Javascript Double Not-Operator(!!)

In javascript Falsy and Truthy values are used all over the place.

For example the following values are always falsy:

• "" (empty string)
• undefined
• false
• 0
• null
• NaN

And any other value is truthy.

Since undefined and empty string are falsy values then we could replace this code below:

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

with a falsy/truthy verification:

if (x) {
  // not emtpy
}

To set the result of falsy/thruthy verification to a variable is possible to use a double not-operator(!!) which will force a type casting to Boolean.

var xIsEmpty = !!x;

Actually !! is not a operator it is only the ! operator twice. The first ! will cast to boolean inverting the result. Then the second ! will invert again the value so it is the expected boolean value.

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