototype.js

@@ -340,6 +340,25 @@ function concat(arg /* ... */)
     return @tailCallForwardArguments(@stringConcatSlowPath, this);
 }
 
+// FIXME: This is extremely similar to charAt, so we should optimize it accordingly.    
+//        https://bugs.webkit.org/show_bug.cgi?id=217139    
+function at(index)    
+{   
+    "use strict";   
+
+    if (@isUndefinedOrNull(this))   
+        @throwTypeError("String.prototype.at requires that |this| not be null or undefined"); 
+
+    var string = @toString(this);   
+    var length = string.length; 
+
+    var k = @toInteger(index);  
+    if (k < 0)  
+        k += length;    
+
+    return (k >= 0 && k < length) ? string[k] : @undefined; 
+}
+
 @globalPrivate
 function createHTML(func, string, tag, attribute, value)
 {

Source/JavaScriptCore/runtime/StringPrototype.cpp

@@ -151,6 +151,9 @@ void StringPrototype::finishCreation(VM& vm, JSGlobalObject* globalObject, JSStr
     JSC_NATIVE_FUNCTION_WITHOUT_TRANSITION("normalize", stringProtoFuncNormalize, static_cast<unsigned>(PropertyAttribute::DontEnum), 0);
     JSC_NATIVE_INTRINSIC_FUNCTION_WITHOUT_TRANSITION(vm.propertyNames->builtinNames().charCodeAtPrivateName(), stringProtoFuncCharCodeAt, static_cast<unsigned>(PropertyAttribute::DontEnum), 1, CharCodeAtIntrinsic);
 
+    if (Options::useAtMethod())
+        JSC_BUILTIN_FUNCTION_WITHOUT_TRANSITION(vm.propertyNames->builtinNames().atPublicName(), stringPrototypeAtCodeGenerator, static_cast<unsigned>(PropertyAttribute::DontEnum));
+
     JSFunction* trimStartFunction = JSFunction::create(vm, globalObject, 0, "trimStart"_s, stringProtoFuncTrimStart, NoIntrinsic);
     JSFunction* trimEndFunction = JSFunction::create(vm, globalObject, 0, "trimEnd"_s, stringProtoFuncTrimEnd, NoIntrinsic);
     putDirectWithoutTransition(vm, Identifier::fromString(vm, "trimStart"), trimStartFunction, static_cast<unsigned>(PropertyAttribute::DontEnum));