why inside function we can override "undefined" but can't in window object in JS...Example is given Below

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();

I know its not a good practice but i just wann know what actually happening under the hood of javascript while its execution — Montu Patel
– Montu Patel, Commented Jun 27, 2017 at 11:08
note: undefined is often passed as an argument to plugin constructors to ensure that the value is preserved and not redefined. it is really not a thing to do — Kaddath
– Kaddath, Commented Jun 27, 2017 at 11:10
@AliSomay he forgot the var before undefined, when it is here, undefined gets actually overriden and the results are what he writes — Kaddath
– Kaddath, Commented Jun 27, 2017 at 11:12

Quentin · Accepted Answer · 2017-06-27 12:36:31Z

3

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.

edited Jun 27, 2017 at 12:36

answered Jun 27, 2017 at 11:14

Quentin

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samanime · Accepted Answer · 2017-06-27 11:10:45Z

1

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.

answered Jun 27, 2017 at 11:10

samanime

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1 Comment

Montu Patel Over a year ago

thnx buddy...i know it is bad practice. i just wann know .Why it is so.

Rakesh Soni · Accepted Answer · 2017-06-27 12:47:51Z

0

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

edited Jun 27, 2017 at 12:47

Rakesh Soni

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answered Jun 27, 2017 at 12:04

vishal

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