从特权方法访问私有成员

Question

为什么不能在draw（）函数中访问Ball对象私有成员？当我登录它们时，私有变量变得不确定。不应该有特权的方法能够访问私人成员？

var ctx; 

(function() { 
    console.log("Hello world!"); 

    var canvas = document.getElementById('gameCanvas'); 
    ctx = canvas.getContext("2d"); 

    var ball1 = new Ball(); 
    ball1.draw(); 

})(); 

function Ball() { 
    var that = this; // for private methods 

    var posY  = 50; 
    var posX  = 50; 
    var velocityX = 0; 
    var velocityY = 0; 
    var radius = 10; 

    this.draw = function() { 
     console.log(this.posY); // outputs 'undefined' 
     ctx.beginPath(); 
     ctx.fillStyle = '#444'; 
     ctx.arc(this.posX, this.posY, this.r, 0, Math.PI*2); 
     ctx.fill(); 
    } 
} 

Answer 1

当你与var定义他们，让他们“私人”，他们不会是在this范围。丢掉this只是引用变量，它会工作。

var ctx; 

(function() { 
    console.log("Hello world!"); 

    var canvas = document.getElementById('gameCanvas'); 
    ctx = canvas.getContext("2d"); 

    var ball1 = new Ball(); 
    ball1.draw(); 

})(); 

function Ball() { 
    var that = this; // for private methods 

    var posY  = 50; 
    var posX  = 50; 
    var velocityX = 0; 
    var velocityY = 0; 
    var radius = 10; 

    this.draw = function() { 
     console.log(posY); 
     ctx.beginPath(); 
     ctx.fillStyle = '#444'; 
     ctx.arc(posX, posY, r, 0, Math.PI*2); 
     ctx.fill(); 
    } 
} 

Answer 2

this.需要我相信：

this.posY  = 50; 
this.posX  = 50; 
this.velocityX = 0; 
this.velocityY = 0; 
this.radius = 10; 

（不var）