api.js

var API = function () {
};

API.prototype.login = function(username, password) {
  console.log("request username and pw to server...");
  this.auth_token = "nhonhonho";
};

API.prototype.getUserById = function(user_id) {
  if (this.auth_token){
    console.log("request user with id " + user_id + " with token " + this.auth_token);
    return { username: 'Bernardo', email: 'bersimoes' }
  }else{
    console.log("you need to login first");
  }
};

var my_api = new API();
my_api.getUserById('1234');
my_api.login('bernardo', 'password');
my_api.getUserById('1234');