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