Bug 291997 – For-In can return duplicate keys in JSC

• Component: JavaScriptCore
• Status: RESOLVED FIXED

[EntryHi]

Hello, I found a bug in JavaScriptCore.
JSC version: c612e507973fe45e1c71c1893376818803781495

poc.js
```
function getKey(x) {
  var arr = [];
  for (var i in x) {
    arr.push(i);
  }
  return arr;
}

function opt() {
    var x = new String("abc");
    function B() {
      this.bar = 5;
      this[0] = 4;
    }
    B.prototype = x;
    var y = new B();
    var elements = getKey(y);
    print(elements);

}
opt();
opt();
opt();
```

Run args: ./jsc poc.js --useConcurrentJIT=0 --jitPolicyScale=0.1

Result in JSC:
0,bar,0,1,2
0,bar,0,1,2
0,bar,1,2


Result in V8:
bar,0,1,2
bar,0,1,2
bar,0,1,2

[Radar WebKit Bug Importer]

<rdar://problem/150420842>

[Yijia Huang]

Pull request: https://github.com/WebKit/WebKit/pull/45818

[Yijia Huang]

Thanks for reporting this issue. Since https://tc39.es/ecma262/#sec-enumerate-object-properties says "The mechanics and order of enumerating the properties is not specified ...". So, the patch fixes duplicate property enumeration.

[EWS]

Committed 295350@main (0a1e72158c24): <https://commits.webkit.org/295350@main>

Reviewed commits have been landed. Closing PR #45818 and removing active labels.