HTMLScriptElement

HTML <script> elements expose the HTMLScriptElement interface, which provides special properties and methods for manipulating the behavior and execution of <script> elements (beyond the inherited HTMLElement interface).

JavaScript files should be served with the application/javascript MIME type, but browsers are lenient and block them only if the script is served with an image type (image/*), video type (video/*), audio type (audio/*), or text/csv. If the script is blocked, its element receives an error event; otherwise, it receives a load event.

Inherits properties from its parent, HTMLElement.

HTMLScriptElement.type

Is a DOMString representing the MIME type of the script. It reflects the type attribute.

HTMLScriptElement.src

Is a DOMString representing the URL of an external script. It reflects the src attribute.

HTMLScriptElement.event

Is a DOMString; an obsolete way of registering event handlers on elements in an HTML document.

HTMLScriptElement.charset

Is a DOMString representing the character encoding of an external script. It reflects the charset attribute.

HTMLScriptElement.async, HTMLScriptElement.defer

The async and defer attributes are boolean attributes that control how the script should be executed. The defer and async attributes must not be specified if the src attribute is absent.

There are three possible execution modes:

1. If the async attribute is present, then the script will be executed asynchronously as soon as it downloads.
2. If the async attribute is absent but the defer attribute is present, then the script is executed when the page has finished parsing.
3. If neither attribute is present, then the script is fetched and executed immediately, blocking further parsing of the page.

The defer attribute may be specified with the async attribute, so legacy browsers that only support defer (and not async) fall back to the defer behavior instead of the default blocking behavior.

Note: The exact processing details for these attributes are complex, involving many different aspects of HTML, and therefore are scattered throughout the specification. These algorithms describe the core ideas, but they rely on the parsing rules for <script> start and end tags in HTML, in foreign content, and in XML; the rules for the document.write() method; the handling of scripting; and so on.

HTMLScriptElement.crossOrigin

Is a DOMString reflecting the CORS setting for the script element. For scripts from other origins, this controls if error information will be exposed.

HTMLScriptElement.text

Is a DOMString that joins and returns the contents of all Text nodes inside the <script> element (ignoring other nodes like comments) in tree order. On setting, it acts the same way as the textContent IDL attribute.

Note: When inserted using the document.write() method, <script> elements execute (typically synchronously), but when inserted using innerHTML or outerHTML, they do not execute at all.

HTMLScriptElement.fetchpriority

An optional DOMString representing a hint given to the browser on how it should prioritize fetching of an external script relative to other external scripts. If this value is provided, it must be one of the possible permitted values: high to fetch at a high priority, low to fetch at a low priority, or auto to indicate no preference (which is the default).

HTMLScriptElement.noModule

Is a boolean value that if true, stops the script's execution in browsers that support ES2015 modules — used to run fallback scripts in older browsers that do not support JavaScript modules.

HTMLScriptElement.referrerPolicy

Is a DOMString that reflects the referrerpolicy HTML attribute indicating which referrer to use when fetching the script, and fetches done by that script.

HTMLScriptElement.supports()

Returns true if the browser supports scripts of the specified type and false otherwise. This method provides a simple and unified method for script-related feature detection.

No specific methods; inherits methods from its parent, HTMLElement.

Let's create a function that imports new scripts within a document creating a <script> node immediately before the <script> that hosts the following code (through document.currentScript). These scripts will be asynchronously executed. For more details, see the defer and async properties.

function loadError(oError) {
  throw new URIError("The script " + oError.target.src + " didn't load correctly.");
}

function prefixScript(url, onloadFunction) {
  var newScript = document.createElement("script");
  newScript.onerror = loadError;
  if (onloadFunction) { newScript.onload = onloadFunction; }
  document.currentScript.parentNode.insertBefore(newScript, document.currentScript);
  newScript.src = url;
}

This next function, instead of prepending the new scripts immediately before the document.currentScript element, appends them as children of the <head> tag.

function loadError(oError) {
  throw new URIError("The script " + oError.target.src + " didn't load correctly.");
}

function affixScriptToHead(url, onloadFunction) {
  var newScript = document.createElement("script");
  newScript.onerror = loadError;
  if (onloadFunction) { newScript.onload = onloadFunction; }
  document.head.appendChild(newScript);
  newScript.src = url;
}

Sample usage:

affixScriptToHead("myScript1.js");
affixScriptToHead("myScript2.js", function () { alert("The script \"myScript2.js\" has been correctly loaded."); });

HTMLScriptElement.supports() provides a unified mechanism for checking whether a browser supports particular types of scripts.

The example below shows how to check for module support, using the existence of the noModule attribute as a fallback.

function checkModuleSupport() {
  if ('supports' in HTMLScriptElement) {
    return HTMLScriptElement.supports('module');
  }
  return 'noModule' in document.createElement('script');
}

Classic scripts are assumed to be supported on all browsers.

• HTML <script> element
• HTML <noscript> element
• document.currentScript
• Web Workers (code snippets similar to scripts but executed in another global context)
• Ryan Grove's <script> and <link> node event compatibility chart