Join MDN and developers like you at Mozilla's View Source conference, 12-14 September in Berlin, Germany. Learn more at https://viewsourceconf.org

Summary

The HTML Script Element (<script>) 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.

Content categories Metadata content, Flow content, Phrasing content.
Permitted content Dynamic script such as text/javascript.
Tag omission None, both the starting and ending tag are mandatory.
Permitted parent elements Any element that accepts metadata content, or any element that accepts phrasing content.
DOM interface HTMLScriptElement

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.
integrity
Contains inline metadata that a user agent can use to verify that a fetched resource has been delivered free of unexpected manipulation. See Subresource Integrity.
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. If a script element has a src attribute specified, it should not have a script embedded inside 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.
If the MIME type specified is not a JavaScript type the content embedded within its tags is treated as a data block which won't be processed by the browser.
If the type specified is module the code is treated as a JavaScript module . See ES6 in Depth: Modules

Note that in Firefox you can use advanced features such as let statements and other features in later JS versions, by using type=application/javascript;version=1.8  . Beware, however, that as this is a non-standard feature, this will most likely break support for other browsers, in particular Chromium-based browsers.
For how to include exotic programming languages, read about Rosetta.
text
Like the textContent attribute, this attribute sets the text content of the element.  Unlike the textContent attribute, however, this attribute is evaluated as executable code after the node is inserted into the DOM.
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, but before firing DOMContentLoaded. The defer attribute shouldn't be used on scripts that don't have the src attribute.
crossorigin
Normal script elements 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, use this attribute.

Examples

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

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

Specifications

Specification Status Comments
WHATWG HTML Living Standard
The definition of '<script>' in that specification.
Living Standard Adds the module type
HTML5
The definition of '<script>' in that specification.
Recommendation  
HTML 4.01 Specification
The definition of '<script>' in that specification.
Recommendation  
Subresource Integrity
The definition of '<script>' in that specification.
Recommendation Adds the integrity attribute.

Browser compatibility

Feature Chrome Firefox (Gecko) Internet Explorer Opera Safari
Basic support 1.0 1.0 (1.7 or earlier)[2] (Yes) (Yes) (Yes)
async attribute (Yes)[1] 3.6 (1.9.2)[1] 10[1] No support[1] (Yes)[1]
defer attribute (Yes) 3.5 (1.9.1)[6]

4[3]
10

No support (Yes)
crossorigin attribute 30.0 13 (13) No support 12.50 (Yes)[4]
integrity attribute 45.0 43 (43)     No support[5]
Feature Android Android Webview Firefox Mobile (Gecko) IE Mobile Opera Mobile Safari Mobile Chrome for Android
Basic support (Yes) (Yes) 1.0 (1.0)[2] (Yes) (Yes) (Yes) (Yes)
async attribute (Yes)[1] (Yes)[1] 1.0 (1.0)[1] No support[1] ?[1] (Yes)[1] (Yes)[1]
defer attribute (Yes) (Yes) 1.0 (1.0) No support ? (Yes) (Yes)
integrity attribute No support 45.0 43 (43)       45.0

[1] 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).

[2] 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.

[3] In versions prior to Internet Explorer 10 Trident implemented <script> by a proprietary specification. Since version 10 it conforms to the W3C specification.

[4] The crossorigin attribute was implemented in WebKit in WebKit bug 81438.

[5] WebKit bug 148363 tracks WebKit implementation of Subresource Integrity (which includes the integrity attribute).

[6] Since Gecko 1.9.2 (Firefox 3.6 / Thunderbird 3.1 / Fennec 1.0), the defer attribute is ignored on scripts that don't have the src attribute. However, in Gecko 1.9.1 (Firefox 3.5 / Thunderbird 3.0 / SeaMonkey 2.0) even inline scripts are deferred if the defer attribute is set.

See also