Chaining ajax requests with jQuery's deferred

Question

I have a web app which must call the server multiple times. So far, I had a long nested callback chain; but I would like to use jQuery\'s when,then etc

Answer 1

The answer cdr gave, which has the highest vote at the moment, is not right.

When you have functions a, b, c each returns a $.Deferred() object, and chains the functions like the following:

a().then(b).then(c)

Both b and c will run once the promise returned from a is resolved. Since both then() functions are tied to the promise of a, this works similiar to other Jquery chaining such as:

$('#id').html("
hello
").css({display:"block"})

where both html() and css() function are called on the object returned from $('#id');

So to make a, b, c run after the promise returned from the previous function is resolved, you need to do this:

a().then(function(){
    b().then(c)
});

Here the call of function c is tied to the promise returned from function b.

You can test this using the following code:

function a() {
    var promise = $.Deferred();
    setTimeout(function() {
        promise.resolve();
        console.log("a");
    }, 1000);
    return promise;
}

function b() {
    console.log("running b");
    var promise = $.Deferred();
    setTimeout(function () {
        promise.resolve();
        console.log("b");
    }, 500);
    return promise;
}

function c() {
    console.log("running c");
    var promise = $.Deferred();
    setTimeout(function () {
        promise.resolve();
        console.log("c");
    }, 1500);
    return promise;
}

a().then(b).then(c);
a().then(function(){
    b().then(c)
});

Change the promise in function b() from resolve() to reject() and you will see the difference.