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 stackoverflow.com/questions/784929/… 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
– Barmar, Commented Mar 3, 2016 at 17:49
The really simple explanation, is that the string "false" is in fact true — adeneo
– adeneo, Commented Mar 3, 2016 at 17:50
Plus the fact that ! always converts its argument to a boolean, so !!<anything> is not the same as <anything> unless <anything> is already a boolean. — Barmar
– Barmar, Commented Mar 3, 2016 at 17:52

Barmar · Accepted Answer · 2016-03-03 17:51:13Z

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.

Barmar

789k57 gold badges555 silver badges669 bronze badges

answered Mar 3, 2016 at 17:46

67.2k17 gold badges107 silver badges133 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Halcyon · Accepted Answer · 2016-03-03 17:46:39Z

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 3, 2016 at 17:46

Halcyon

57.8k10 gold badges93 silver badges131 bronze badges