How does the double exclamation (!!) work in javascript?
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! 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, ie !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
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