CODEWITHSUNDEEP
javascriptnode.jsq
sowdri
sowdri

Reputation: 2243

Resolve promise based on another promise

I've a function, which returns a promise, inside that I call another function, and the status of this promise is based on the status of the inner promise.

Is there a way to shorthand this process. Look at the example below.

    function foo(bar) {
        var deferred = Q.defer();

        switch (bar) {
            case 'baz1':
                deferred.resolve();
                break;
            case 'baz2':
                deferred.reject();
                break;
            case 'this_is_how_i_do_it':
                funReturningPromise().then(function (value) {
                    deferred.resolve(value);
                }, function (err) {
                    deferred.reject(err);
                });
                break;
            case 'can_we_do_it_like_this':
                // can we do something like this, which will resolve or reject the deferred, 
                // based on the status promise returned by funReturningPromise().
                // 'chain' is just a name
                funReturningPromise().chain(deferred);
                break;
        }

        return deferred;
    }

Thanks,

Upvotes: 0

Views: 3707

Answers (3)

chevett
chevett

Reputation: 728

If you already have the deferred, then you can resolve it with another promise.

deferred.resolve(funReturningPromise())

Otherwise, what Kris said.

Upvotes: 0

Kris Kowal
Kris Kowal

Reputation: 3846

If you return a value, throw an exception, or return a promise from inside any function managed by Q, particularly the callbacks given to then, the chained promise will “become” that value, exception, or promise.

var foo = Q.fbind(function (bar) {
    switch (bar) {
        case 'baz1':
            return;
        case 'baz2':
            throw new Error("");
        case 'this_is_how_you_can_do_it':
            return funReturningPromise();
    }
});

In this case, I’m using Q.fbind to guarantee that the return result is a promise, but if foo were called in the context of a promise handler, just like funReturningPromise, the fbind would be unnecessary. If you want to invoke a function immediately and get a promise for the result, you can also use fcall.

Upvotes: 1

arghbleargh
arghbleargh

Reputation: 3160

It doesn't work as well for more complex functions, but in your example you could do something like:

        case 'can_we_do_it_like_this':
            return funReturningPromise();

You can also try adding your own promise.prototype.chain method if you're using only q promises.

Upvotes: 2

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