Safari 10.1 `nomodule` support

safari-nomodule.js

// UPDATE: In 2023, you should probably stop using this! The narrow version of Safari that
// does not support `nomodule` is probably not being used anywhere. The code below is left
// for posterity.

/**
 * Safari 10.1 supports modules, but does not support the `nomodule` attribute - it will
 * load <script nomodule> anyway. This snippet solve this problem, but only for script
 * tags that load external code, e.g.: <script nomodule src="nomodule.js"></script>
 *
 * Again: this will **not** prevent inline script, e.g.:
 * <script nomodule>alert('no modules');</script>.
 *
 * This workaround is possible because Safari supports the non-standard 'beforeload' event.
 * This allows us to trap the module and nomodule load.
 *
 * Note also that `nomodule` is supported in later versions of Safari - it's just 10.1 that
 * omits this attribute.
 */
(function() {
  var check = document.createElement('script');
  if (!('noModule' in check) && 'onbeforeload' in check) {
    var support = false;
    document.addEventListener('beforeload', function(e) {
      if (e.target === check) {
        support = true;
      } else if (!e.target.hasAttribute('nomodule') || !support) {
        return;
      }
      e.preventDefault();
    }, true);

    check.type = 'module';
    check.src = '.';
    document.head.appendChild(check);
    check.remove();
  }
}());

safari-nomodule.min.js

/**
 * Minified-ish version of the above.
 */
(function() {
  var d = document;
  var c = d.createElement('script');
  if (!('noModule' in c) && 'onbeforeload' in c) {
    var s = false;
    d.addEventListener('beforeload', function(e) {
      if (e.target === c) {
        s = true;
      } else if (!e.target.hasAttribute('nomodule') || !s) {
        return;
      }
      e.preventDefault();
    }, true);

    c.type = 'module';
    c.src = '.';
    d.head.appendChild(c);
    c.remove();
  }
}());

FWIW I've added an Apache-2.0 license at the top of this file for anyone who is concerned about that.

Hey everyone! This approach works but has its challenges—Safari might still download the script, even though it won't execute, which is not ideal (but it is a limited version of Safari that this issue effects).

A simpler approach, although it involves adding more hacks to your actual code, is to create your ES Modules / nomodule scripts in this order:

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

The key here is the defer attribute. As ES Modules scripts are deferred by default, you want your older code to run in the same way—after the page is loaded—so they will run in a consistent order.

And then inside the nomodule.js file, check whether your code has already executed: e.g. if (window._yourCodeLoaded) { throw new Error("safari 10.1 has a bug"); }.

To avoid adding a new property in window for browsers other than safari 10.1 may be can check for safari 10.1 before adding the global property

var check = document.createElement('script');
  if (!('noModule' in check) && 'onbeforeload' in check) {
      if (window._yourCodeLoaded)  throw new Error("safari 10.1 has a bug"); 
      window._yourCodeLoaded = true;
  }

why appendChild(check) and remove it?

why (e.target === check),can prove support 'type=module' ?
the support variable seems to have never been used by anyone ?
Thanks for help:)

Is it safe to stop using this script? I'm doing differential serving with the following browserslist queries:

• > 0.25% and not supports es6-module for legacy browsers
• > 0.25% and supports es6-module, not dead, Firefox ESR for the modern ones

Safari is not included in the first list, while the second one only includes Safari from 14.1 to 16.6, so I guess I'm not supporting Safari 10.1 at all. Is my assumption correct or do you still recommend we include the script?

This code is ancient, stop using it.

The narrow window of Safari that is broken here is likely not being used anywhere.