JS: Promise doesn't return value

Question

I need to get a value of an asynchronous function. I tried to use Promise, but that does not work:

const res = new Promise(function (resolve, reject) {
  gm(readStream).size({ bufferStream: true }, function (err, size) {
    if (!err) resolve(size)
  })
})
console.log(res)

The result I get is Promise { <pending> }

Using promises doesn't make your code synchronous. `res.then(function (ret) { console.log(ret); })` — Keith, Sep 08 '17 at 16:23
Don't need to be sarcastic here @adeneo. You were a beginner once. — Duncan Thacker, Sep 08 '17 at 16:25
Well, you've asked question before where you've actually used promises, and async/await, and callbacks, so you clearly know what asynchronous is, unless you're just copy/pasting random questions for the fun? — adeneo, Sep 08 '17 at 16:42
I mean -> https://stackoverflow.com/questions/45948918/nodejs-restify-how-can-i-recieve-file-upload-in-api come on ! — adeneo, Sep 08 '17 at 16:44
Obviously I've some problems understanding those things. I think for me there are some misunderstandings. That's why I'asking this basic question. Still I'm not able to convert the answer of nadavvadan to a async/await version — user3142695, Sep 08 '17 at 17:36

nadavvadan · Accepted Answer · 2017-09-08T23:49:03.440

10

Promises are an abstraction for callbacks, not magic. They can't make asynchronous code synchronous.

The correct solution is:

const res = new Promise(function (resolve, reject) {
  gm(readStream).size({ bufferStream: true }, function (err, size) {
    if (err) reject(err);
    else resolve(size);

  })
});

res.then(function(promiseResolutionValue) {
    console.log(res)
})

You could also use async / await here:

const getSize = readStream => {
    return new Promise(function (resolve, reject) {
    gm(readStream).size({ bufferStream: true }, function (err, size) {
      if (err) reject(err);
      else resolve(size);
    })
  });
}


let printSize = async readStream => {
  console.log(`Size is ${await getSize(readStream)}`);
}

Or, if you're using NodeJS (Version 8+), you might be able to adapt your function to use util.promisify.
Other Promise libraries, such as Bluebird, also offer such functions, to easily convert 'standard' node-style functions (functions that have a callback with err, data as arguments) into promise-returning equivalents.

Or just use the callback.

edited Sep 08 '17 at 23:49

answered Sep 08 '17 at 16:25

nadavvadan

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I think you could have been nicer, but this is the correct answer! – Duncan Thacker Sep 08 '17 at 16:26
You could also add `async` / `await` to the example, which kind of is magic and does sort of _make asynchronous code synchronous._ – msanford Sep 08 '17 at 16:26
2

I did not mean to be rude at all, sorry if it came out that way. – nadavvadan Sep 08 '17 at 16:27
I do believe async/await is the way to go; Will edit and add an example later. – nadavvadan Sep 08 '17 at 16:28
1

Would be great if you could add an async/await example – user3142695 Sep 08 '17 at 16:40
@user3142695 - added – nadavvadan Sep 08 '17 at 23:49

score 1 · Answer 2 · answered Sep 08 '17 at 16:30

Your code should be looked like this:

const res = new Promise(function (resolve, reject) {
  gm(readStream).size({ bufferStream: true }, function (err, size) {
    if (!err) resolve(size)
    else reject(err)
  })
})

function onResolved(data) {
  console.log('data: ', data);
}

function onRejected(err) {
  console.log('err:', err);
}

res.then(onResolved, onRejected);

Promise does not make your code synchronous, it lets you control when you want to get the result, not immediately like callback style.

JS: Promise doesn't return value

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