问题
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.
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])Again, if I simply declare arr using var instead of let I can log it just fine.
回答1:
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])
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.
回答2:
The behavior you're getting confused about is not specific to 'this'. It is related to scope. 'var' has a different scope than 'let' or 'const'.
var - has function level scoping and can change the value reference
let - has block level scoping and can change the value reference
const - has block level scoping but cannot change the value reference
https://love2dev.com/blog/javaScript-var-let-const/
Be precise about how and where you declare your variables as this is an important part of JavaScript syntax/interpretation.
来源：https://stackoverflow.com/questions/56908194/im-trying-to-better-understand-the-use-of-this-the-example-is-verbose-but-i