javascript - Return value from a Promise constructor - Stack Overflow

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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.

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.

• javascript
• promise

Share Improve this question Follow edited Sep 25, 2015 at 11:50 vitaly-t asked Sep 25, 2015 at 10:41 vitaly-tvitaly-t 26k1717 gold badges127127 silver badges150150 bronze badges 5

• which Promise library is this? – Jamiec Commented Sep 25, 2015 at 10:49
• @jamiec es6 promise. – Daniel A. White Commented Sep 25, 2015 at 10:49
• @Jamiec ES6 Promise within NodeJS 4.1.1 – vitaly-t Commented Sep 25, 2015 at 10:49
• ok, I thought so but wasnt 100% – Jamiec Commented Sep 25, 2015 at 10:50
• 1 The returned value is to be ignored per the spec (as it obviously has, to be able to wait for asynchronous resolve calls). Check out the design of the constructor pattern. If you are looking for the differences between thenables and promises, see this post. – Bergi Commented Sep 26, 2015 at 15:03

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2 Answers 2

Sorted by: Reset to default 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

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 bees

return Promise.resolve(123);

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