operator in javascript with different type and value

Question

I am trying to understand a small piece of code
I thought !!in javascript does the not operator two times
```
var result = !!String("false");
```
so I thought for the above code it will return as false
but it returns true..and can you tell me why the type is boolean?

This was closed as a duplicate of http://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript/784946#784946 but I reopened it. The question isn't about what `!!` does, but why it returns a different result than he expected with this particular argument. — Barmar, Mar 03 '16 at 17:49
The really simple explanation, is that the string `"false"` is in fact true — adeneo, Mar 03 '16 at 17:50
Plus the fact that `!` always converts its argument to a boolean, so `!!` is not the same as `` unless `` is already a boolean. — Barmar, Mar 03 '16 at 17:52

score 5 · Answer 1 · edited Mar 03 '16 at 17:51

5

That is because,

String("false") will return "false". A string.
!"false" will be evaluated to false. Since a non empty string will be considered as true when it coerces to boolean.
!false will be evaluated to true.

edited Mar 03 '16 at 17:51

Barmar

answered Mar 03 '16 at 17:46

score 2 · Answer 2 · answered Mar 03 '16 at 17:46

2

String("false")

Resolves to:

"false"

Which is a string that is non-empty, so

!"false"

Resolves to

false

And finally

!false

Resolves to

true

answered Mar 03 '16 at 17:46

Halcyon

2 Answers2