jQuery Uncaught TypeError in $(selector).on()

Question

I am getting the following error:

Uncaught TypeError: ((jQuery.event.special[handleObj.origType] || (intermediate value)).handle || handleObj.handler).apply is not a function

This is my code and it's somewhere in between the beforeSend and the success calls that the error is happening:

$('#main-container').on('click', '#sign-in-btn', function (event) {
    var username = $('#username-textbox').val();
    var password = $('#password-textbox').val();

    $.ajax({
        url: '/login',
        method: 'POST',

        beforeSend: function (xhr) {
            xhr.setRequestHeader('Authorization', 'Basic ' + btoa(username + ":" + password));
        },

        success: function (data) {
        }
    });

    event.preventDefault();
});

I also tried to put inside a $(document).ready(function () { ... } call and I get the exact same error.

Answer 1

Having the event function return false fixed it because the error was somewhere during the propagation of the event and if an event returns false then event.stopPropagation() is "called".