I've put together a jsperf test that compares for loops that iterate over an array with caching the array length condition vs not caching. I thought that caching the variable before to avoid recalculating the array length each iteration would be faster, but the jsperf test says otherwise. Can someone explain why this is? I also thought that including the array length variable definition (when cached) within the for loop's initialization would reduce the time since the parse doesn't need to look up the "var" keyword twice, but that also doesn't appear to be the case.
example without caching:
for(var i = 0; i < testArray.length; i++){
//
}
example with caching
var len = testArray.length;
for(var i = 0; i < len; i++){
//
}
example with caching variable defined in for loop initialization
for(var i = 0, len=testArray.length; i < len; i++){
//
}
http://jsperf.com/for-loop-condition-caching
Can someone explain why this is?
This optimization and case is extremely common so modern JavaScript engines will automatically perform this optimization for you.
Some notes:
This is not the case when iterating a NodeList (such as the result of querySelectorAll
This is an overkill micro optimization for most code paths anyway, usually the body of the loop takes more time than this comparison anyway.
The performance of the scenarios that you posted depends on how smart the JS engine's optimizer is. An engine with a very dump optimizer (or no optimizer at all) will likely be faster when you are using the variable, but you can't even rely on that. After all, length's type is well-known, while a variable can be anything and may require additional checks.
Given a perfect optimizer, all three examples should have the same performance. And as your examples are pretty simple, modern engines should reach that point soon.
Related
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.
Are there any differences between the standard for loop (the first example above) and the Array.prototype.forEach implementation by modern browsers?
Are there any difference between modern browser implementations and Mozilla's implementation linked to above (with special regard to IE8)?
Performance is not as much of an issue, just consistency with which properties are iterated over.
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.
It is suggested by many developers (e.g. Kyle Simpson) to use .forEach to indicate that the array will have a side effect and .map for pure functions. for loops fit well as a general-purpose solution for known number of loops or any other case that doesn't fit as it is easier to communicate because of its broad support across the majority of programming languages.
e.g.
/* For Loop known number of iterations */
const numberOfSeasons = 4;
for (let i = 0; i < numberOfSeasons; i++) {
//Do Something
}
/* Pure transformation */
const arrayToBeUppercased = ['www', 'html', 'js', 'us'];
const acronyms = arrayToBeUppercased.map((el) => el.toUpperCase));
/* Impure, side-effects with .forEach */
const acronymsHolder = [];
['www', 'html', 'js', 'us'].forEach((el) => acronymsHolder.push(el.toUpperCase()));
Convention wise, this seems best, however the community hasn't really settled on a convention on the newer iteration protocol for in loops. Generally, I think it's a good idea to follow the FP concepts that the JS community seems to be open to adopting.
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.
Are there any differences between the standard for loop (the first example above) and the Array.prototype.forEach implementation by modern browsers?
Are there any difference between modern browser implementations and Mozilla's implementation linked to above (with special regard to IE8)?
Performance is not as much of an issue, just consistency with which properties are iterated over.
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.
It is suggested by many developers (e.g. Kyle Simpson) to use .forEach to indicate that the array will have a side effect and .map for pure functions. for loops fit well as a general-purpose solution for known number of loops or any other case that doesn't fit as it is easier to communicate because of its broad support across the majority of programming languages.
e.g.
/* For Loop known number of iterations */
const numberOfSeasons = 4;
for (let i = 0; i < numberOfSeasons; i++) {
//Do Something
}
/* Pure transformation */
const arrayToBeUppercased = ['www', 'html', 'js', 'us'];
const acronyms = arrayToBeUppercased.map((el) => el.toUpperCase));
/* Impure, side-effects with .forEach */
const acronymsHolder = [];
['www', 'html', 'js', 'us'].forEach((el) => acronymsHolder.push(el.toUpperCase()));
Convention wise, this seems best, however the community hasn't really settled on a convention on the newer iteration protocol for in loops. Generally, I think it's a good idea to follow the FP concepts that the JS community seems to be open to adopting.
I am not very well parsed in javascript, but do you call the following micro optimization?
for(var j=0;j < document.getElementsByTagName('a').length; j++)
{
//...
}
var Elements = document.getElementsByTagName('a');
var ElementsLength = Elements.length;
for(var j=0;j < ElementsLength ; j++)
{
//...
}
var Elements = document.getElementsByTagName('a');
for(var j=0;j < Elements.length; j++)
{
//...
}
does document.getElementByTagName really get called in every loop
cycle in the first case?
do browsers try to optimize the javascript we write?
is there any difference between the second and third case considering
that the collection will never change?
does document.getElementByTagName really get called in every loop cycle in the first case?
Yes.
do browsers try to optimize the javascript we write?
It won't change it functionally. There's a difference between calling a function once and calling it every time in a loop; perhaps you meant to call the function every time. "Optimising" that away is actually changing what the program does.
is there any difference between the second and third case considering that the collection will never change?
Not functionally, but performance wise yes. Accessing the length attribute is a little more overhead than reading the number from a simple variable. Probably not so much you'd really notice, but it's there. It's also a case that cannot be optimised away, since Elements.length may change on each iteration, which would make the program behave differently. A good optimiser may be able to detect whether the attribute is ever changing and optimise in case it's certain it won't; but I don't know how many implementations really do that, because this can become quite complex.
