Explore how to judge the double exclamation mark in javascript-javascript tutorial

Determine the double exclamation mark in js. I checked some information on the Internet. He is equivalent to a ternary operator and returns a boolean value. Today, I read the qunit source code and found a strange piece of code. Although I can understand what he meant, but I do not understand the role of the Two exclamation points.

The Code is as follows:

Function id (name ){
Return !! (Typeof document! = "Undefined" & document. getElementById )&&
Document. getElementById (name );
}

Then I checked some information on the Internet, which is equivalent to a ternary operator and returns a boolean value.

The Code is as follows:

Var ret = !! Document. getElementById

It is equivalent:

The Code is as follows:

Var ret = document. getElementById? True: false;

If the value is a non-null string or a non-zero number, true is returned. If the value is a null String, 0, or null, false is returned.

The Code is as follows:

Var a = ""; alert (!! A); // true
Var a = "s"; alert (!! A); // true
Var a = true; alert (!! A); // true
Var a = 1; alert (!! A); // true
Var a =-1; alert (!! A); // true
Var a =-2; alert (!! A); // true

Var a = 0; alert (!! A); // false
Var a = ""; alert (!! A); // false
Var a = false; alert (!! A); // false
Var a = null; alert (!! A); // false