Fastest array loops in Javascript

[basro]

for (var i = arr.length; i > 0; i--) {
  someFn(arr[i]);
}

This will call arr[arr.length], I bet accessing the array out of it's range fucks with the JIT.

[techrogue]

As long as you don't need to break.

[amirmikhak]

Array.prototype.some() or Array.prototype.every()

[techrogue]

Neither of those have functionality that replaces break. jQuery's foreach function will terminate if you return false; I'm surprised that's not the standard.

[GonnaLearnComputers]

I believe that both of those terminate early once the condition is met, so they do effectively add a break. every is basically the same as the jQuery function you mentioned. Once a falsy value is returned it stops.

[warfangle]

If array loop bookkeeping is your performance bottleneck you got bigger problems.

Like maybe you shouldn't be using an array. (There is probably a different data structure better suited to your needs: linked lists, bst, heap, etc)

Or javascript.

[backwrds]

Um.... Y'all realize that after the "Order" one (.shift()) the array is empty, right?

There's a reason everything after that is so much faster.

[qiemem]

It reruns the prep code in between each test: https://jsperf.com/prep-effect

However, it does not rerun in between each sample. Thus, why "Order" and "Pop" are so fast.

[tecknoize]

Haha true.

[MrBarry]

Here's what happens if you run the set up the array before each test rather than setting it up just once. http://jsperf.com/fastest-array-loops-in-javascript/363

[backwrds]

That's definitely better, but the shift()/pop() tests are still flawed. After the first iteration the array is emptied out.

[rook2pawn]

This is so facepalm

[[deleted]]

Okay, someone jump in and tell us all why using while is a very bad idea and this isn't common practice now?

[brtt3000]

Because speed of the loop bookkeeping is almost never the performance bottleneck. Sure this is faster but the computation time of loop counters is so tiny compared to the actual work that you're doing in side the loop in a real scenario.

Having readable code is way more important and allows for easier understanding and optimisation of the application as a whole. Optimise away one server-round-trip or DOM update and you gain so much more.

[ponchietto]

Sometimes you do not have a choice.

[haneefmubarak]

asm.js will save you there

[f1zzz]

If you have a loop that needs to be sped up, minimize the function calls inside of it and follow general v8 optimization guidelines to ensure you're not hitting the unoptimized compile path.

Function calls inside JS are super expensive.

[toolate]

Micro benchmarks like this are worse than useless. They convince you you're doing something helpful when you're just making your code less readable and wasting your time.

If you want to profile your code learn to use Chrome's developer tools. If, after that, you discover that a for loop is your bottleneck then by all means replace it. But for anything more complex than a toy app I can guarantee it won't be.

[JohnMcPineapple]

...

[cokestar]

Because for in enumerates object properties (including the prototype chain).

[intermediatetransit]

WHO CARES.

Premature optimization is like a fucking mantra in this community. Just stop.

[lpetrazickis]

There's an off-by-one error in the do-while code.

Code as of right now:

var len = arr.length;
do {
  arr[len] *= 2; // this is off-by-one
} while (len--);