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. functionality. However, I can't seem to get stuff running again after using a then.

$
.when ($.get('pages/run-tool.html'))
.then (function (args)
{
    // This works fine
    alert(args);
    $('#content').replaceWith (args);
    $('#progress-bar').progressbar ({value: 0});
})
.then ($.get('pages/test.html'))
.done (function(args)
{
    // This prints the same as the last call
    alert (args);
});

What am I doing wrong? I guess its some scoping issue, as I can see the second get call being executed. Using two different args variables does not help as the argument passed to the done function is still the first get request.

Answer 1

I thought I would leave this little exercise here for anyone who may find it useful, we build an array of requests and when they are completed, we can fire a callback function:

var urls = [{
    url: 'url1',
    data: 'foo'
}, {
    url: 'url2',
    data: 'foo'
}, {
    url: 'url3',
    data: 'foo'
}, {
    url: 'url4',
    data: 'foo'
}];
var requests = [];
var callback = function (result) {
    console.log('done!');
};

var ajaxFunction = function () {
    for (var request, i = -1; request = urls[++i];) {
        requests.push($.ajax({
            url: request.url,
            success: function (response) {
                console.log('success', response);
            }
        }));
    }
};

// using $.when.apply() we can execute a function when all the requests 
// in the array have completed
$.when.apply(new ajaxFunction(), requests).done(function (result) {
    callback(result)
});

Answer 2

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("<div>hello</div>").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.

Answer 3

All three callbacks (the two with then and the one with done) are applied to the same request – the original when call. This is because then returns the same Deferred object, rather than a new one, so that you can add multiple event handlers.

You need to use pipe instead.

$
.when ($.get('pages/run-tool.html'))
.then (function (args)
{
    // This works fine
    alert(args);
    $('#content').replaceWith (args);
    $('#progress-bar').progressbar ({value: 0});
})
.pipe (function() { 
    return $.get('pages/test.html'); // the return value creates a new Deferred object
})
.done (function(args)
{
    alert (args);
});

Answer 4

Here is an wonderfully simple and highly effective AJAX chaining / queue plugin. It will execute you ajax methods in sequence one after each other.

It works by accepting an array of methods and then executing them in sequence. It wont execute the next method whilst waiting for a response.

//--- THIS PART IS YOUR CODE -----------------------

$(document).ready(function () {

var AjaxQ = [];
AjaxQ[0] = function () { AjaxMethod1(); }
AjaxQ[1] = function () { AjaxMethod2(); }
AjaxQ[3] = function () { AjaxMethod3(); }

//Execute methods in sequence
$(document).sc_ExecuteAjaxQ({ fx: AjaxQ });

});

//--- THIS PART IS THE AJAX PLUGIN -------------------

$.fn.sc_ExecuteAjaxQ = function (options) {

//? Executes a series of AJAX methods in dequence

var options = $.extend({

    fx: [] //function1 () { }, function2 () { }, function3 () { }

}, options);

if (options.fx.length > 0) {

    var i = 0;

    $(this).unbind('ajaxComplete');
    $(this).ajaxComplete(function () {

        i++;
        if (i < options.fx.length && (typeof options.fx[i] == "function")) { options.fx[i](); }
        else { $(this).unbind('ajaxComplete'); }

    });

    //Execute first item in queue
    if (typeof options.fx[i] == "function") { options.fx[i](); }
    else { $(this).unbind('ajaxComplete'); }

} 

}

Answer 5

As an update:

With modern jquery (1.8+) you don't need the preliminary when because get returns a Deferred Promise.

Also, pipe is deprecated. Use then instead. Just be sure to return the result of the new get which becomes the Promise attached to by subsequent then/*done*/fail calls.

So:

$.get('pages/run-tool.html')
.then (function (args) { // this will run if the above .get succeeds
    // This works fine
    alert(args);
    $('#content').replaceWith (args);
    $('#progress-bar').progressbar ({value: 0});
})
.then (function() { // this will run after the above then-handler (assuming it ran)
    return $.get('pages/test.html'); // the return value creates a new Deferred object
})
.done (function(args) { // this will run after the second .get succeeds (assuming it ran)
    alert (args); 
});

Answer 6

<script type="text/javascript">

    var promise1 = function () {
        return new
        $.Deferred(function (def) {
            setTimeout(function () {
                console.log("1");
                def.resolve();
            }, 3000);
        }).promise();
    };

    var promise2 = function () {
        return new
        $.Deferred(function (def) {
            setTimeout(function () {
                console.log("2");
                def.resolve();
            }, 2000);
        }).promise();
    };

    var promise3 = function () {
        return new
        $.Deferred(function (def) {
            setTimeout(function () {
                console.log("3");
                def.resolve();
            }, 1000);
        }).promise();
    };

    var firstCall = function () {
        console.log("firstCall");
        $.when(promise1())
        .then(function () { secondCall(); });
    };

    var secondCall = function () {
        console.log("secondCall")
        $.when(promise2()).then(function () { thirdCall(); });
    };

    var thirdCall = function () {
        console.log("thirdCall")
        $.when(promise3()).then(function () { console.log("done"); });
    };


    $(document).ready(function () {
        firstCall();
    });
</script>