jQuery 11 Performance Tips

jQuery 11 Performance Tips:

Be remember the following tips for increasing the performance by reducing the work the browser has to do.

1. Always Descend From an #id

2. Use Tags Before Classes

3. Cache jQuery Objects

4. Harness the Power of Chaining

5. Use Sub-queries

6. Limit Direct DOM Manipulation

7. Leverage Event Delegation (a.k.a. Event Bubbling)

8. Eliminate Query Waste

9. Defer to $(window).load

10. Compress Your JS

11. Learn the Library

1. Always Descend From an #id

The fastest selector in jQuery is the ID selector ($('#someid')). This is because it maps directly to a native JavaScript method, getElementById().

<h2>Traffic Light</h2>

<li><input type="radio" class="TrunOff" name="light" value="yellow" /> Yellow</li>

<li><input type="radio" class="TurnOff" name="light" value="green" /> Green</li>

</ul>

</form>

</div>

Selecting the button like this is slower:

var traffic_button = $('#MyDivID .button');

Always select the button directly:

var traffic_button = $('#btntraffic');

Selecting Multiple Elements

Once we start talking about selecting multiple elements, we are really talking about DOM traversal and looping, something that is slow. To minimize the performance hit, always descend from the closest parent ID:

var traffic_lights = $('#ulTrafficLight input');

2. Use Tags Before Classes

The second fastest selector in jQuery is the Tag selector ($('div')). Again, this is because it maps to a native JavaScript method, getElementsByTagName()

<h2>Traffic Light</h2>

<li><input type="radio" class="TrunOff" name="light" value="yellow" /> Yellow</li>

<li><input type="radio" class="TurnOff" name="light" value="green" /> Green</li>

</ul>

</form>

</div>

Always prefix a class with a tag name (and remember to descend from an ID):

var active_light = $('#ulTrafficLight input.TurnOn');

Note: The class selector is among the slowest selectors in jQuery; in IE it loops through the entire DOM. Avoid using it whenever possible. Never prefix an ID with a tag name. For example, this is slow because it will loop through all <div> elements looking for the ‘MyDivID’ ID:

var content = $('div#MyDivID');

Along the same lines, it is redundant to descend from multiple IDs:

var traffic_light = $('#MyDivID #ulTrafficLight');

3. Cache jQuery Objects

Get in the habit of saving your jQuery objects to a variable (much like our examples above). For example, never do this:

$('#ulTrafficLight input.on).bind('click', function(){...});

$('#ulTrafficLight input.on).css('border', '3px dashed yellow');

$('#ulTrafficLight input.on).css('background-color', 'orange');

$('#ulTrafficLight input.on).fadeIn('slow');

Instead, first save the object to a local variable, and continue your operations:

var $active_light = $('#ulTrafficLight input.on');

$active_light.bind('click', function(){...});

$active_light.css('border', '3px dashed yellow');

$active_light.css('background-color', 'orange');

$active_light.fadeIn('slow');

Tip: Since we want to remember that our local variable is a jQuery wrapped set, we are using $ as a prefix. Remember, never repeat a jQuery selection operation more than once in your application.

Note – Storing jQuery results for later

If you intend to use the jQuery result object(s) in another part of your program, or should your function execute more than once, cache it in an object with a global scope. By defining a global container with jQuery results, we can reference them from within other functions:

// Define an object in the global scope (i.e. the window object)

window.$my =

{

// Initialize all the queries you want to use more than once

head : $('head'),

trafficLight : $('#ulTrafficLight'),

btntraffic : $('#btntraffic')

};

function doSomething()

{

// Now you can reference the stored results and manipulate them

var script = document.createElement('script');

$my.head.append(script);

// When working inside functions, continue to save jQuery results

// to your global container.

$my.someResults = $('#some_ul li');

$my.otherResults = $('#some_table td');

// Use the global functions as you would a normal jQuery result

$my.otherResults.css('border-color', 'red');

$my.trafficLight.css('border-color', 'green');

}

4. Exploit the Power of Chaining

var $active_light = $('#ulTrafficLight input.TurnOn');

$active_light.bind('click', function(){...});

$active_light.css('border', '3px dashed yellow');

$active_light.css('background-color', 'orange');

$active_light.fadeIn('slow');

The above code can also be accomplished like this:

var $active_light = $('#ulTrafficLight input.TurnOn');

$active_light.bind('click', function(){...})

.css('border', '3px dashed yellow')

.css('background-color', 'orange')

.fadeIn('slow');

This allows us to write less code, making our JavaScript more lightweight.

5. Use Sub-queries

jQuery allows us to run additional selector operations on a wrapped set. This reduces performance overhead on subsequent selections since we already grabbed and stored the parent object in a local variable.

<h2>Traffic Light</h2>

<li><input type="radio" class="TrunOff" name="light" value="yellow" /> Yellow</li>

<li><input type="radio" class="TurnOff" name="light" value="green" /> Green</li>

</ul>

</form>

</div>

For example, we can leverage sub-queries to grab the active and inactive lights and cache them for later manipulation.

var $trafficLight = $('#ulTrafficLight '),

$activeLight = $ trafficLight.find('input.TurnOn'),

$inactiveLights = $ trafficLight.find('input.TurnOff');

Note: You can declare multiple local variables by separating them with commas – save those bytes!

