Question

我以为我理解call在javascript中是如何工作的，但显然不是。

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

为什么这不起作用？我怎样才能使它工作（不将a作为变量传递）

Answer 1

你做不到。

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;

.call(this)的第一个参数之后的任何参数都是你在{/ 1}}上调用的函数的参数

Answer 2

为什么这不起作用？我怎么能让它工作（没有通过as 变量）

您在此处所做的是与Function.prototype.call分享上下文。共享上下文不共享范围变量。范围变量无法从范围外部访问，pig()运行的范围与bear()不同。

你能做的是

a。）在参数中发送公共变量：

// recommended

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

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

bear();

b。）将对象定义为公共上下文：

// 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();

c。）定义一个类

// 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。）在较高范围内定义公共变量

// 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();

或关闭

// recommended

(function (){
    var a;

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

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

    bear();
})();

Answer 3

您需要查看scope of variables：

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

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

function pig(){
    alert(a);
}

bear();
</script>

为什么我的上下文不会被共享？

3 个答案: