janhancic / fasterforeach.shim

A hack to see if I can make a faster forEach shim.

fasterForEach

A hack to see if I can make a faster forEach shim for IE8.

About

This is a hack that adds support for Array#forEach to IE8 but in a different way than the ES5 shim.

This basically rewrites the item processor (callback you pass into forEach) into a regular for loop and executes that (as oposed to executing the callback on each iteration). The result is that instead of 1 + n function invocations that you get with native or shimed forEach you get only two function invocations.

I created two nearly identical versions, the only difference is how the rewriten code gets executed. One uses eval and the other uses new Function().

Testing shows that both versions are faster than the ES5 shim. However I only have access to a VM from modern.ie running on a Macbook Air. But please visit this jsPerf test page and give it a spin so I can collect more data.

Is it usable?

I haven't tested this in any real project, but in theory I don't see why it couldn't be used.

License

Licensed under MIT. See LICENSE.md file for details.

fasterforeach.shim's Issues

Context in eval version

In eval version, the item processor has access to variables defined inside fasterForEachEval.

Can be fixed with eval.call(window, .... Unfortunately, this needs to create a temporary function on the window object which needs to be removed after.

Function params

Ierator functions do not allways take all the arguments and the might be named differently too.

Line 19: the replace "function ... {" should be a regexp
Or maybe the first occurence of the "{".

Also if I give in a function that is

myArray.forEach(function (model) { ... }),; // maintains a collection of models

this will be stringified to "function (model) { ... }" so the variables "item", "idx", "array" defined in the string will not work.

I think it would be beneficial if the provided iterator method would be parsed before any other operation.

Iterator context

Missing (new Function( ... )).call(context || this);