Promise rejected with non-error warning

This just means that the error that was thrown is not an instanceof Error. For example, the following is not an error, but I can throw it because... well... you can throw anything in JavaScript.

throw 42;

Which gives us a wonderful uncaught exception: 42.

To throw an actual error, use an Error:

throw new Error('An actual error');

Now in your specific case, you need to pass the error that jQuery supplies, which isn't the first argument it passes. It gives you a string, so wrap it in an error...

.fail(function(jqXHR, textStatus, errorThrown ) {  reject(new Error(errorThrown));}

For example, as I write this I also think it's a bit redundant to have a Promise wrapping an ajax object, but honestly I don't know how to re-write it otherwise

You could define a function that returns the AJAX object, which stated in the jQuery docs, returns an object that uses the JavaScript Promise interface.

The jqXHR objects returned by $.ajax() as of jQuery 1.5 implement the Promise interface, giving them all the properties, methods, and behavior of a Promise (see Deferred object for more information).

Here is a code example of how to implement such a function. I am using the Random User API for example data.

function getRandomUser() {  return $.ajax({    url: 'https://randomuser.me/api/',    dataType: 'json'  })  .fail(error => console.error(error));}// getRandomUser's return value is a Promise like object, which we can chain ontogetRandomUser().then(  function(random_user) {    console.log("success with value = ", random_user)  },  function(error) {    console.log("error with value = ", error)  });

<script src="https://ajax.googleapis.com/ajax/libs/jquery/2.1.1/jquery.min.js"></script>