Tuesday, June 09, 2009

Nicholas C. Zakas: Speed Up Your JavaScript

Nicholas C. Zakas delivers the seventh Web Exponents tech talk at Google. Nicholas is a JavaScript guru and author working at Yahoo!. Most recently we worked together on my next book, Even Faster Web Sites. Nicholas contributed the chapter on Writing Efficient JavaScript, containing much of the sage advice found in this talk. Check out his slides and watch the video.

Nicholas starts by asserting that users have a greater expectation that sites will be fast. Web developers need to do most of the heavy lifting to meet these expectations. Much of the slowness in today's web sites comes from JavaScript. In this talk, Nicholas gives advice in four main areas: scope management, data access, loops, and DOM.

Scope Management: When a symbol is accessed, the JavaScript engine has to walk the scope chain to find that symbol. The scope chain starts with local variables, and ends with global variables. Using more local variables and fewer global variables results in better performance. One way to move in this direction is to store a global as a local variable when it's referenced multiple times within a function. Avoiding with also helps, because that adds more layers to the scope chain. And make sure to use var when declaring local variables, otherwise they'll end up in the global space which means longer access times.

Data Access: In JavaScript, data is accessed four ways: as literals, variables, object properties, and array items. Literals and variables are the fastest to access, although the relative performance can vary across browsers. Similar to global variables, performance can be improved by creating local variables to hold object properties and array items that are referenced multiple times. Also, keep in mind that deeper object property and array item lookup (e.g., obj.name1.name2.name3) is slower.

Loops: Nicholas points out that for-in and for each loops should generally be avoided. Although they provide convenience, they perform poorly. The choices when it comes to loops are for, do-while, and while. All three perform about the same. The key to loops is optimizing what is performed at each iteration in the loop, and the number of iterations, especially paying attention to the previous two performance recommendations. The classic example here is storing an array's length as a local variable, as opposed to querying the array's length property on each iteration through a loop.

DOM: One of the primary areas for optimizing your web application's interaction with the DOM is how you handle HTMLCollection objects: document.images, document.forms, etc., as well as the results of calling getElementsByTagName() and getElementsByClassName(). As noted in the HTML spec, HTMLCollections "are assumed to be live meaning that they are automatically updated when the underlying document is changed." Any idea how long this code takes to execute?

var divs = document.getElementsByTagName("div");
for (var i=0; i < divs.length; i++) {
var div = document.createElement("div");

This code results in an infinite loop! Each time a div is appended to the document, the divs array is updated, incrementing the length so that the termination condition is never reached. It's best to think of HTMLCollections as live queries instead of arrays. Minimizing the number of times you access HTMLCollection properties (hint: copy length to a local variable) is a win. It can also be faster to copy the HTMLCollection into a regular array when the contents are accessed frequently (see the slides for a code sample).

Another area for improving DOM performance is reflow - when the browser computes the page's layout. This happens more frequently than you might think, especially for web applications with heavy use of DHTML. If you have code that makes significant layout changes, consider making the changes within a DocumentFragment or setting the className property to alter styles.

There is hope for a faster web as browsers come equipped with JIT compilers and native code generation. But the legacy of previous, slower browsers will be with us for quite a while longer. So hang in there. With evangelists like Nicholas in the lead, it's still possible to find your way to a fast, efficient web page.

Check out other blog posts and videos in the Web Exponents speaker series:

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