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.
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 .
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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