CODEWITHSUNDEEP
javascript
Mike Sav
Mike Sav

Reputation: 15291

Difference in using "in" operator and dot notation in JavaScript

I have a question that is just bothering me. If I declare a var a and I then try to test it using the "in" operator I get a true result, if I do the same test using the dot notation I get a false result, for example...

var a;

if('a' in window) {
  console.log('a in window'); // this is written to the console
}

if (window.a) {
  console.log('window.a'); // nothing happening here
}

now I noticed that when I give a a value like so both outputs work... look at this and notice how I check if it doesn't exist:

var a;

if('a' in window) {
  console.log('a in window'); // this is written to the console
}

if (!window.a) {
  console.log('!window.a'); // this is written to the console
}

a = 1;

if (window.a) {
  console.log('window.a'); // this is written to the console
}

So why does the dot notation only work when the variable is assigned a value? A silly question I know but so far I can't get a definite answer!

Upvotes: 2

Views: 87

Answers (3)

falsarella
falsarella

Reputation: 12437

Go to your console and play like this:

var a;

undefined

'a' in window

true

window.a

undefined

a = 1

1

window.a

1

'a' in window

true

The in keyword checks if there is a variable in a context. So, an expression using in, will always return true or false based on its existence, not on its value.

The . notaction access the variable's value in a context. So, an expression using ., might return any value, including undefined, if it wasn't.

Now that you know what's being returned for each expression, mix it with what the evaluation done when each value is returned inside an if statement.

As you might know, undefined will evaluate to false, and both 1 and true will evaluate to true, resulting on the behavior you're facing.

Upvotes: 1

Zach Sadler
Zach Sadler

Reputation: 580

window.a is undefined before you give it a value, and

if (undefined) === if (Boolean(undefined))

Since Boolean(undefined) === false, the console.log within your if statement is not reached.

Upvotes: 2

Paul Rowe
Paul Rowe

Reputation: 788

As Johan pointed out, the check is getting a falsey value.

if ('a' in window) console.log("a exists");
if (window.a) console.log("window.a is a truthy value");
if (!window.a) console.log("window.a is a falsey value");
if (typeof window.a != "undefined") console.log("window.a is not undefined");

Upvotes: 3

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