rel=modulepreload

The modulepreload keyword, for the rel attribute of the <link> element, provides a declarative way to preemptively fetch a module script, parse and compile it, and store it in the document's module map for later execution.

Preloading allows modules and their dependencies to be downloaded early, and can also significantly reduce the overall download and processing time. This is because it allows pages to fetch modules in parallel, instead of sequentially as each module is processed and its dependencies are discovered. Note however that you can't just preload everything! What you choose to preload must be balanced against other operations that might then be starved, adversely affecting user experience.

Links with rel="modulepreload" are similar to those with rel="preload". The main difference is that preload just downloads the file and stores it in the cache, while modulepreload gets the module, parses and compiles it, and puts the results into the module map so that it is ready to execute.

When using modulepreload the fetch request mode is always cors, and the crossorigin property is used to determine the request credential mode. If crossorigin is set to anonymous or "" (default), then the credentials mode is same-origin, and user credentials such as cookies and authentication are only sent for requests with the same-origin. If crossorigin is set to use-credentials then the credentials mode is include, and user credentials for both single- and cross-origin requests.

The as attribute is optional for links with rel="modulepreload", and defaults to "script". It can be set to "script" or any script-like destination, such as "audioworklet", "paintworklet", "serviceworker", "sharedworker", or "worker". An Event named "error" is fired on the element if any other destination is used.

A browser may additionally also choose to automatically fetch any dependencies of the module resource. Note however that this is a browser-specific optimization — the only approach to ensure that all browsers will try to preload a module's dependencies is to individually specify them! Further, the events named load or error fire immediately following success or failure of loading the specified resources. If dependencies are automatically fetched, no additional events are fired in the main thread (although you might monitor additional requests in a service worker or on the server).

Examples

Consider the basic-modules example (live version), introduced in the JavaScript modules guide.

This has a file structure as shown below, consisting of the top level module main.js, which statically imports two dependency modules modules/canvas.js and modules/square.js using the import statement.

index.html
main.js
modules/
    canvas.js
    square.js

The HTML for the example below shows how main.js is fetched in a <script> element. Only after main.js has loaded does the browser discover and fetch the two dependency modules.

html

<!doctype html>
<html lang="en-US">
  <head>
    <meta charset="utf-8" />
    <title>Basic JavaScript module example</title>
    <style>
      canvas {
        border: 1px solid black;
      }
    </style>
    <script type="module" src="main.js"></script>
  </head>
  <body></body>
</html>

The HTML below updates the example to use <link> elements with rel="modulepreload" for the main file and each of the dependency modules. This is much faster because the three modules all start downloading asynchronously and in parallel before they are needed. By the time main.js has been parsed and its dependencies are known, they have already been fetched and downloaded.

html

<!doctype html>
<html lang="en-US">
  <head>
    <meta charset="utf-8" />
    <title>Basic JavaScript module example</title>
    <link rel="modulepreload" href="main.js" />
    <link rel="modulepreload" href="modules/canvas.js" />
    <link rel="modulepreload" href="modules/square.js" />
    <style>
      canvas {
        border: 1px solid black;
      }
    </style>

    <script type="module" src="main.js"></script>
  </head>
  <body></body>
</html>

Specifications

Specification
HTML Standard
# link-type-modulepreload

Browser compatibility

BCD tables only load in the browser

See also