JavaScript !! Operator – What is the !! (Not Not) Operator?

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.

Any falsy value will satisfy the if(!insert_variable_here) condition, including:

• false
• null
• undefined
• The empty string ''
• The number 0
• NaN

If callback return evaluates any of those values, the condition will be satisfied.

Even though null != false, the following will give you an alert:

x = null;
if(!x) {
    alert('"!null" does evaluate to true');
}

Regardless of whether or not null != false makes sense to you or anyone else, the point is that in JavaScript null is a falsy value, and thus a value that would satisfy the condition in my first bit of code listed above. This, it seems, is the question you have asked--not, rather, if null should or should not == false.