using ! and the in operator return false when it should be true, why?

Question

In Javascript, the following code works like so:

> var foo = { bar: 1 }
undefined
> foo
{ bar: 1 }
> 'bar' in foo
true
> !'bar' in foo
false
> 'baz' in foo
false
> !'baz' in foo
false

Why does using ! and in not return true when checking if an object does not contain a property?

Answer 1

You need to do !('baz' in foo), otherwise it's the same as (!'baz') in foo (which looks up 'false' in foo), reason being operator precedence.

!'foo' in {false: true} //as in 'false' in {false: true}
//true

!('foo' in {false: true}) //as in, does a key "foo" not exist in the object
//true

Answer 2

You need to use the ! against the result of the in

!("key" in object)

Right now it is like this.

(!"key") in object

And that is the same as doing

false in object

Whcih becomes

"false" in object