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.

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.

• javascript
• promise
• q

Share Improve this question Follow edited Apr 11, 2015 at 11:44 Bergi 667k161161 gold badges1k1k silver badges1.5k1.5k bronze badges asked Apr 11, 2015 at 11:28 mushroommushroom 1,94533 gold badges1616 silver badges3333 bronze badges 5

• Yes, this looks fine. Even ES6 Promises have similar structure only. – thefourtheye Commented Apr 11, 2015 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 Commented Apr 11, 2015 at 11:36
• You are not returning resolve, so accessToken is undefined – Krzysztof Safjanowski Commented Apr 11, 2015 at 11:37
• @KrzysztofSafjanowski Updated! ^ – mushroom Commented Apr 11, 2015 at 11:42
• I highly remend reading the Promises/A+ specification. It isn't long, pretty understandable and applies to almost any promise library: promisesaplus. – Zenorbi Commented Apr 11, 2015 at 11:47

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1 Answer 1

Sorted by: Reset to default 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 :-)}