myVar = !!someOtherVar [duplicate]

Answer 1

It's a double negation, but it also works for type casting. !somevar will return a boolean (true, if somevar is "truthy" and false if it is "falsey", as per Crockford's lectures). So, !!somevar will be not(bool) and hence it will be boolean.

Answer 2

(Rewritten to clarify, simplify)

That statement performs a couple different actions:

myVar = // This portion is a regular assignment, it will store the value of the suffix
        !!someOtherVar; // This portion is evaluated to a boolean result

The !!someOtherVar, I assume, is what you're really asking about. The answer is simple: it performs two logical NOT operations against the truthiness (a Javascript'ism) of someOtherVar.

In other words, if you understand the ! operator, this just combines two of them (!! isn't a different operator). By doing this it essentially returns the boolean evaluation of someOtherVar--in other words, it's a cast from whatever type someOtherVar is to boolean.

So... to walk through this, and pay attention to the result of myVar:

myVar = someOtherVar; // myVar will be whatever type someOtherVar is
myVar = !someOtherVar; // myVar will *always be boolean, but the inverse of someOtherVar's truthiness
myVar = !!someOtherVar; // myVar will *always be boolean, and be the equivalent of someOtherVar's truthiness

Answer 3

If you need to pass a boolean value to a function, or are anal about evaluating only booleans in conditional statements, that casts someOtherVar to a boolean for you by double-negating it.

Answer 4

In non-strictly typed languages, the ! operator converts a value to a boolean. Doing it twice would be equivalent to saying

myVar = (boolean)someOtherVar

Note that this is not recommended for code clarity.