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What is the best way to do loops in JavaScript
What’s the best way to loop through a set of elements in JavaScript?
Any ideas?
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6 Answers
Check this JavaScript loop benchmarks.
answered Jan 7, 2009 at 19:49
Christian C. Salvadó
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3 Comments
Christian C. Salvadó
@Plan B: Yeah, look this article also: moddular.org/log/javascript-loops
Michael Haren
Hopefully this encourages you to go with whatever is clearest--not fastest. They're all fast.
rkj
@Micheal - Yeah in my Opera the first without any optimization is often the fastest...
What's wrong with a good old-fashioned for loop?
for( var i = 0; i < list.length; i++ ) {
// do something with list[i]
}
The semantics of for...in and for...each...in tend to confuse people and lead to unexpected results.
1 Comment
steve_c
In Javascript variable resolution is late bound. In your example above you would need to resolve length every iteration of the loop. You should cache the length variable by doing for (var i=0, l=list.length;i<l;i++). Good rule of thumb is the less dots the better in JS.
CMS's link should show you for small data sets they're all fast, the only thing i'd suggest is that you avoid for (a in b) as it has a much higher memory overhead than any other looping construct and can potentially be much slower due to its "interesting" semantics.
Anyhoo, with the exception of for(in) any real JS should be spending quite a bit more time actually doing real work than is spent handling the looping itself, so minor variation in cost of the loop shouldn't be too important.
Comments
As answered elsewhere (why does this thread exist?): the reverse while:
var i = foo.length; while (i--) { /* do something with foo[i] */ }
..is the fastest loop if you can deal with the caveats that:
reverse order isn't always suitable
it's a bit hard to read
it adds a variable to the footprint of whatever scope it's in
it's only slightly faster than the smaller footprinted and more readable cached length
forloop
2 Comments
aaronfarr
If the array is 0 based then you should use --i in the while loop otherwise it will be out of bounds var i = foo.length; while (--i) { /* do something with foo[i] */ }
Blaise
You can see here that this is not always true: jsperf.com/loops/33. Most loops caching the array's length are more or less equally fast.
Supposedly objects are faster... For example:
var array = {"0": "foo", "1": "bar"}
for(var i in array){
var val = array[i];
// do something
}
Comments
This was covered recently in Greg Reimer's Weblog.
The quick answer is this:
for (var i=0, node; node = hColl[i++];) {
// do something with node
}
1 Comment
ajax333221
why the
i++ there? you will need to use (i-1) to "do something with node"