DEV Community

Cover image for HTML tags | script
Carlos Espada
Carlos Espada

Posted on

HTML tags | script

It is used to embed executable code or data; 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 and JSON.

The <script> element either contains scripting statements, or it points to an external script file through the src attribute.

async

For classic scripts, if the async attribute is present, then the classic script will be fetched in parallel to parsing and evaluated as soon as it is available.

For module scripts, if the async attribute is present then the scripts and all their dependencies will be executed in the defer queue, therefore they will get fetched in parallel to parsing and evaluated as soon as they are available.

This attribute allows the elimination of parser-blocking JavaScript where the browser would have to load and evaluate scripts before continuing to parse. defer has a similar effect in this case.

This is a boolean attribute: the presence of a boolean attribute on an element represents the true value, and the absence of the attribute represents the false value.

crossorigin

Normal <script> elements pass minimal information to the window.onerror for scripts which do not pass the standard CORS checks. Use this attribute to allow error logging for sites which use a separate domain for static media.

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.

The defer attribute has no effect on module scripts — they defer by default.

Scripts with the defer attribute will execute in the order in which they appear in the document.

This attribute allows the elimination of parser-blocking JavaScript where the browser would have to load and evaluate scripts before continuing to parse. 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.

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 allow 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: 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 (HTTPS→HTTPS), but don't send it to a less secure destination (HTTPS→HTTP).
  • strict-origin-when-cross-origin (default): send a full URL when performing a same-origin request, only send the origin when the protocol security level stays the same (HTTPS→HTTPS), and send no header to a less secure destination (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.

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 strict-origin-when-cross-origin.

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.

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. Unlike classic scripts, module scripts require the use of the CORS protocol for cross-origin fetching.
  • 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.

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.

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.

You can also use the <script> element to embed data in HTML with server-side rendering by specifying a valid non-JavaScript MIME type in the type attribute.

<!-- Generated by the server -->
<script id="data" type="application/json">{"userId":1234,"userName":"John Doe","memberSince":"2000-01-01T00:00:00.000Z"}</script>

<!-- Static -->
<script>
  const userInfo = JSON.parse(document.getElementById("data").text);
  console.log("User information: %o", userInfo);
</script>
Enter fullscreen mode Exit fullscreen mode
  • Type: -
  • Self-closing: No
  • Semantic value: No

Definition | Example | Support

Discussion (0)