6. Limit Direct DOM Manipulation

The basic idea here is to create exactly what you need in memory, and then update the DOM. This is not a jQuery best practice, but a must for efficient JavaScript. Direct DOM manipulation is slow. For example, if you need to dynamically create a list of elements, do not do this:

var ListesItems = [...], // assume this has 100 unique strings

$mylist = $('#mylist'); // jQuery selects our <ul> element

for (var i=0, l= ListesItems.length; i<l; i++)

{

$mylist.append('<li>' + ListesItems [i] + '</li>');

}

Instead, we want to create the entire set of elements in a string before inserting into the DOM:

var ListesItems = [...], // assume this has 100 unique strings

$mylist = $('#mylist'), // jQuery selects our <ul> element

Li_List = ""; // This will store our list items

for (var i=0, l= ListesItems.length; i<l; i++)

{

// storing in a string variable.

Li_List += '<li>' + ListesItems [i] + '</li>';

}

//Insert after a loop.

$mylist.html(Li_List);

Even faster, we should always wrap many elements in a single parent node before insertion:

var ListesdItems = [...], // assume this has 100 unique strings

$mylist = $('#mylist'), // jQuery selects our <ul> element

Ul_List = '<ul id="#mylist">'; // This will store our entire unordered list

for (var i=0, l= ListesdItems.length; i<l; i++)

{

Ul_List += '<li>' + ListesdItems [i] + '</li>';

}

Ul_List += '</ul>'; // Close our unordered list

$mylist.replaceWith(Ul_List);

If you do the above and are still concerned about performance:

· Give jQuery’s clone() method a try. This creates a copy of the node tree, which you can manipulate “off-line” and then insert back in when you are ready.

· Use DOM DocumentFragments. As the creator of jQuery points out, they perform much better than direct DOM manipulation. The idea would be to create what you need (similar to what we did above with a string), and use the jQuery insert or replace methods.

7. Leverage Event Delegation (a.k.a. Event Bubbling)

Unless told otherwise, every event (e.g. click, mouseover, etc.) in JavaScript “bubbles” up the DOM tree to parent elements. This is incredibly useful when we want many elements (nodes) to call the same function. Instead of binding an event listener function to many nodes—very inefficient—you can bind it once to their parent, and have it figure out which node triggered the event. For example, say we are developing a large form with many inputs, and want to toggle a class name when selected. A binding like this is inefficient:

$('#myList li).bind('click', function(){

$(this).addClass('clicked');

// do stuff

});

Instead, we should listen for the click event at the parent level:

$('#myList).bind('click', function(e){

var target = e.target, // e.target grabs the node that triggered the event.

$target = $(target); // wraps the node in a jQuery object

if (target.nodeName === 'LI') {

$target.addClass('clicked');

// do stuff

}

});

Important Note: As of jQuery 1.7, the .on() method is the preferred method for attaching event handlers to a document. For earlier versions, the .bind() method is used for attaching an event handler directly to elements.

The parent node acts as a dispatcher and can then do work based on what target element triggered the event. If you find yourself binding one event listener to many elements, you are doing something wrong (and slow).

8. Eliminate Query Waste

Although jQuery fails nicely if it does not find any matching elements, it still takes time to look for them. If you have one global JavaScript for your entire site, it may be tempting to throw every one of your jQuery functions into $(document).ready(function(){ // all my glorious code }). Don’t you dare. Only run functions that are applicable to the page. The most efficient way to do this is to use inline initialization functions so your template has full control over when and where JavaScript executes. For example, in your “article” page template, you would include the following code before the body close:

mylib.article.init();

</script>

</body>

If your page template includes any variety of modules that may or may not be on the page, or for visual reasons you need them to initialize sooner, you could place the initialization function immediately after the module.

<li><input type="radio" class="TurnOff" name="light" value="yellow" /> Yellow</li>

<li><input type="radio" class="TurnOff" name="light" value="green" /> Green</li>

</ul>

mylib.trafficLight.init();

</script>

Your Global JS library would look something like this:

var mylib =

{

article_page :

{

init : function()

{

// Article page specific jQuery functions.

}

trafficLight :

{

init : function()

{

// Traffic light specific jQuery functions.

}

9. Defer to $(window).load

There is a temptation among jQuery developers to hook everything into the $(document).ready pseudo event. After all, it is used in most examples you will find. Although $(document).ready is incredibly useful, it occurs during page render while objects are still downloading. If you notice your page stalling while loading, all those $(document).ready functions could be the reason why. You can reduce CPU utilization during the page load by binding your jQuery functions to the $(window).load event, which occurs after all objects called by the HTML (including <iframe> content) have downloaded.

$(window).load(function(){

// jQuery functions to initialize after the page has loaded.

});

Superfluous functionality such as drag and drop, binding visual effects and animations, pre-fetching hidden images, etc., are all good candidates for this technique.

10. Compress Your JS

This isn’t jQuery related, but very useful technique to make your javascript lightweight. There is a tendency to make JavaScript functions and variables overly descriptive, which is essential for developers but irrelevant to end-users. No more excuses, it’s time to build JS compression into our workflows. Comment the heck out of your code, and run it through a compression tool before launching to production. Use YUICompressorto squeeze out wasteful bytes from your code. In our experience, it safely compresses JavaScript as small as it can possibly get without a CPU penalty (such as Base62 encoding with Packer).

Note: For maximum compression in YUICompressor, always declare your variables (e.g. var my_long_variable_name;).

11. Learn the Library

Learn more and more about jQuery. JQuery API Documentation is available at http://api.jquery.com/. There are many other cheat sheets available on internet to explore the jQuery API.

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