從特權方法訪問私有成員

Question

爲什麼不能在draw（）函數中訪問Ball對象私有成員？當我登錄它們時，私有變量變得不確定。不應該有特權的方法能夠訪問私人成員？

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

當你與var定義他們，讓他們「私人」，他們不會是在this範圍。丟掉this只是引用變量，它會工作。

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）