What does the exclamation mark do before the function?

Question

!function () {}();

Answer 1

It returns whether the statement can evaluate to false. eg:

!false      // true
!true       // false
!isValid()  // is not valid

You can use it twice to coerce a value to boolean:

!!1    // true
!!0    // false

So, to more directly answer your question:

var myVar = !(function(){ return false; })();  // myVar contains true

Answer 2

An exclamation mark in Javascript is the "not" operator. It the function returns false (or null or zero), then !f will be true. Alternatively if the result is true (or any non-null, non-zero value) then !f will be false.

Answer 3

The function:

function () {}

returns nothing (or undefined).

So, this expression calls that function:

function () {}()

and using the ! operator on the result converts the undefined result to true:

!function () {}()

As this will show you:

alert( !function () {}());