Common Pitfalls of jQuery

Introduction

Nowadays, jQuery is the de facto JavaScript library. CodeProject uses jQuery, Microsoft uses jQuery, Google uses jQuery, and if you've ever made an interactive client-side web application, the chances are that you used jQuery to do it.

This means that the jQuery community is huge, active and vibrant--and that's a great thing. But it also means that people often start creating things with jQuery without first becoming familiar with Javascript and the DOM. It's wonderful, of course, that jQuery is so accessible that even newbies can use it. That said, people will use the most convoluted things with jQuery that could be done simply with just the DOM library, by using a slightly more obscure jQuery feature, or by just being more familiar with how Javascript itself works.

The goal of this article is to exhibit the most common pitfalls people encounter with jQuery. This article assumes that you know what jQuery is and have some basic familiarity with using it.

Excessive jQuery Calls

It's unfortunately common to see code like this

$("#example").text("Hello, world!");
$("#example").css("color", "red");
$("#example").addClass("fun");

This code is unnecessarily expensive, because it is making repeated calls to jQuery to look for elements that match the "#example" selector.

There are two alternative ways to do this. First, you can cache the query.

var $example = $("#example");
$example.text("Hello, world!");
$example.css("color", "red");
$example.addClass("fun");

When assigning a jQuery object to a variable, the convention is to put a dollar sign ($) before the name to demarcate it as a jQuery object.

Second, you can use method chaining. jQuery methods will return new jQuery objects that you can call more jQuery methods on.

$("#example").text("Hello, world!").css("color", "red").addClass("fun");

Mistakes with selectors

Missing parts of selectors

There is one single silly mistake with jQuery selectors that I've made so many times and that has wasted much more debugging time than I would like to admit. Do you see the problem with the following sample?

$("example").text("Hello, world!");

It's easy to miss. In case you didn't notice, there is no hash (#) or dot (.) in the selector to specify that it is searching for an element with a particular id or a set of elements with a particular class. The given selector will look for elements with the tag name example -- which won't exist in any normal circumstances.

Unscoped selectors

Let's suppose you had some markup like this, in the midst of lots of other markup

<!-- lots of HTML here -->
<div id="container">
  <div class="box">
  </div>
  <div class="lid">
  </div>
  <div class="box">
  </div>
  <div class="lid">
</div>
<!-- lots of HTML here -->

and you decided to select all the divs with the class box. So you just use the class selector.

$(".box")

This will work, but it will search the entire page for elements with the box class. This can be a very expensive operation. If you know that all the elements with the box class are inside of the container div, you can speed things up by limiting your search boundaries. There are a few different ways to do this in jQuery.

$("#container .box")
$("#container").find(".box")
$(".box", $("#container"))

Repetitive Selectors

It's fairly common to see the same effect being performed on multiple selections of elements.

$("#main").css("color", "red");
$("#content").css("color", "red");
$(".important").css("color", "red");

It's better to use the comma (,) selection operator to match multiple selections

$("#main, #content, .important").css("color", "red");

Unused Shortcuts

These lines of code are unfortunately common

$("#el").css("display", "none");
$("#el").css("display", "");

if($("#el").is(":visible")) $("#el").css("display", "none");
else $("#el").css("display", "");

$("#el").html("");
$("#el").prop("class", $("#el").prop("class") + " " + newClass);

when there are faster and more readable shortcuts

$("#el").hide();
$("#el").show();
$("#el").toggle();
$("#el").empty();
$("#el").addClass(newClass);

Unfamiliarity with the DOM

It's a good idea to get acquainted with the DOM, because there are some things that using jQuery for is excessive. For example,

 $('img').click(function() { 
      alert($(this).attr('src'));
}); 

is excessive when you can just use

$('img').click(function() { 
    alert(this.src);
}); 

Similarly, you can use this.id instead of $(this).attr("id"). Be careful, though: some properties are not standard will not work across browsers. For example, the non-standard .class property will work on some browsers but not others, but the standard .className should work universally.

The A in AJAX stands for Asynchronous

You should take advantage of asynchronous requests. Too often people will be frustrated with functions not executing in the order they expected and then disable the asynch property

$.ajax({
  async: false
});

Disabling async this makes things slower and locks up the browser during the request--so it's usually a bad idea. You should use callbacks instead to manage how you want things to be executed. Explaining how callbacks work is outside the scope of thise article, but there are good reources explaining it.

Understanding Dynamic Changes

Suppose that you have this code,

$("button").click(doSomething);

and then you dynamically add some buttons to the page. You will notice that the added buttons will not fire the doSomething function. Why is this? The click function only bind the event to the button elements that already exist. If you want it to also bind the event for every button that will be added to the page, you need to use the delegate jQuery method

$(document).delegate("button", "click", doSomething);

This will bind the doSomething function to every button that will ever exist inside of the document. If you only wanted to bind it with with buttons inside a certain selector, then you would replace document with whatever selector you wanted.

That's all, folks!

These are the most common jQuery pitfalls that I have seen and been bit by. If there is an obvious one that I am missing, let me know about it and I might update this article.