为什么在函数内部我们可以覆盖“ undefined”，但不能在JS的window对象中使用…示例如下

Question

I have given two different code. One inside global window object and second inside foo function

Here my First code:

 var undefined = 2; // 2 console.log(undefined == 2); // it gives me FALSE var window = 5; // 5 console.log(window == 5); // it gives me FALSE var a; console.log(a); // it give me UNDEFINED 

Here my second code:

 function foo() { var undefined = 2; // 2 console.log(undefined == 2); // true var window = 5; // 5 console.log(window == 5); // true var a; console.log(a); } foo(); 

Answer 1

window and undefined are predefined (by the JS engine / browser) variables.

You can't overwrite a read only variable (if a variable already exists, then using var in the scope it exists in does nothing).

You can declare a new variable, in a narrower scope, that masks one with the same name in a wider scope.

Answer 2

The global scope won't let you mess with just anything. That would basically cause all sorts of bad things to happen. That's why you can't change the things outside.

Inside the function, in an isolated scope, it'll let you declare variables with other names. While this is an absolutely horrible practice and you should avoid it at all costs, it won't affect anything beyond the one function.

function foo() {
    var window = 5;
}

The window in the function is not the same as the window at the global level. The window in the function is simply masking the outer window, making it inaccessible from inside of foo .

Answer 3

In Your function foo() console.log(undefined == 2) you mentioned //true is returning. But actually it return "false". Because "undefined" is a reserved keyword. So you can't use this as a variable. See images: When using undefined keyword it's not working

Better use some other variable name in your foo() When using some other variable