<script>: The Script element

The HTML <script> element is used to embed or reference executable code; this is typically used to embed or refer to JavaScript code. The <script> element can also be used with other languages, such as WebGL's GLSL shader programming language.

Attributes

This element includes the global attributes.

asyncHTML5

This is a Boolean attribute indicating that the browser should, if possible, load the script asynchronously and then execute it as soon as it’s downloaded.

This attribute must not be used if the src attribute is absent (i.e. for inline scripts). If it is included in this case it will have no effect.

Browsers usually assume the worst case scenario and load scripts synchronously, (i.e. async="false") during HTML parsing.

Dynamically inserted scripts (using document.createElement()) load asynchronously by default, so to turn on synchronous loading (i.e. scripts load in the order they were inserted) set async="false".

This attribute allows the elimination of render-blocking JavaScript where the page would have to load and execute scripts before finishing to render the page. defer has a similar effect in this case.

See Browser compatibility for notes on browser support. See also Async scripts for asm.js.

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. See CORS settings attributes for a more descriptive explanation of its valid arguments.

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.

Scripts with the defer attribute will prevent the DOMContentLoaded event from firing until the script has loaded and finished evaluating.

This attribute must not be used if the src attribute is absent (i.e. for inline scripts), in this case it would have no effect.

To achieve a similar effect for dynamically inserted scripts use async="false" instead. Scripts with the defer attribute will execute in the order in which they appear in the document.

This attribute allows the elimination of render-blocking JavaScript where the page would have to load and execute scripts before finishing to render the page. async has a similar effect in this case.

integrity

This attribute 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.

nomodule

This Boolean attribute is set to indicate that the script should not be executed in browsers that support ES2015 modules — in effect, this can be used to serve fallback scripts to older browsers that do not support modular JavaScript code.

nonce

A cryptographic nonce (number used once) to whitelist inline scripts in a script-src Content-Security-Policy. The server must generate a unique nonce value each time it transmits a policy. It is critical to provide a nonce that cannot be guessed as bypassing a resource's policy is otherwise trivial.

referrerpolicy

Indicates which referrer to send when fetching the script, or resources fetched by the script:

• no-referrer: The Referer header will not be sent.
• no-referrer-when-downgrade (default): The Referer header will not be sent to origins without TLS (HTTPS).
• origin: The sent referrer will be limited to the origin of the referring page: its scheme, host, and port.
• origin-when-cross-origin: The referrer sent to other origins will be limited to the scheme, the host, and the port. Navigations on the same origin will still include the path.
• same-origin: A referrer will be sent for same origin, but cross-origin requests will contain no referrer information.
• strict-origin: Only send the origin of the document as the referrer when the protocol security level stays the same (e.g. HTTPS→HTTPS), but don't send it to a less secure destination (e.g. HTTPS→HTTP).
• strict-origin-when-cross-origin: Send a full URL when performing a same-origin request, but only send the origin when the protocol security level stays the same (e.g.HTTPS→HTTPS), and send no header to a less secure destination (e.g. HTTPS→HTTP).
• unsafe-url: The referrer will include the origin and the path (but not the fragment, password, or username). This value is unsafe, because it leaks origins and paths from TLS-protected resources to insecure origins.

Note: An empty string value ("") is both the default value, and a fallback value if referrerpolicy is not supported. If referrerpolicy is not explicitly specified on the <script> element, it will adopt a higher-level referrer policy, i.e. one set on the whole document or domain. If a higher-level policy is not available, the empty string is treated as being equivalent to no-referrer-when-downgrade.

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 since it can lead to unexpected behavior. The unexpected behavior is because it is only the JavaScript in the file referenced in the src attribute that will be added to the HTML page.

type

This attribute indicates the type of script represented. The value of this attribute will be in one of the following categories:

• Omitted or a JavaScript MIME type: This indicates the script is JavaScript. The HTML5 specification urges authors to omit the attribute rather than provide a redundant MIME type. In earlier browsers, this identified the scripting language of the embedded or imported (via the src attribute) code. JavaScript MIME types are listed in the specification.
• module: Causes the code to be treated as a JavaScript module. The processing of the script contents is not affected by the charset and defer attributes. For information on using module, see our JavaScript modules guide.
• Any other value: The embedded content is treated as a data block which won't be processed by the browser. Developers must use a valid MIME type that is not a JavaScript MIME type to denote data blocks. The src attribute will be ignored.

Note: in Firefox you could specify the version of JavaScript contained in a <script> element by including a non-standard version parameter inside the type attribute — for example type="text/javascript;version=1.8". This has been removed in Firefox 59 (see bug 1428745).

Deprecated attributes

charset

If present, its value must be an ASCII case-insensitive match for "utf-8". It’s unnecessary to specify the charset attribute, because documents must use UTF-8, and the script element inherits its character encoding from the document.

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.

Notes

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

The script should be served with the text/javascript MIME type, but browsers are lenient and only block them if the script is served with an image type (image/*); a video type (video/*); an audio (audio/*) type; or text/csv. If the script is blocked, an error is sent to the element, if not a load event is sent.

Examples

Basic usage

These examples show how to import (an external) script using the <script> element in both HTML4 and HTML5:

<!-- HTML4 -->
<script type="text/javascript" src="javascript.js"></script>

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

And the following examples show how to put (an inline) script inside the <script> element in both HTML4 and HTML5:

<!-- HTML4 -->
<script type="text/javascript">
  alert("Hello World!");
</script>

<!-- HTML5 -->
<script>
  alert("Hello World!");
</script>

Module fallback

Browsers that support the module value for the type attribute ignore any script with a nomodule attribute. That enables you to use module scripts while also providing nomodule-marked fallback scripts for non-supporting browsers.

<script type="module" src="main.js"></script>
<script nomodule src="fallback.js"></script>

Specifications

Browser compatibility

Compatibility notes

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 Firefox. In Firefox 4, the async DOM property defaults to true for script-created scripts, so the default behaviour matches the behaviour 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), set async="false" on the scripts you want to maintain order.

See also

• document.currentScript
• Ryan Grove's <script> and <link> node event compatibility chart
• Flavio Copes' article on loading JavaScript efficiently and explaining the differences between async and defer
• Javascript tips and tricks.
• JavaScript modules guide