I've seen plenty of questions that suggest using:
for (var i = 0; i < myArray.length; i++){ /* ... */ }
instead of:
for (var i in myArray){ /* ... */ }
for arrays, due to inconsistent iteration (see here).
However, I can't seem to find anything that seems to prefer the object oriented loop:
myArray.forEach(function(item, index){ /* ... */ });
Which seems way more intuitive to me.
For my current project, IE8 compatibilty is important, and I'm considering using Mozilla's polyfill, however I'm not 100% sure how this will work.
for...of loops are the fastest when it comes to small data sets, but they scale poorly for large data sets.
This foreach loop is faster because the local variable that stores the value of the element in the array is faster to access than an element in the array. The forloop is faster than the foreach loop if the array must only be accessed once per iteration.
ForEach is 96% slower than for loop.
The most substantive difference between the for
loop and the forEach
method is that, with the former, you may break
out of the loop. You can simulate continue
by simply returning from the function passed to forEach
, but there is no way to stop looping altogether.
Aside from that, the two accomplish effectively the same functionality. Another minor difference involves the scope of the index (and all containing variables) in the for loop, due to variable hoisting.
// 'i' is scoped to the containing function for (var i = 0; i < arr.length; i++) { ... } // 'i' is scoped to the internal function arr.forEach(function (el, i) { ... });
However, I find that forEach
is much more expressive—it represents your intent to iterate through each element of an array, and it provides you with a reference to the element, not just the index. Overall, it mostly comes down to personal taste, but if you can use forEach
, I would recommend using it.
There are a few more substantial differences between the two versions, specifically regarding performance. In fact, the simple for loop performs considerably better than the forEach
method, as demonstrated by this jsperf test.
Whether or not such performance is necessary for you is up to you to decide, and in most cases, I would favor expressiveness over speed. This speed difference is likely due to the minor semantic differences between the basic loop and the method when operating on sparse arrays, as noted in this answer.
If you don't need the behavior of forEach
and/or you need to break out of the loop early, you can use Lo-Dash's _.each
as an alternative, which will also work cross-browser. If you're using jQuery, it also provides a similar $.each
, just note the differences in arguments passed to the callback function in each variation.
(As for the forEach
polyfill, it should work in older browsers without problems, if you choose to go that route.)
You can use your custom foreach function which will perform much better then Array.forEach
You should add this once to your code. This will add new function to the Array.
function foreach(fn) { var arr = this; var len = arr.length; for(var i=0; i<len; ++i) { fn(arr[i], i); } } Object.defineProperty(Array.prototype, 'customForEach', { enumerable: false, value: foreach });
Then you can use it anywhere like the Array.forEach
[1,2,3].customForEach(function(val, i){ });
The only difference it is 3 times faster. https://jsperf.com/native-arr-foreach-vs-custom-foreach
UPDATE: In new Chrome version the performance of .forEach() was improved. However, the solution can give the additional performance in other browsers.
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