Javascript Double Not-Operator(!!)

example1.js

if (typeof x !=== 'undefined' && x !=== '') {
  // not emtpy
}

example2.js

if (x) {
  // not emtpy
}

example3.js

var xIsEmpty = !!x;

example4.js

var x = 0;        // falsy
console.log(!x);  // true (it is not what we want)
console.log(!!x); // false