Friday 10 February 2012

`forEach` and runtime cost

Just a tiny one today, mostly to write this down somewhere:

As you all know, ECMAScript5 adds forEach to arrays, where you supply a function that gets called for each element in the array. There are lots of benefits to this, not least variable scoping on the index and value variables and a bit less typing, but don't all of those function calls add up to a significant runtime cost?

No, they don't.

I got curious about it (I have a tendency to micro-optimize, which I'm trying to break myself of), so I tested it on the slowest (desktop) browser currently in use: IE6. Specifically, IE6 running in an old Windows 2000 VM I have lying around. I tested the performance of looping through a 100-entry array both with and without calling a function. Without the function call, IE6 looped through the array ~4,500 times in a second (that's 450,000 loop iterations/second). With the function, it managed ~2,000 times a second (200,000 loop iterations/second). So while that's a big relative difference (56% slower!), in real terms it falls into fergedaboudit territory: 2.78 microseconds of overhead. You heard, me, microseconds. That's 0.00278 milliseconds.

Now, I'm not going to say there aren't places where it could matter. I am going to say that they're going to be extraordinarily rare, I'd wager I'll never run into a situation where it makes a difference and nor will you. Whatever else you're doing in the loop body is totally going to wash out the function calls. Really.

Happy coding!


cyberplus1 said...

I will probably safe this few microseconds. Microptimizations...

Tim said...

I would actually not test this in IE6 since the JavaScript engine is so poorly implemented that it might not be even able to optimize a (function call in a) for-loop. Today's browser do extremely good jobs at analyzing and optimizing critical parts of the code, I would say that there wouldn't be any significant difference for wrapping a block of code in a function call.
Besides IE6 doesn't represent today's browsers anymore.

Still a interesting article!

T.J. Crowder said...

@Tim - That was the whole point (as I said in the post): I used the oldest, slowest browser I could find, figuring if it didn't cause enough overhead to matter on that thing, it wouldn't matter on the rather better browsers we have today.

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