TypeError: $.ajax(...) is not a function?

Neither of the answers here helped me. The problem was: I was using the slim build of jQuery, which had some things removed, ajax being one of them.

The solution: Just download the regular (compressed or not) version of jQuery here and include it in your project.

Double-check if you're using full-version of jquery and not some slim version.

I was using the jquery cdn-script link that comes with jquery. The problem is this one by default is slim.jquery.js which doesn't have the ajax function in it. So, if you're using (copy-pasted from Bootstrap website) slim version jquery script link, use the full version instead.

That is to say use <script src="https://code.jquery.com/jquery-3.1.1.min.js"> instead of <script src="https://code.jquery.com/jquery-3.1.1.slim.min.js"

Not sure, but it looks like you have a syntax error in your code. Try:

$.ajax({
  type: 'POST',
  url: url,
  data: postedData,
  dataType: 'json',
  success: callback
});

You had extra brackets next to $.ajax which were not needed. If you still get the error, then the jQuery script file is not loaded.