JAVASCRIPT

js double exclamation mark

// "!!" converts a value to a boolean, then inverts it and then inverts it again.
// So let's say you want to check the value for a given variable:
const num = null; 

if (num) { // false -> value is null -> evaluated as falsy
}
if (!num) { // true
}
if (!!num) { // false
}
// another example
const str = "Hello World";

if (str) { // true -> value is a "proper" (not empty) string -> evaluated as truthy
}
if (!str) { // false
}
if (!!str) { // true
}

double exclamation mark js

// Converts anything to boolean. 

!!false === false
!!true === true

!!0 === false
!!1 === true

!!parseInt("foo") === false // NaN is falsy
!!-1 === true               // -1 is truthy
!!(1/0) === true            // Infinity is truthy

!!"" === false              // empty string is falsy
!!"foo" === true            // non-empty string is truthy
!!"false" === true          // ...even if it contains a falsy value

!!window.foo === false      // undefined is falsy
!!null === false            // null is falsy

!!{} === true               // an (empty) object is truthy
!![] === true               // an (empty) array is truthy; PHP programmers beware!

double exclamation mark javascript

Double Exclamation

javascript double exclamation mark

console.log(!!navigator.userAgent.match(/MSIE 8.0/));  
// returns either true or false

javascript double exclamation mark

const isIE8 = !! navigator.userAgent.match(/MSIE 8.0/);  
console.log(isIE8); // returns true or false