user model mongoose with password

user_model.js

var mongoose = require("mongoose"),
    Schema = mongoose.Schema,
    bcrypt = require("bcrypt"),
    SALT_WORK_FACTOR = 10;

var UserSchema = new Schema({
    username: { type: String, required: true, index: { unique: true } },
    password: { type: String, required: true }
});

UserSchema.pre(save, function(next) {
    var user = this;
  if (!user.isModified('password')) return next();
  bcrypt.genSalt(SALT_WORK_FACTOR, function(err, salt) {
      if (err) return next(err);
      bcrypt.hash(user.password, salt, function(err, hash) {
          if (err) return next(err);
          user.password = hash;
          next();
      });
  });


});

UserSchema.methods.comparePassword = function(candidatePassword, cb) {
    bcrypt.compare(candidatePassword, this.password, function(err, isMatch) {
        if (err) return cb(err);
        cb(null, isMatch);
    });
};