In JavaScript, why does (null + null === 0) return true?

#1

Post by swozniak » Mon, May 05, 2014, 5:30 PM UTC

I could be convinced into thinking == would return true if null was treated as 0 (so 0 + 0 = 0), but with ===, this doesn't seem right at all, especially considering:

typeof null === 'object'

and

typeof 0 === 'number'

Can someone explain?

Re: In JavaScript, why does (null + null === 0) return true?

#2

Post by zamfi » Mon, May 05, 2014, 5:45 PM UTC

> null + null

0

> +null

0

Looks like JS converts null to 0 when it's used in a number context.

Re: In JavaScript, why does (null + null === 0) return true?

#3

Post by singold » Mon, May 05, 2014, 5:52 PM UTC

I don't really know (I don't have that in depth knowledge of javascript) but it is probably like you said, because you are doing a sum, it treats null as 0, so (0 + 0 === 0) is true.

You can do the same with false: (false + false === 0)

And also (true + true === 2) is true

Edit: The same way as if you do "" + 1 you get "1" because it assumes + as string concatenation

Re: In JavaScript, why does (null + null === 0) return true?

#4

Post by joelbirchler » Mon, May 05, 2014, 8:48 PM UTC

The + operation is evaluated first, and + coerces nulls to zeros. This looks like:

(null + null) === 0

(0) === 0

true

Re: In JavaScript, why does (null + null === 0) return true?

#5

Post by swozniak » Tue, May 06, 2014, 4:58 PM UTC

joelbirchler wrote: post #4

The + operation is evaluated first, and + coerces nulls to zeros. This looks like: (null + null) === 0 (0) === 0 true

Ah, it looks like JavaScript coerces null to the type against which it is being compared.

Thanks!