user.model.js

var mongoose = require('mongoose');

// User Schema
var userSchema = mongoose.Schema({
  email:{
    type: String,
    required: true,
    unique: true
  },
  password:{
    type: String,
    required: true
  },
  createdAt:{
    type: Date,
    default: Date.now
  }
});

module.exports = mongoose.model('User', userSchema);

userSchema.pre('save', function(next) {
  var newUser = this;
  console.log('password 1: ', this.password);
  console.log('newUser.password 1: ', newUser.password);
  // only hash the password if it has been modified (or is new)
  if (!this.isModified('password')) {
    return next();
  }
  // generate a salt
  return bcrypt.genSalt(10, function(error, salt) {
    console.log('password 2: ', this.password);
    console.log('newUser.password 2: ', newUser.password);
    if (error) {
      return next(error);
    }

    // hash the password using the new salt
    return bcrypt.hash(newUser.password, salt, function(error, hash) {
      console.log('password 3: ' + this.password);
      console.log('newUser.password 3: ', newUser.password);
      if (error) {
        return next(error);
      }
      // override the cleartext password with the hashed one
      newUser.password = hash;

      console.log('hash user password: ', newUser.password);

      return next();
    });
  });
});

Running console log at user schema 3 line 37, this.password is undefined

what if you run console.log(this) ?

Looks like the this.password is out of scope in the return function. So just made a new object and passing that through is working as expected.

Line 50, never returns a promise back to the routes