JavaScript 中全局和局部变量的奇怪行为 [英] Strange Behavior on Global and Local Variable in JavaScript

问题描述

我尝试了以下代码:

var a = 5;

function x() {
  console.log(a);
}

x();

它按预期运行并打印 5.

It runs as expected and prints 5.

但我更改了代码，因此全局变量 a 将被覆盖如下:

But i changed the code so the global variable a will be overwrite as follows:

var a = 5;

function x() {
  console.log(a);
  var a = 1;
}

x();

它打印未定义.这对我来说没有意义，因为覆盖应该在 console.log(a) 之后发生.那么问题出在哪里?

It prints undefined. It doesn't make sense for me since the overwrite should be happened right after console.log(a). So what is the problem?

推荐答案

发生这种情况是因为您的第二个 a 变量被提升"到函数的顶部并且它隐藏了第一个 >a.实际发生的事情是这样的:

This is happening because your second a variable is being 'hoisted' to the top of the function and it hides the first a. What is actually happening is this:

var a = 5;

function x() {
  var a;
  console.log(a);
  a = 1;
}

x();

这是一篇关于从足够好的提升提升的文章，以进一步阅读该主题.

Here is an article on hoisting from adequately good for further reading on the subject.

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