Bluebird coroutine usage

Question

I am attempting to use Bluebird's coroutines as follows:

var p = require('bluebird');
//this should return a promise resolved to value 'v'
var d = p.coroutine(function*(v) { yield p.resolve(v); });
//however this prints 'undefined'
d(1).then(function(v){ console.log(v); });

What is incorrect here?

Answer 1

Quoting the documentation of coroutine,

Returns a function that can use yield to yield promises. Control is returned back to the generator when the yielded promise settles.

So, the function can make use of yield, but yield is not used to return value from the function. Whatever you are returning from that function with return statement will be the actual resolved value of the coroutine function.

Promise.coroutine simply makes yield statement wait for the promise to resolve and the actual yield expression will be evaluated to the resolved value.

In your case, the expression

yield p.resolve(v);

will be evaluated to 1 and since you are returning nothing from the function explicitly, by default, JavaScript returns undefined. That is why you are getting undefined as the result.

---

To fix this, you can actually return the yielded value, like this

var p = require('bluebird');

var d = p.coroutine(function* (v) {
    return yield p.resolve(v);
});

d(1).then(console.log);