What does the !! operator (double exclamation point) mean in JavaScript?

Question

I've just walked into this code:

val.enabled = !!enable

and have no idea what does "!!" do... I googled JavaScript operators but haven't found this one.

Answer 1

!! converts the value to the right of it to its equivalent boolean value. (Think poor man's way of "type-casting"). Its intent is usually to convey to the reader that the code does not care what value is in the variable, but what it's "truth" value is.

Answer 2

It converts the suffix to a Boolean value.

Answer 3

It's a horribly obscure way to do a type conversion.

! is NOT. So !true is false and !false is true. !0 is true and !1 is false.

So you're converting a value to a bool, then inverting it, then inverting it again.

//Maximum Obscurity:
val.enabled = !!userId;

//Partial Obscurity:
val.enabled = (userId != 0) ? true : false;

//And finally, much easier to understand:
val.enabled = userId != 0;

Answer 4

It's a double not operation. The first ! converts the value to boolean and inverts its logical value. The second ! inverts the logical value back.

Answer 5

! is "boolean not", which essentially typecasts the value of "enable" to its boolean opposite. The second ! flips this value. So, !!enable means "not not enable," giving you the value of enable as a boolean.

Answer 6

!!foo applies the unary not operator twice and is used to cast to boolean type similar to the use of unary plus +foo to cast to number and concatenating an empty string ''+foo to cast to string.

Instead of these hacks, you can also use the constructor functions corresponding to the primitive types (without using new) to explicitly cast values, ie

Boolean(foo) === !!foo
Number(foo)  === +foo
String(foo)  === ''+foo

Answer 7

It's not a single operator, it's two. It's equivalent to the following and is a quick way to cast a value to boolean.

val.enabled = !(!enable);