Javascript ! and !! differences [duplicate]

This question already has answers here: Closed 11 years ago.

Possible Duplicate:

What is the !! operator in JavaScript?

What is the difference between these two operators? Does !! have special meaning, or does it simply mean yo开发者_如何学运维u are doing two '!' operations. I know there are "Truth" and "Truthy" concepts in Javascript, but I'm not sure if !! is meant for "Truth"

---

!! is just double !

!true // -> false
!!true // -> true

!! is a common way to cast something to boolean value

!!{}  // -> true
!!null // -> false

---

Writing !! is a common way of converting a "truthy" or "falsey" variable into a genuine boolean value.

For example:

var foo = null;

if (!!foo === true) {
    // Code if foo was "truthy"
}

After the first ! is applied to foo, the value returned is true. Notting that value again makes it false, meaning the code inside the if block is not entered.