Where does the callback come from?

Hi Robin,

callback will get invoked once the promise is resolved.

Let's assume this is the code:

//define myCallback function which accepts the ajax request's data as a parameter
var myCallback = function (data) {
    console.log('below we are logging out the data from the AJAX request!');
    console.log(data);
};

//define the function that handles the ajax request
var getTodos = function(callback){
        $http.get('todos.json').then(callback)
};

//invoke our function
getTodos(myCallback);

In the above snippet, we invoke the function that handles the ajax request and we pass it the myCallvack function as its callback parameter.

The GET request returns a promise, which is a 'thenable' object. Once the ajax request completes (success or failure) it resolves the promise and invokes the function within the .then.

Let's assume that the GET request succeeds. In that case, it will pass your todos data to the callback within the .then as a parameter. (As an aside, you would pass a second callback to handle cases where a failure occurs - or use .catch() with the failureCallback).

A different style, but one that would yield the same results, is to write the callback directly into the .then, like this:

var getTodos = function(){
        $http.get('todos.json')
            .then( function (data) {
                    console.log('below we are logging out the data from the AJAX request!');
                    console.log(data);
             })
};

getTodos();