7

Consider two examples below...

TEST 1

function test1() {
    return new Promise(function () {
        return 123;
    });
}

test1()
    .then(function (data) {
        console.log("DATA:", data);
        return 456;
    })
    .then(function (value) {
        console.log("VALUE:", value);
    });

It outputs nothing.

TEST 2

function test2() {
    return new Promise(function (resolve, reject) {
        resolve(123);
    });
}

test2()
    .then(function (data) {
        console.log("DATA:", data);
        return 456;
    })
    .then(function (value) {
        console.log("VALUE:", value);
    });

It outputs:

DATA: 123
VALUE: 456

What are the drawbacks or spec contradictions for a promise constructor not to simply resolve a returned value in TEST 1?

Why does it have to be a different result than in TEST 2?

I'm trying to understand how a constructed promise object is different from a then-able object as per the promise spec.

vitaly-t
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  • which `Promise` library is this? – Jamiec Sep 25 '15 at 10:49
  • @jamiec es6 promise. – Daniel A. White Sep 25 '15 at 10:49
  • @Jamiec ES6 Promise within NodeJS 4.1.1 – vitaly-t Sep 25 '15 at 10:49
  • ok, I thought so but wasnt 100% – Jamiec Sep 25 '15 at 10:50
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    [The returned value is to be ignored per the spec](http://stackoverflow.com/q/31651610/1048572) (as it obviously has, to be able to wait for asynchronous `resolve` calls). Check out [the design of the constructor pattern](http://stackoverflow.com/q/28687566/1048572). If you are looking for the differences between thenables and promises, see [this post](http://stackoverflow.com/q/29435262/1048572). – Bergi Sep 26 '15 at 15:03

2 Answers2

7

The function passed to Promise isn't a callback for onFulfilled or onRejected. MDN calls it the executor. Think of it as the async context that the promise is attempting to capture. Returning from an async method doesn't work (or make sense), hence you have to call resolve or reject. For example

var returnVal = new Promise(function() {
     return setTimeout(function() {
         return 27;
     });
});

... does not work as intended. If you were to return a value from the executor before your async calls finished, the promise couldn't be re-resolved.

Also, it could be ambigous with the implicit return undefined; at the end of the function. Consider these executors that function the same way.

// A
function a() { return undefined; }

// B
function b() { }

What would tell the Promise constructor that you really wanted to resolve with undefined?

a() === b(); // true
Daniel A. White
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2

It's worth mentioning that there is a shorthand for returning a resolved promise, Promise.resolve.

return new Promise(function (resolve, reject) {
    resolve(123);
});

simply becomes

return Promise.resolve(123);
Roy Miloh
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  • The result is the same, but what's happening underneath isn't. The second one will always perform better. And depending on the promise implementation, the second one can perform much better. – vitaly-t Sep 25 '15 at 17:36
  • Absolutely. The flow is similar (although it probably depends on implementation, I've taken a look in bluebird) but it skips some unnecessary steps. – Roy Miloh Sep 25 '15 at 17:59