我不明白JavaScript中call（）方法的意义 [英] I don't understand the point of the call() method in JavaScript

问题描述

请看下面的代码并解释：我做错了什么？

Please look at the code below and explain: what am I doing wrong?

    function doStuff(a, b){
      return a + b + 1 ;
    }

    var myContext = {
       c: 1, 
       d: 3
    };

// myContext = this (result => 5)
    doStuff.call(myContext,myContext.c,myContext.d)

// ... so why doesn't the below work? (result => NaN)
    doStuff.call(myContext,this.c,this.d)

// To make the above work, i must replace "this" with "myContext" (result => 5)...
    doStuff.call(myContext,myContext.c,myContext.d)

// ...which is no different to... 
    doStuff(myContext.c,myContext.d)

// ...so what was the point of call() method?

我厚吗？

推荐答案

调用的要点是设置这个 的值功能。由于您的 doStuff 未在函数中使用此 ，使用打电话，这是毫无意义的。

The main point of call is to set the value of this within the function. Since your doStuff doesn't use this within the function, using call with it is pointless.

以下是一个重要的例子：

Here's an example where it matters:

function doStuff(a, b) {
    return this.sum(a, b);
}
var obj = {
    sum: function(a, b) {
        return a + b;
    }
};
console.log(doStuff.call(obj, 3, 4)); // 7, because `obj` has a `sum` property
console.log(doStuff(3, 4));           // Fails with an error that `this.sum` is not a function

---

---

所以为什么以下不起作用？ （result => NaN）

so why doesn't the below work? (result => NaN)

doStuff.call（myContext，this.c，this.d）

因为 this.c 在电话之前评估到 doStuff ，使用当前这个值。如何调用该代码，这个是（在松散模式下）全局对象（浏览器上的窗口对象），它可能没有 c 或 d 属性。 （在严格模式下，再次假设你如何调用它，你会得到一个例外，因为这个是 undefined 并且您无法从 undefined 中检索属性。）

Because this.c is evaluated before the call to doStuff, using whatever the current this value is. How you're probably calling that code, this is (in loose mode) the global object (the window object, on browsers), which probably doesn't have a c or d property. (In strict mode, again making assumptions about how you're calling that, you'd get an exception, because this is undefined and you can't retrieve properties from undefined.)

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