Use promises for flatter code

src/models/users.js

@@ -9,36 +9,43 @@ const User = new mongoose.Schema({
   salt: { type: String }
 })
 
-User.pre('save', function(next) {
+User.pre('save', function preSave(next) {
   const user = this
 
-  if(!user.isModified('password')) {
+  if (!user.isModified('password')) {
     return next()
   }
 
-  bcrypt.genSalt(10, (err, salt) => {
-    if(err) { return next(err) }
-
+  new Promise((resolve, reject) => {
+    bcrypt.genSalt(10, (err, salt) => {
+      if (err) { return reject(err) }
+      return salt
+    })
+  })
+  .then(salt => {
     bcrypt.hash(user.password, salt, (err, hash) => {
-      if(err) { return next(err) }
+      if (err) { throw new Error(err) }
 
       user.password = hash
       user.salt = salt
-      next()
+      return user
     })
   })
+  .then(() => {
+    next()
+  })
 })
 
-User.methods.validatePassword = function(password) {
+User.methods.validatePassword = function validatePassword(password) {
   const user = this
 
   return new Promise((resolve, reject) => {
     bcrypt.compare(password, user.password, (err, isMatch) => {
-      if(err) { return reject(err) }
+      if (err) { return reject(err) }
 
       resolve(isMatch)
-    });
+    })
   })
-};
+}
 
 export default mongoose.model('user', User)