I have read in several places that for-in loops are slower than looping over an array... Though I understand that moving forward in sizeof (type) blocks is practically effortless compared to whatever happens behind to the scenes to iterate over an object's keys, I am still curious, what the exact reason is that it's so slow...
Is it having to do a reverse hash function to get the key, and that process is what is slow?
The real answer to this in the case of any particular engine will likely depend on that engine's implementation. (As will the size of the difference, if any.)
However, there are invariants. For instance, consider:
var obj = {a: "alpha", b: "beta"};
var name;
for (name in obj) {
console.log(obj[name]);
}
var arr = ["alpha", "beta"];
var index;
for (index = 0; index < arr.length; ++index) {
console.log(arr[index]);
}
In the case of obj, the engine has to use a mechanism to keep track of which properties you've already iterated over and which ones you haven't, as well as filtering out the non-enumerable properties. E.g., there's some kind of iterator object behind the scenes (and the way the spec is defined, that may well be a temporary array).
In the case of arr, it doesn't; you're handling that in your code, in a very simple, efficient way.
The content of the block of each loop is the same: A property lookup on an object. (In the latter case, in theory, there's a number-to-string conversion as well.)
So I expect the only non-implementation-specific answer to this is: Additional overhead.
for..each loops use iterators and generators.
Iterator is an object that has a next() method. Generator is a factory function that contains yield() expressions. Both constructs are more complicated than an integer index variable.
In a typical for(var i = 0; i < arr.length; i++) loop, the two commands that execute in almost all iterations are i++ and i < arr. This is arguably much faster than making a function call (next() or yield()).
Moreover, the loop initiation (var i = 0) is also faster than creating the iterator object with next() method or calling the generator to create the iterator. However, it highly depends on the implementation and the creators of Javascript engines do their best to accelerate such commonly used language features.
I would say the difference is so marginal that I might want to spend my time optimizing other parts of the code. The choice of syntax should consider code readability and maintainability more than performance, when the performance gain is so small for adding complexity. Having said that, use the syntax that makes more sense to you and other developers who maintain your code after you get rich and famous! ;)
In my quest to optimize my game engine I have discovered optimization i have been doing affecting each browser differently, in a lot of cases making one browser worse and the other better!
Currently I'm trying to optimize looping as i do alot of it and depending on the way this is done can have a big effect on the performance of my engine.
Based on the results here http://jsperf.com/for-vs-while-loop-iterating/3
It seems a reverse for loop in chrome is the fastest
var l = foo.length;
for (var i = l; i--;) {
}
And in firefox a forward for loop is fastest
var l = foo.length;
for (var i = 0; i < l; i++) {
}
Now in order to use the correct one per browser I'm doing something like this
function foreach(func, iterations){
var browser = $.browser;
var i;
if (browser.webkit)
{
for(i=iterations;i--;)
{
func(i);
}
}
else
{
for (i = 0; i < iterations; i++)
{
func(i);
}
}
}
but it seems there may be alot of overhead here that may hurt performance.
If you were to provide different ways of looping for different browsers what would you do?
EDIT: seems there was a bug in my testing where i was doing one too many loops on the forward loop and now chrome seems to be the fastest there also, I may not need to optimize the loops but it may still be worth while as mention in another comment incase browser versions change performance again
Unfortunately, if your goal is the best performance loop on each browser, the very last thing you want to do is introduce function calls into it. There's no way you can define your foreach function such that it will be anything like as fast as the straight for loop. Calling the iteration function will wash out any gains you might get.
A quick jsperf can confirm this easily enough. For instance, run this one on Chrome or a recent version of Firefox or Opera. It compares looping forward with for, backward with for, or using the browser's built-in Array#forEach function (part of ECMAScript5). (I think we can assume any foreach function you build will be slower than the built-in one.) As you can see, the forEach version is dramatically slower than either of the for loops, which makes sense: Calling functions isn't very expensive, but it isn't free.
Mind you, what you're doing in the loop probably washes out the difference between counting up and count down. What I'd do is figure out what's fastest on the slower browsers, and use that. You said, for instance, that a reverse loop is faster in Chrome but a forward loop is faster in Firefox. As Chrome's V8 is dramatically faster than Firefox's SpiderMonkey (at the moment, these things are constantly in flux), pick the forward loop, as it's faster on the slower engine.
Otherwise, you're into needing to do preprocessing on your code and churning out a different version tailored to each browser.
I don't think your overhad is as big as you feel, but what you can do is do your test only once:
var foreach;
if (forwardIsFaster) {
foreach = function (func, iterations) {
// loop forwards...
};
} else {
foreach = function (func, iterations) {
// loop backwards...
};
}
That said, I'm not sure using browser sniffing is the best solution here; maybe instead do a test loop on startup, measure which solution is faster, and choose the one that turns out to be faster.
When you start doing something significant inside the loop (e.g. just printing the variable), the order of iteration doesn't matter, see:
http://jsperf.com/for-vs-while-loop-iterating/4
So stop caring about this and if (and only if) your code is slow in any place, just profile it and optimize that part.
You could make foreach return a function, which would be an iterator. So you would have, once in your script, var iterator = foreach($.browser.webkit);. From then on, you would just use iterator(iterations, callback), which would no longer be executing any conditionals.
The key, basically, is that the user's browser won't change, so the result of the execution of that conditional needs to be evaluated only once per script.