what does double exclamation mark mean javascript code example

Example 1: double question mark javascript

//Similar to || but only returns the right-hand operand if the left-hand is null or undefined
0 ?? "other" // 0
false ?? "other" // false
null ?? "other" // "other"
undefined ?? "other" // "other"

Example 2: 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!