爲什麼我的上下文不能共享？

Question

我想我明白瞭如何在javascript中工作call，但顯然不是。

function bear(){ 
    var a = 1 
    pig.call(this) 
} 

function pig(){ 
    alert(a) //throws an error, 'a is not defined' 
} 

bear() 

http://codepen.io/anon/pen/jEYGOv

爲什麼不這項工作？我怎樣才能使它工作（沒有通過a作爲變量）

Answer 1

你不能做到這一點。

function bear(){ 
    pig.call(this); 
} 

function pig(){ 
    this.a = 5; //same as window.a = 5 unless used as a constructor; 
} 
bear(); //window.a == 5; 
a = new bear(); //this keyword is now referring to variable object a so a.a = 5; 

而且第一個爲.call(this)後的任何參數將是功能的，你在

Answer 2

你需要看一看在scope of variables：

<script type="text/javascript"> 
var a; 

function bear(){ 
    a = 1 
    pig.call(this) 
} 

function pig(){ 
    alert(a); 
} 

bear(); 
</script> 

Answer 3

調用call()參數爲什麼不這項工作？我怎樣才能使它發揮作用（而沒有經過作爲 變量）

你在這裏做什麼是Function.prototype.call共享的上下文。共享上下文不共享範圍變量。範圍變量不能從範圍之外訪問，並且pig()在與bear()不同的範圍內運行。

你可以做的是

一）發送的參數的公共變量：。

// recommended 

function bear(){ 
    var a = 1; 
    pig(a); 
} 

function pig(a){ 
    alert(a); 
} 

bear(); 

B）定義對象是共同環境：

// not recommended 
// hard to follow where the context is coming from 

function bear(){ 
    this.a = 1; 
    pig.call(this); 
} 

function pig(){ 
    alert(this.a); 
} 

var o = {}; 
bear.call(o); 

或

// recommended 

var o = { 
    a: undefined, 
    bear: function(){ 
     this.a = 1; 
     this.pig(); 
    }, 
    pig: function pig(){ 
     alert(this.a); 
    } 
}; 

o.bear(); 

角）定義一個類

// not recommended 
// you are defining the methods (bear and pig) by each instantiation 

var My = function(){ 
    this.bear = function(){ 
     this.a = 1; 
     this.pig(); 
    }; 
    this.pig = function pig(){ 
     alert(this.a); 
    }; 
}; 

var o = new My(); 
o.bear(); 

或

// recommended 

var My = function(){}; 
My.prototype = { 
    constructor: My, 
    a: undefined, 
    bear: function(){ 
     this.a = 1; 
     this.pig(); 
    }, 
    pig: function pig(){ 
     alert(this.a); 
    } 
}; 

var o = new My(); 
o.bear(); 

d。）在上部範圍

// not recommended 
// (unless we are talking about a commonjs module which has its own scope) 
// don't pollute the global scope with local variables 

var a; 

function bear(){ 
    a = 1; 
    pig(a); 
} 

function pig(a){ 
    alert(a); 
} 

bear(); 

或閉合定義公共變量

// recommended 

(function(){ 
    var a; 

    function bear(){ 
     a = 1; 
     pig(a); 
    } 

    function pig(a){ 
     alert(a); 
    } 

    bear(); 
})();