Request: allow override to retrieve auto-imported script element

Description of Proposed Feature

Auto-import is a great feature we rely on to load our app inlined. However, in some cases, the retrieval of the current running script will not be correct, as the last script might not always be the running script. (https://github.com/systemjs/systemjs/blob/master/src/features/script-load.js#L40)

systemJSPrototype.register = function (deps, declare) {
  if (hasDocument && document.readyState === 'loading' && typeof deps !== 'string') {
    var scripts = document.getElementsByTagName('script');
    var lastScript = scripts[scripts.length - 1]; // not always the running script
    var url = lastScript && lastScript.src;
    if (url) {
      lastAutoImportUrl = url;

This proposal would extract the retrieval of the current script into a separate, overridable, hook:

systemJSPrototype.getCurrentScript = function () {
  var scripts = document.getElementsByTagName('script');
  return scripts[scripts.length - 1];
};

systemJSPrototype.register = function (deps, declare) {
  if (hasDocument && document.readyState === 'loading' && typeof deps !== 'string') {
    var lastScript = systemJSPrototype.getCurrentScript();
    var url = lastScript && lastScript.src;
    if (url) {
      lastAutoImportUrl = url;

In what way would you use it?

In the end state, when we drop support for IE 11 in our app, we would use document.currentScript in order to always have the correct current script:

systemJSPrototype.getCurrentScript = function () {
  return document.currentScript;
};

In the meantime, we would use the hook to return the correct script element dynamically. It would also allow us to prevent the import of incorrectly matched last scripts when dynamic loads have been used.

[tmsns]

I was finally able to reproduce it! 😃 https://plnkr.co/edit/dMPuci6LJh0smbyu

Some remarks about the repro:

• the document.write in index.html is used in order to fake a sync third party library, but I wanted to use a file in the same project for it (animals/chicken.js)
• in order to micic the slower call I used flash.siwalik.in (a https variant of Slowwly)
• the core of the issue lies in the retrieval of the dependencies of a dynamic import (eg. triton.js). These dependencies are retrieved with a url that is constructed using the wrong base url. That wrong base url is coming from document.getElementsByTagName('scripts') which retrieves the last script instead of the current script.
• notice that nothing is wrong on the page. In most cases the wrong url will not resolve (if it does not exist). Users do get errors though.

Let me know if can help more!

[guybedford]

@tmsns this fix was released in 6.6.0 and 6.6.1, just let me know if you hit any further issues here or need to reopen.