I'm trying to get access to the password stored in the user instance using keyword this and it doesn't work?

Here is the code when I'm calling it.

Users.find({where: {$or: [{ email: email} ,{ username: username }]}})
  .then(user => {
    if (!user) return res.status(401).send(new errors.Unauthorized('Email/Username or Password Not Found!'));

    user.authenticate(password, (err, match) => {
      if (err) return res.status(401).send(err);
      res.send(user);
    });
  })
  .catch(err => res.send(err));

Here is the code for my instance method.

    instanceMethods :{
      authenticate: (password, cb) => {
        return bcrypt.compare(password, this.password, (err, match) => {
          if (err) return cb(new errors.Unauthorized());
          return cb(null, match);
        });
      }
    }