How can I use `Q.set`?

Question

According to the API reference Q provides promise extensions for working with objects, but they don't behave as I would expect.

Consider the following line of code:

Q({foo:"bar"}).set("foo","baz").then(console.log);

I would have expected {"foo": "baz"} to be printed, but it's actually undefined.

Do I misuderstand the set method, or am I just using it in a wrong way?

Answer 1

The Q.set is used to augment an object, in the future and it doesn't return the augmented object (in fact, it doesn't return anything at all). That is why you are getting undefined in the console.log.

You need to use it like this

var obj = {
    foo: "bar"
};

Q(obj).set("foo", "baz").then(function () {
    console.log(obj);
});

This would print

{ foo: 'baz' }

The property foo is set as baz in the obj and since it is passed along the promise chain, we need to explicitly print it.

---

You can use the sister function, Q.get, like this

Q(obj).set("foo", "baz").then(function () {
    return Q.get(obj, 'foo');
}).then(console.log);

You might be wondering why you cannot do

Q(obj).set("foo", "baz").get('foo').then(console.log);

It will not work, because of the same reason why set didn't work in the first place. set doesn't resolve with the modified object. That's why we need to explicitly get it from the actual object.

---

FWIW, I found this, set not being chainable, bit odd. So, I submitted a pull request to the Q library to change this, and it got rejected.