'this'在这种情况下如何工作？
这个问题在这里已经有了答案：
mainState是一个对象文字。 'prototype'是javascript中用于原型继承的函数对象的一个属性。 Javascript原型
有一件事总是帮助我记住this将是什么，以查找谁在调用函数，请检查此代码段
var sayHi = function() { console.log(this.name) }
var yoda = { name: "Yoda", sayHi: sayHi }
var darthVader = { name: "Anakin Skywalker", sayHi: sayHi }
// the `this` here will be the yoda object
yoda.sayHi()
// the `this` here will be the darthVader object
darthVader.sayHi()
window.name = "Global name"
// here, since nothing was specified, the object is the global object, or the window on browsers, this the same as "window.sayHi()"
sayHi()如果你想在原型上使用你的方法，你可以创建一个MainState构造函数，并附上你的原型方法：
function MainState(game) {
this.game = game;
}
MainState.prototype.create = function() {
this.bird = this.game.add.sprite(100, 245, 'bird');
};
MainState.prototype.myFunction = function() { };
// etc.
var mainState = new MainState(game);
mainState.myFunction();