how to use Promise with express in node.js?

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.

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.