What does a !! in JavaScript mean?

2018-06-26 01:05:29

Possible Duplicates:
myVar = !!someOtherVar
What does the !! operator (double exclamation point) mean in JavaScript?

Came across this line of code

strict = !!argStrict

... here and wondered what effect the !! has on the line? Pretty new to JS!

It converts your value to a boolean type:

var x = '1';
var y = !!x;

// (typeof y === 'boolean')

Also note the following:

var x = 0;
var y = '0';       // non empty string is truthy
var z = '';

console.log(!!x);  // false
console.log(!!y);  // true
console.log(!!z);  // false

It converts the value to a value of the boolean type by negating it twice. It's used when you want to make sure that a value is a boolean value, and not a value of another type.

In JS everything that deals with booleans accepts values of other types, and some can even return non-booleans (for instance, || and && ). ! however always returns a boolean value so it can be used to convert things to boolean.

It is a pair of logical not operators.

It converts a falsey value (such as 0 or false ) to true and then false and a truthy value (such as true or "hello" ) to false and then true .

The net result is you get a boolean version of whatever the value is.

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