!! Negation in Javascript

Double negation casts a string or a number to a boolean:

var bool = !!num;
The following values are equivalent to false in conditional statements:

• false
• null
• undefined
• The empty string "" (\ '')
• The number 0
• The number NaN

All other values are equivalent to true.
var x = "somevalue" var isNotEmpty = !!x.length;
Let’s break it to pieces:

x.length   // 9
!x.length  // false
!!x.length // true