理解JavaScript函数范围 [英] Understanding JavaScript function scoping

问题描述

以下代码是JavaScript代码。我试图了解JavaScript中的函数范围，并关注此处的文章。我正在复制下面的代码 -

The code below is JavaScript code. I am trying to understand function scope in JavaScript and following the article over here. I am reproducing the code below -

var cow = "purple"; // just a random cow

var f = function (x) {
    var r = 0;
    cow = "glue";
    if (x > 3) {
        var cow = 1; // a local variable
        r = 7;
    }
    return r;
};

var z = f(2);
alert(cow); // returns purple

我不太明白为什么字符串purple会被警告。行 cow =glue; 应该将cow变量的值设置为glue。如果我删除了if块，然后在最后一个语句中警告cow，我看到字符串glue被警告。

I don't quite understand why the string "purple" is alerted. The line cow = "glue"; should set the value of the cow variable to "glue". If I remove the if block, and then alert cow in the last statement, I see that the string "glue" is alerted.

当调用f（2）时，如果没有输入if代码块并且其中没有任何内容被执行，那么为什么我会看到不同的结果呢？即为什么在最后一个语句中警告牛现在返回字符串紫色？

When f(2) is called, the if code block is not entered and nothing in it gets executed, so why do I see different results ? i.e why does alerting cow in the last statement return the string "purple" now ?

推荐答案

函数内的变量声明总是被提升到顶端。所以你的代码实际上是：

Variable declarations inside functions are always hoisted to the top. So your code is actually:

var f = function (x) {
    var cow, r;
    r = 0;
    cow = "glue";
    if (x > 3) {
        cow = 1; // a local variable
        r = 7;
    }
    return r;
};

在函数内部，您总是分配给 local cow ，绝不是全球。

Inside the function you're always assigning to the local cow, never the global.

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