Sep 17th, 2010
WebKit nightly builds now support the HTML5
Normally when the parser encounters an external script, parsing is paused, a request is issued to download the script, and parsing is resumed only after the script has fully downloaded and executed.
During the download the browser is blocked from doing other useful work, such as parsing HTML, executing other scripts and performing layout. Although partially mitigated by WebKit’s preload scanner, which speculatively looks ahead for resources to download during the down time, network latency still slows down some pages.
There are many clever techniques for working around this performance bottleneck, but they all involve extra code and browser-specific hacks. Instead, scripts which do not require synchronous execution can now be marked as either
defer. Here’s how they work.
<script async src="myAsyncScript.js" onload="myInit()"></script>
<script defer src="myDeferScript.js" onload="myInit()"></script>
defer scripts begin to download immediately without pausing the parser and both support an optional
onload handler to address the common need to perform initialization which depends on the script. The difference between
defer centers around when the script is executed. Each
async script executes at the first opportunity after it is finished downloading and before the
load event. This means it’s possible (and likely) that
async scripts are not executed in the order in which they occur in the page. The
defer scripts, on the other hand, are guaranteed to be executed in the order they occur in the page. That execution starts after parsing is completely finished, but before the
Here is an example of an external script which takes 1 second to download followed by an inline script which takes 1 second to execute. We can see that the page loads in about 2 seconds.
Here is the same example, but this time the external script is deferred. Since the second script can execute while the first is downloading, the page now loads about twice as fast.
In addition to upcoming versions of WebKit-based browsers, Firefox has long supported the
onload attributes and support for
async was added in version 3.6. Internet Explorer has also long supported the
defer attribute. While
async is not yet supported, support for the
onload attribute was added in version 9.