Using this keyword in Javascript ES6 [duplicate]

This question already has an answer here:

• When should I use Arrow functions in ECMAScript 6? 5 answers
• Methods in ES6 objects: using arrow functions 2 answers

I am developing Node.js application with ES6.

Using bcrypt-nodejs to encrypt and decrypt password before saving to MongoDB.

There is Schema defination:

import mongoose from 'mongoose'
import bcrypt from 'bcrypt-nodejs'

const Schema = mongoose.Schema

let userSchema = new Schema({
  username: {
    type: String,
    min: 6,
    max: 20,
    unique: true,
    required: true,
    dropDups: true
  },
  password: {
    type: String,
    min: 6,
    max: 20
  }
})

// generate password hash
userSchema.methods.generateHash = password => bcrypt.hashSync(password, bcrypt.genSaltSync(8), null)

// checking if password is valid
userSchema.methods.validPassword = password => bcrypt.compareSync(password, this.password)

export default mongoose.model('User', userSchema)

When I want to verify user password I call below function:

if (!user.validPassword(req.body.password)) {
          console.error(`Authentication failed. Wroong password for user     '${req.body.username}'`)
        }

Then the error occurred:

events.js:182
      throw er; // Unhandled 'error' event
      ^

TypeError: Cannot read property 'password' of undefined

I think this problem is caused by ES6 syntax, this is undefined. But I don't know how I can fix it.

Thank you all in advance.

via Toan Tran