How do I return the response from an asynchronous call.....

callback.js

function foo(callback) {
  $.ajax({
    url: '...',
    success: function(response) {
      callback(null, response); // Returning the response
    }
  });
}

var result 

foo(function callback(data) {
  result = data;
});

issue-this-is-broken.js

function foo() {
  var result;

  $.ajax({
    url: '...',
    success: function(response) {
      result = response;
      // return response; // <- I tried that one as well
    }
  });

  return result;
}

var result = foo(); // It always ends up being `undefined`.

Promise.js

function foo() {
  return new Promise(function(resolve) {
    $.ajax({
      url: '...',
      success: function(response) {
        resolve(response); // Returning the response
      }
    });
  });
}

var result 

foo().then(function(data) {
  result = data;
});