how to use Promise with express in node.js?

Question

I am using Promise with Express.

router.post('/Registration', function(req, res) {
  var Promise = require('promise');
  var errorsArr = [];
  function username() {
    console.log("1");
    return new Promise(function(resolve, reject) {
      User.findOne({ username: req.body.username }, function(err, user) {
        if(err) {
          reject(err)
        } else {
          console.log("2");
          errorsArr.push({ msg: "Username already been taken." });
          resolve(errorsArr);
        }
      });
    });
  }
  var username = username();
  console.log(errorsArr);
});

When I log errorsArray, it is empty and I don't know why. I am new in node.js. Thanks in advance.

Yes, you are retuning a Promise. We will need to do something like `username.then(function(data){console.log(...)});` — anolsi, Jan 27 '16 at 10:19
Do you understand the concept of a Promise? Because it looks that first you will need to understand some concepts behind it... — anolsi, Jan 27 '16 at 10:29

anolsi · Accepted Answer · 2017-08-19T17:54:14.130

21

Try the following, and after please read the following document https://www.promisejs.org/ to understand how the promises work.

var Promise = require('promise');
router.post('/Registration',function(req,res,next) {
    function username() {
        console.log("agyaaa");
        return new Promise(function(resolve,reject) {
            User.findOne({"username":req.body.username}, function(err,user) {
                if (err) {
                    reject(err)
                } else {
                    console.log("yaha b agyaaa");
                    var errorsArr = [];
                    errorsArr.push({"msg":"Username already been taken."});
                    resolve(errorsArr);
                }

            });
        });
    }
    username().then(function(data) {
        console.log(data);
        next();
    });
});

You can have other errors also (or things that shouldn't be done that way). I'm only showing you the basic use of a Promise.

edited Aug 19 '17 at 17:54

answered Jan 27 '16 at 10:36

anolsi

529
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No, @stackdave, it does not as of Node v0.12 (!) - see [node.green](http://node.green/#ES2015-built-ins-Promise). – Erwin Wessels Aug 10 '17 at 07:16
@anolsi but how do you write your data into res after the promise has resolved? Express is returning res as soon as the function exits. – Emanuele Paolini Aug 19 '17 at 10:37
@EmanuelePaolini I never used Express, but for what I see, you only need to use the next function. See my new edited example. – anolsi Aug 19 '17 at 17:54
I'm surprised that you don't pass back the promise to some express handler that is going to make sure the `then` is called. Don't we need "something" to wait over the promise? – Laurent May 19 '18 at 18:28

score 1 · Answer 2 · edited Sep 26 '17 at 20:39

1

router.post('/Registration', function(req, res) {
    return User
        .findOne({ username: req.body.username })
        .then((user) => {
            if (user) {
                return console.log({ msg:"Username already been taken" });
            }
            return console.log({ msg: "Username available." });
        })
        .catch((err)=>{
            return console.error(err);
        });
});

you can write a clean code like this. Promise is a global variable available you don't need to require it.

edited Sep 26 '17 at 20:39

Ilya Volodin

10,929
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answered Aug 24 '17 at 13:03

Gokul Thulaseedharan

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Thanks but why do we return a result of console.log() which is undefined? What's the role of next() that we do not call? – eel ghEEz Mar 16 '18 at 21:42

how to use Promise with express in node.js?

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