jQuery.Deferred().then, how to resolve with multiple parameters

deferred, jquery

Solution

You will have to invoke `resolve()` or `reject()` in order to pass multiple arguments.

`.then()` doesn't include any mechanisms for "spreading" a returned collection. It'll just keep the collection intact as the 1st argument.

But, it will interact with a `Deferred` or promise that's returned. From the paragraph starting with "As of jQuery 1.8":

These filter functions can return a new value to be passed along to the promise's .done() or .fail() callbacks, or they can return another observable object (Deferred, Promise, etc) which will pass its resolved / rejected status and values to the promise's callbacks.

So, you could use your example of `other()` as the basis for `fn()` to keep it fairly succinct with another `Deferred()`:

function fn() {
    return other().then(function () {
        return $.Deferred().resolve(1, 2).promise();
    });
}

fn().then(function (a, b) {
    console.log(arguments.length, a, b); // 2 1 2
});

http://jsfiddle.net/cqac2/

Problem

So my API expects that when a particular deferred is resolved it gets 2 params. ``` fn().done(function(arg1, arg2) { console.log(arg1, arg2); }).fail(function(err) { console.error(err); }); ``` Now relating to the `fn` function above, it needs to first wait for some other deferred to return before resolve. ``` function other() { // stubbed out to always resolve return $.Deferred().resolve().promise(); } function fn() { return other().then(function() { return [1, 2]; }); } ``` But this is not working, because `arg1` will come as `[1, 2]` and `arg2` will be `undefined`. I cannot figure out how to return something from `Deferred.then()` first success filter function argument so that the resulting piped deferred resolves with multiple arguments. Of course I can do this: ``` function fn() { var done = $.Deferred(); other().done(function(){ done.resolve(1, 2); }).fail(function(){ done.reject.apply(done, arguments); }); return done.promise(); } ``` But that is not nearly as elegant as using `.then()` and I now need to worry about the negative failure case API each time even though I know that I'm merely piping the rejected state through. And yes, I could also change `fn()` api to resolve with an array, but I am really hoping there is an elegant solution to this.

Original source