Return another promise from a promise

Question

I have a promise for an object and would like to get a promise for a property of that object. How should I do that?

var user = Q.nfcall(User.findOne, {
    _id: userId
});
var accessToken = Q.Promise(function (resolve, reject) {
    user.then(function (user) {
        if (!user) return reject(new Error('User not found.'));
        if (!user.github.accessToken) return reject(new Error('Access token not found.'));
        return resolve(user.github.accessToken);
    }, function(err) {
        return reject(err);
    });
});

This is what I tried so far, but I'm not sure if its the best (or most correct) way.

Yes, this looks fine. Even ES6 Promises have similar structure only. — thefourtheye, Apr 11 '15 at 11:32
Why don't you chain `user` instead? `user.then(function() {}, function() {})`. You also may return a scalar value from the promise. — zerkms, Apr 11 '15 at 11:36
You are not returning `resolve`, so accessToken is `undefined` — Krzysztof Safjanowski, Apr 11 '15 at 11:37
I highly recommend reading the Promises/A+ specification. It isn't long, pretty understandable and applies to almost any promise library: https://promisesaplus.com — Zenorbi, Apr 11 '15 at 11:47

score 7 · Accepted Answer · edited May 23 '17 at 12:15

7

Do not use the deferred antipattern¹! There's no need to use Promise constructor, .then already returns you a promise for the result of its callback:

var accessToken = user.then(function(user) {
    if (!user) throw new Error('User not found.');
    if (!user.github.accessToken) throw new Error('Access token not found.');
    return user.github.accessToken;
});

_{[1]: You've seen yourself how errorprone it is :-)}

edited May 23 '17 at 12:15

Community

1
1

answered Apr 11 '15 at 11:42

Bergi

630,263
148
957
1,375

If later accessToken is not "used" or "called", will user be called? – mushroom Apr 11 '15 at 11:46
@mushroom "promises" do not provide laziness. – zerkms Apr 11 '15 at 11:47
@mushroom `user` is a promise. It cannot be called. If you mean `User.findOne`, *you* already did call it via `Q.nfcall` explicitly to create the promise. – Bergi Apr 11 '15 at 11:47
1

@Bergi Ah yes, I was momentarily confused and should have thought more first. So `Q.nfcall()` immediately executes `User.findOne()` and returns a promise for `user`. It all makes sense now, thanks! – mushroom Apr 11 '15 at 11:49

Return another promise from a promise

1 Answers1