Bug 199663 – A defect of Array.prototype.lastIndexOf method

• Status: RESOLVED
• Resolution: DUPLICATE
• Component: JavaScriptCore

Version: <d940b47>
OS: <ubuntu 16.04>

#Testcase:
var foo = function () {
	var targetObj = ['hello', 'world'];
	var obj = {
		0: targetObj,
		1: 1234,
		2: targetObj,
		4294967294: targetObj,
		4294967295: targetObj,
		length: 4294967299
	};
	return Array.prototype.lastIndexOf.call(obj, targetObj);
};
var result = foo();
print(result);

#Command:
./webkit/WebKitBuild/Release/bin/jsc testcase.js

#Output:
2

#Expected output:
4294967295


#Description:
When using the above testcase, JSC outputs 2 which is the wrong result. This is caused by a defect of ES5.1. If the length of an object is more than 2^32, the result will be wrong because the ToUint32 method will let len%2^32 which makes the lastIndexOf method search the target object from index 2 in this testcase.
So we suggest that it's better to update the Array.prototype.lastIndexOf method according to ES2015.
The ECMAScript standard references are as follow:
http://www.ecma-international.org/ecma-262/5.1/index.html#sec-15.4.4.15
http://www.ecma-international.org/ecma-262/6.0/index.html#sec-array.prototype.lastindexof

Other more, the output of other engines is 4294967295, such as v8, spiderMonkey, chakra.

[Radar WebKit Bug Importer]

<rdar://problem/52894578>

[Alexey Shvayka]

Thank you for detailed report.
Apart from Array.prototype.lastIndexOf, other Array methods (mostly ones that are implemented in C++) are affected.
I am merging all bug reports to fix all (observable) cases with a single patch.

*** This bug has been marked as a duplicate of bug 163417 ***