@jchip this should be reliable, if it is not that is a bug. There were a bunch of refactorings to this code in eg #2116 and #2114.

If you would like to work on a fix that would be very welcome.

@guybedford I observed this periodically. It happens a little more as the number of <script> tags increases. Looking at James Greene's work https://github.com/JamesMGreene/document.currentScript and https://github.com/JamesMGreene/currentExecutingScript it seems we can improve this a bit.

I think a potential change could be:

1. Use document.currentScript https://developer.mozilla.org/en-US/docs/Web/API/Document/currentScript
2. Add a getCurrentScript hook in case someone wants to provide different logic, such as use James' currentExecutingScript
3. Add documentation to note this

A sample proposal for 1 and 2:

systemJSPrototype.getCurrentScript = function () {
  if (hasDocument && document.readyState === 'loading') {
    if (document.currentScript) {
      return document.currentScript;
    }
    var scripts = document.querySelectorAll('script[src]');
    return scripts[scripts.length - 1];
  }
  return undefined;
};

I'm open to it - this project once had to support IE, so the constraints may have changed. It's important that whatever is done is tested and correct though!