auth_example.js

let auth = (user, pwd) => {
   return new Promise((resolve, reject) => {
      if( user !== "foo" && pwd !== "bar"){
         return reject({ message : "Wrond credential"})   
      }
      return resolve({
         user : user
      });
   });
}
let promoteUser = (user) => {
    return new Promise((resolve, reject) => {
      user.admin = true;
      return resolve(user);
   });
}
auth("foo", "bar")
     .then(promoteUser)
     .then((user) => {
        console.log(user)
     }).catch((e) => {
        console.log(e);
     });