When to use the double not (!!) operator in JavaScript

I understand what the double not operator does in JavaScript. I'm curious about it's use though and whether or not a recent assertion that I made is correct.

I said that if (!!someVar) is never meaningful nor is (!!someVar && ... because both the if and the && will cause someVar to be evaluated as a boolean so the !! is superfluous.

In fact, the only time that I could think of that it would be legitimate to use the double not operator is if you wanted to do a strict comparison to another boolean value (so maybe in return value that expects true or false explicitly).

Is this correct? I started to doubt myself when I noticed jQuery 1.3.2 used both if (!!someVar) and return !!someVar && ...

Does the double not have any actual effect in these situations?

My personal opinion is that it just leads to confusion. If I see an if statement, I know it's evaluating it as a boolean.

Answers:

Answer

In the context of if statements I'm with you, it is completely safe because internally, the ToBoolean operation will be executed on the condition expression (see Step 3 on the spec).

But if you want to, lets say, return a boolean value from a function, you should ensure that the result will be actually boolean, for example:

function isFoo () {
  return 0 && true;
}

console.log(isFoo()); // will show zero
typeof isFoo() == "number";

In conclusion, the Boolean Logical Operators can return an operand, and not a Boolean result necessarily:

The Logical AND operator (&&), will return the value of the second operand if the first is truly:

true && "foo"; // "foo"

And it will return the value of the first operand if it is by itself falsy:

NaN && "anything"; // NaN
0 && "anything"; // 0

On the other hand, the Logical OR operator (||) will return the value of the second operand, if the first one is falsy:

false || "bar"; // "bar"

And it will return the value of the first operand if it is by itself non-falsy:

"foo" || "anything"; // "foo"

Maybe it's worth mentioning that the falsy values are: null, undefined, NaN, 0, zero-length string, and of course false.

Anything else (that is not falsy, a Boolean object or a Boolean value), evaluated in boolean context, will return true.

Answer

Yes, !!var is used when you want 0||1 return value.

One is simple comparison of bool values, when you want "a == b" be equivalent of "a xor not b" except a=5 and b=7 would both be true but not be equal.

Another is when you want to coerce a set of conditions into bits of a variable:

 var BIT_NONEMPTY=1;
 var BIT_HASERRORS=2;
 var BIT_HASCHILDREN=4;
 var BIT_HASCONTENT=8;

 result_bitfields = 
    (!!countLines())*BIT_NOTEMPTY +
    (!!errorCode())*BIT_HASERRORS +
    (!!firstChild())*BIT_HASCHILDREN +
    (!!getContent())*BIT_HASCONTENT;

Not very useful in Javascript which lives pretty far from bit values, but may be useful at times.

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