Should I use jQuery.each()?

问题:

I'm doing very frequent iterations over arrays of objects and have been using jQuery.each(). However, I'm having speed and memory issues and one of the most called methods according to my profiler is jQuery.each(). What's the word on the street about its performance? Should I switch to a simple for loop? Of course I'm fixing the many issues in my own code too.

回答1:

The source code for jQuery's each is as follows (Thank you John Resig and MIT License):

each: function( object, callback, args ) {
    var name, i = 0, length = object.length;

    if ( args ) {
        if ( length === undefined ) {
            for ( name in object )
                if ( callback.apply( object[ name ], args ) === false )
                    break;
        } else
            for ( ; i < length; )
                if ( callback.apply( object[ i++ ], args ) === false )
                    break;

    // A special, fast, case for the most common use of each
    } else {
        if ( length === undefined ) {
            for ( name in object )
                if ( callback.call( object[ name ], name, object[ name ] ) === false )
                    break;
        } else
            for ( var value = object[0];
                i < length && callback.call( value, i, value ) !== false; value = object[++i] ){}
    }

    return object;
}

As you can trace and see, in most cases it is using a basic for loop where the only overhead is really just the callback itself. Shouldn't make a difference in performance.

EDIT: This is with the realization that selector overhead has already occurred and you are given a populated array object.

回答2:

This article (#3) ran some performance tests and found that the jQuery .each function was about 10x as slow as the native javascript for loop.

^{From 10 Ways to Instantly Increase Your jQuery Performance - 3. Use For Instead of Each}
Using Firebug, it's possible to measure the time each of the two functions takes to run.

var array = new Array ();
for (var i=0; i<10000; i++) {
    array[i] = 0;
}

console.time('native');
var l = array.length;
for (var i=0;i<l; i++) {
    array[i] = i;
}
console.timeEnd('native');

console.time('jquery');
$.each (array, function (i) {
    array[i] = i;
});
console.timeEnd('jquery');

The above results are 2ms for native code, and 26ms for jQuery's "each" method. Provided I tested it on my local machine and they're not actually doing anything (just a mere array filling operation), jQuery's each function takes over 10 times as long as JS native "for" loop. This will certainly increase when dealing with more complicated stuff, like setting CSS attributes or other DOM manipulation operations.

回答3:

One way to make sure you are getting the most out of jquery is to store the returned array of results in a variable rather than re-traversing the DOM everytime you need to get something.

Example of what NOT to do:

$('.myLinks').each(function(){
    // your code here
});
$('.myLinks').each(function(){
    // some more code here
});

Better practice:

var myLinks = $('.myLinks');
myLinks.each(function(){
    // your code here
});
myLinks.each(function(){
    // some more code here
});

回答4:

This method should give you a dramatic speed improvement.

var elements = $('.myLinks').get(), element = null;
for (var i = 0, length = elements.length; i < length; i++) {
    element = elements[i];
    element.title = "My New Title!";
    element.style.color = "red";
}

Caching will also improve performance.

function MyLinkCache() {
    var internalCache = $('.myLinks').get();

    this.getCache = function() {
        return internalCache;  
    }

    this.rebuild = function() {
        internalCache = $('.myLinks').get();
    }
}

In use:

var myLinks = new MyLinkCache();

function addMyLink() {
    // Add a new link.
    myLinks.rebuild();
}

function processMyLinks() {
    var elements = myLinks.getCache(), element = null;
    for (var i = 0, length = elements.length; i < length; i++) {
        element = elements[i];
        element.title = "My New Title!";
        element.style.color = "red";
    }
}

回答5:

Using native functionality will generally be faster than an abstraction, such as JQuery.each() method. However, the JQuery.each() method is easier to use and requires less coding on your part.

Truthfully, neither one is the right or wrong choice without any context. I'm of the oppinion that we should be writing easier code first, assuming it's good/legilble/clean code. If you come into a situation, such as the one you're describing, where there's a noticeable lag, than it's time to find out where your bottlenecks are and write faster code.

Replacing the JQuery.each() method in these scenarios might help, however, having not seen your code, it's possible you have bottlenecks unrelated to the JQuery method.