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Created June 19, 2011 16:34
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polyfill an ES5-compatibile Array.prototype.lastIndexOf where needed.
Raw
README.md

Polyfill for ES5: Array.prototype.lastIndexOf

A tweet-sized ES5-compatible polyfill for Array.prototype.lastIndexOf based on a similar function in my own tiny.js

Raw
annotated.js
// avoid overwriting Array.prototype.lastIndexOf
// otherwise it becomes enumerable, which leads to
// errors in Chrome
[].lastIndexOf || (Array.prototype.lastIndexOf = function(
a, // item to be found
b // index placeholder
) {
for (
// initialize index
b=this.length;
// if the index decreased by one is not already -1
// index is not set (sparse array)
// and the item at index is not identical to the searched one
~--b && (!(b in this) || this[b] !== a););
// return index of last found item or -1
return b
})
Raw
index.js
[].lastIndexOf||(Array.prototype.lastIndexOf=function(a,b){for(b=this.length;~--b&&(!(b in this)||this[b]!==a););return b})
Raw
LICENSE.txt
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
Version 2, December 2004
Copyright (C) 2011 Alex Kloss <alexthkloss@web.de>
Everyone is permitted to copy and distribute verbatim or modified
copies of this license document, and changing it is allowed as long
as the name is changed.
DO WHAT THE FUCK YOU WANT TO PUBLIC LICENSE
TERMS AND CONDITIONS FOR COPYING, DISTRIBUTION AND MODIFICATION
0. You just DO WHAT THE FUCK YOU WANT TO.
Raw
package.json
{
"name": "lastIndexOf",
"description": "polyfill an ES5-compatibile Array.prototype.lastIndexOf where needed.",
"keywords": [
"array",
"lastIndexof",
"es5",
"polyfill"
]
}
Raw
test.html
<!DOCTYPE html>
<title>Foo</title>
<div>Expected value: <b>0,1,2,-1</b></div>
<div>Actual value: <b id="ret"></b></div>
<script>
var lastIndexOf =
[].lastIndexOf||(Array.prototype.lastIndexOf=function(a,b){for(b=this.length;~--b&&(!(b in this)||this[b]!==a););return b})
var testdata=[1,2,3,5];
document.getElementById( "ret" ).innerHTML = [lastIndexOf.call(testdata,1), lastIndexOf.call(testdata,2), lastIndexOf.call(testdata,3), lastIndexOf.call(testdata,4)];
</script>
@tsaniel
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tsaniel commented Dec 5, 2011

Seems we don't need b?

function(a,c){for(c=this.length;this[--c]!==a&&~c;);return c}

@atk
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atk commented Dec 5, 2011

Now we have as much ES5-compatibility as possible; the length is resolved correctly, sparse arrays are taken care of and the Chrome bug is resolved by the more secure shim logic.

@jdalton
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jdalton commented Dec 5, 2011

This is still missing fromIndex support.

@tsaniel
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tsaniel commented Dec 5, 2011

What about

function(a,b){for(++b>0?0:b=this.length+~~b;~--b&&(!(b in this)||this[b]!==a););return b}

?

Updated.

@tsaniel
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tsaniel commented Dec 5, 2011

I think it still has room to be shortened.

@atk
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atk commented Dec 9, 2011

@tsaniel: nice one! I think that'll work!

@tsaniel
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tsaniel commented Dec 9, 2011

By the way, i wonder if

[].lastIndexOf||(Array.prototype.lastIndexOf=...)

and

[].lastIndexOf?0:Array.prototype.lastIndexOf=...

are equal.

@jdalton
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jdalton commented Dec 9, 2011

@tsaniel It's not clamping the fromIndex correctly:

5: If argument fromIndex was passed let n be ToInteger(fromIndex); else let n be len.
6: If n ≥ 0, then let k be min(n, len – 1).
7: Else, n < 0
---- a. Let k be len - abs(n).

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