I’ve been primarily coding in Javascript for a couple months now, and the language’s quirks have become familiar, almost normal, to me. IIFEs, truthy/falsy, ===, and other weird language features make sense now. When I came across !! in a Javascript library, I thought I’d missed some obscure syntax. Searching online provided no clues as to what it did. I read through the list of Javascript expressions and operators and only found the familiar ! logical not operator.
Finally, it hit me. !! is not a single operator but ! applied twice. But what’s the point? Doesn’t it just return the same boolean value? The answer is yes for most programming languages, but in Javascript, it is a useful shorthand for converting truthy/falsy values into boolean true/false values. Regardless of the operand, ! always returns a boolean value. You can use !! instead of a longer if statement or ternary operator to make the conversion. Below are some examples of this in action.
> !!undefined
false
> !!null
false
> !!''
false
> !!0
false
> !!'hello world'
true
> !!25
true
> !!-25
true
> !![]
true
> !!{}
true
Instead of var someValue = someValue ? true : false;, you write var someValue = !!someValue;. Only in Javascript!