How does the double exclamation (!!) work in javascript?
How does the double exclamation (!!) work in javascript?
! is the logical negation or not operator. !! is ! twice. Its 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 Heres 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