在JavaScript中在函数体顶部声明变量的好处 | 在JavaScript中在函数体顶部声明变量的好处

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问题描述

我正在阅读Douglas Crockford的书。他正在讨论范围，并说JS没有块范围：

I am reading Douglas Crockford's book "Javascript: The Good Parts". He is talking about scope and saying that JS doesn't have block scope:

但是，我想不出一个很好的例子，为什么这个语句有意义，看到一些验证这个语句的例子真的很好。

However, I couldn't think of a good example why this statement makes sense and it would be really nice to see some examples which verify this statement.

推荐答案

在顶部声明变量有助于避免这样的情况：

Declaring variables at the top helps you avoid situations like this:

function outer() {
  var i = 50;

  function inner() {
    alert(i);
    var i= 30;
  }
  inner();
}

outer();

很多人都希望警报显示 50 ，他们会惊讶地看到 undefined 。那是因为 i 变量是在内部函数中声明的，但是直到<$ c之后它才被初始化$ C>警报。因此它具有完整的功能范围，即使它在初次使用后被声明。

Many people would expect the alert to show 50, and they would be surprised to see undefined. That's because the i variable is declared within the inner function, but it isn't initialized until after the alert. So it has full function scope, even though it's declared after its initial use.

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