[JavaScript] double exclamation mark (!!)

! is the logical negation or "not" operator. !! is ! twice. It's a way of casting a "truthy" or "falsy" value to true or false, respectively. Given a boolean, ! will negate the value, i.e. !true yields false and vice versa. Given something other than a boolean, the value will first be converted to a boolean and then negated. For example, !undefined will first convert undefined to false and then negate it, yielding true. Applying a second ! operator (!!undefined) yields false, so in effect !!undefined converts undefined to false.

In JavaScript, the values false, null, undefined, 0, -0, NaN, and '' (empty string) are "falsy" values. All other values are "truthy."(1):7.1.2 Here's a truth table of ! and !! applied to various values:

 value     │  !value  │  !!value
━━━━━━━━━━━┿━━━━━━━━━━┿━━━━━━━━━━━
 false     │ ✔ true   │   false
 true      │   false  │ ✔ true
 null      │ ✔ true   │   false
 undefined │ ✔ true   │   false
 0         │ ✔ true   │   false
 -0        │ ✔ true   │   false
 1         │   false  │ ✔ true
 -5        │   false  │ ✔ true
 NaN       │ ✔ true   │   false
 ''        │ ✔ true   │   false
 'hello'   │   false  │ ✔ true

from https://stackoverflow.com/questions/29312123/how-does-the-double-exclamation-work-in-javascript