Chain ajax and execute it in sequence. Jquery Deferred

Question

There are many ways to write this kind of thing.

A flexible approach is separate “actions” from “sequence”, allowing :

functions a, b, c to initiate an asynchronous (ajax) action, with no knowledge of how they are to be sequenced
a, b, c to be reusable, as part of one or more sequences or individually, as required.

Here’s a way to code this approach, using .then() exclusively for the chaining logic :

function a() {
    return $.post(...).then(function(result) {
        if(result)
            return result;//continue on "success" path.
        else
            return $.Deferred().reject('a').promise();//convert success to failure.
    }, function() {
        return 'a';//continue on failure path.
    });
}
function b() {
    return $.post(...).then(function(result) {
        if(result)
            return result;//continue on "success" path.
        else
            return $.Deferred().reject('b').promise();//convert success to failure.
    }, function() {
        return 'b';//continue on failure path.
    });
}
function c() {
    return $.post(...).then(function(result) {
        if(result)
            return result;//continue on "success" path.
        else
            return $.Deferred().reject('c').promise();//convert success to failure.
    }, function() {
        return 'c';//continue on failure path.
    });
}

a().then(b).then(c).then(function() {
    console.log("successful");
}, function(id) {
    console.log("failed: " + id);
});

Alternatively, if you want to have a single asynchronous function, a, called from within a loop then the code could be something like this :

function a(obj) {
    return $.post(...).then(function(result) {
        if(result)
            return result;//continue on "success" path.
        else
            return $.Deferred().reject(obj.id).promise();//convert success to failure.
    }, function() {
        return obj.id;//continue on failure path.
    });
}

var data = [{id:'A', param1:1235, param2:3214},  {id:'B', param1:5432, param2:9876}];
//Note how IDs are included so these data objects can be identified later in failure cases.

var dfrd = $.Deferred();//starter Deferred for later resolution.
var p = dfrd.promise();//A promise derived from the starter Deferred, forming the basis of a .then() chain.

//Build a .then() chain by assignment
$.each(data, function(i, obj) {
    p = p.then( function() {
        return a(obj);
    });//By not including a fail handler here, failures will pass straight through to be handled by the terminal .then()'s fail handler.
});

//Chain a terminal .then(), with success and fail handlers.
p.then(function() {
    console.log("successful");
}, function(id) {
    console.log("failed: " + id);
});

dfrd.resolve();//Resolve the starter Deferred to get things started.

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