我试图更好地理解`this`的用法.该示例很冗长，但其目的是为了更好地理解

问题描述

我试图更好地理解this的用法.在尝试此代码时，我发现可以使用类似console.log(this.arr[4])的方式访问arr中的项目，但前提是必须使用var声明arr.如果我使用let或const声明arr，则会收到TypeError.

I'm trying to better understand the use of this. In experimenting with this code, I found I can access the items in arr by using something like console.log(this.arr[4]), but only if arr is declared using var. If I declare arr using let or const, I get a TypeError.

首先，我知道这很冗长.就像我说的那样，我只是在尝试以寻求更好的理解，并偶然发现了这个引起我好奇的问题.

First, I do understand this is verbose. Like I said, I am just experimenting to try and get a better understanding and came across this issue that piqued my curiosity.

const arr = [
  1,
  false,
  {
    name: 'John',
    address: '123 Peachtree Drive'
  },
  function(name = 'new user') {
    const greeting = 'Hello there, '
    console.log(greeting + name)
  },
  'foo',
]

console.log(this.arr[4])

同样，如果我只是简单地使用var而不是let声明arr，我就可以很好地记录它.

Again, if I simply declare arr using var instead of let I can log it just fine.

推荐答案

在浏览器中，顶级this是window，顶级var声明也使变量可以作为window的属性进行访问，而let和const则没有.引用arr的正确方法很简单

In a browser, top-level this is window, and top-level var declarations also make variables accessible as properties of window, while let and const do not. The correct way to reference arr is simply

console.log(arr[4])

我不鼓励您使用顶级this甚至访问var声明，因为依赖var行为的代码显然令人困惑，因为这种情况是一个很好的例子.

I would discourage you from using top-level this to even access var declarations, because code which relies on that behavior of var is obviously confusing, as this case is a perfect example of.

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