<script>

This article is in need of a technical review.

Summary

The HTML <script> element is used to embed or reference an executable script within an HTML or XHTML document.

Scripts without async or defer attributes, as well as inline scripts, are fetched and executed immediately, before the browser continues to parse the page.

Attributes

This element includes the global attributes.

async HTML5
Set this Boolean attribute to indicate that the browser should, if possible, execute the script asynchronously. It has no effect on inline scripts (i.e., scripts that don't have the src attribute).
See Browser compatibility for notes on browser support. See also Async scripts for asm.js.
src
This attribute specifies the URI of an external script; this can be used as an alternative to embedding a script directly within a document. script elements with an src attribute specified should not have a script embedded within its tags.
type
This attribute identifies the scripting language of code embedded within a script element or referenced via the element’s src attribute. This is specified as a MIME type; examples of supported MIME types include text/javascript, text/ecmascript, application/javascript, and application/ecmascript. If this attribute is absent, the script is treated as JavaScript.
language
Like the type attribute, this attribute identifies the scripting language in use. Unlike the type attribute, however, this attribute’s possible values were never standardized. The type attribute should be used instead.
defer
This Boolean attribute is set to indicate to a browser that the script is meant to be executed after the document has been parsed. Since this feature hasn't yet been implemented by all other major browsers, authors should not assume that the script’s execution will actually be deferred. The defer attribute shouldn't be used on scripts that don't have the src attribute. Since Gecko 1.9.2, the defer attribute is ignored on scripts that don't have the src attribute. However, in Gecko 1.9.1 even inline scripts are deferred if the defer attribute is set.
crossorigin
Normal script tags will pass minimal information to the window.onerror for scripts which do not pass the standard CORS checks. To allow error logging for sites which use a separate domain for static media, several browsers have enabled the crossorigin attribute for scripts using the same definition as the standard img crossorigin attribute. Efforts to standardize this attribute are underway on the WHATWG mailing list.

Examples

<!-- HTML4 and (x)HTML -->
<script type="text/javascript" src="javascript.js">

<!-- HTML5 -->
<script src="javascript.js"></script>

Specifications

Specification Status Comments
HTML5 Candidate Recommendation  
HTML 4.01 Specification Recommendation  

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 1.0 1.0 (1.7 or earlier) (Yes) (Yes) (Yes)
async attribute (Yes) 3.6 (1.9.2) 10 Not supported (Yes)
defer attribute (Yes) 3.5 (1.9.1)

4 (follows a spec of its own)

10 (by the spec)

Not supported (Yes)
crossorigin attribute 30.0 Chromium Bug 159566 13 (13) bug 696301 Not supported 12.50 (Yes)

 (WebKit bug 81438)

Feature Android Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile
Basic support (Yes) 1.0 (1.0) (Yes) (Yes) (Yes)
async attribute (Yes) 1.0 (1.0) Not supported ? (Yes)
defer attribute (Yes) 1.0 (1.0) Not supported ? (Yes)

Async support

In older browsers that don't support the async attribute, parser-inserted scripts block the parser; script-inserted scripts execute asynchronously in IE and WebKit, but synchronously in Opera and pre-4.0 Firefox. In Firefox 4.0, the async DOM property defaults to true for script-created scripts, so the default behavior matches the behavior of IE and WebKit. To request script-inserted external scripts be executed in the insertion order in browsers where the document.createElement("script").async evaluates to true (such as Firefox 4.0), set .async=false on the scripts you want to maintain order. Never call document.write() from an async script. In Gecko 1.9.2, calling document.write() has an unpredictable effect. In Gecko 2.0, calling document.write() from an async script has no effect (other than printing a warning to the error console).

Gecko-specific notes

Starting in Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), inserting script elements that have been created by calling document.createElement("script") into the DOM no longer enforces execution in insertion order. This change lets Gecko properly abide by the HTML5 specification. To make script-inserted external scripts execute in their insertion order, set .async=false on them.

Also, <script> elements inside <iframe>, <noembed> and <noframes> elements are now executed, for the same reasons.

See also