What the meaning of `!!` in JS?

Question

var obj = {
    a: "value",
    b: false
};
// nonexistent properties
console.log(!!obj.nonexistent);

// existing properties
console.log( !! obj.a);

i don't know the meaning of !!.

console.log(obj.nonexistent); -> undefined I can understand this. console.log(!obj.nonexistent); -> true i cann't understand this.?????????

console.log( !! obj.a); i cann't understand this.?????????

Well I guess the last one shows *false* isn't it? ...instead of *undefined*... — Breaking not so bad, Nov 24 '14 at 03:07
See: http://stackoverflow.com/questions/784929/what-is-the-not-not-operator-in-javascript — Dave G, Nov 24 '14 at 03:09

score 1 · Accepted Answer · answered Nov 24 '14 at 03:10

1

Consider, for example:

null  //null

then:

!null  //true

then:

!!null //false

answered Nov 24 '14 at 03:10

sealocal

1 Answers1