How does one test if some javascript library was loaded succesfully from CDN?

Question

When downloading javascript files from a CDN, I'd like to provide a local fallback. It's however not always obvious to me how to test if a certain load has succeeded.

For some popular things this is well documented in other StackOverflow questions, e.g. for Bootstrap one just tests if (window.jQuery), if (window.Popper), if ($.fn.modal) for its three parts respectively. However, for other libraries this is less obvious:

• For jQuery Validate, I find conflicting information: this topic suggests one can do if (window.validator), this one suggests one needs if(typeof $().validate == 'undefined'). What is the difference between these, and why would I need one over the other?
• For less popular libraries, e.g. balanceText, how should I find out if the library was loaded succesfully?

A general answer/approach would be perfect, but if that's not feasible, I'd be grateful for an answer on these two libraries.

Answer 1

If you're using loading the script yourself, you can use the onerror event on the script element to see if the script loaded instead of checking for side effects. Example:

<script>
function cdnLoaded() {
  console.log('loaded');
}

function cdnError() {
  console.log('not loaded');
  // do error handling here
}
</script>

<script onload="cdnLoaded()" onerror="cdnError()" async src="https://unpkg.com/react@17.0.1/umd/react.production.min.js"></script>
<script onload="cdnLoaded()" onerror="cdnError()" async src="https://unpkg.com/nothing-here.js"></script>