Boolean operators in if statement not working

Question

Could someone explain what's happening here? I'm trying to use the javascript !! (double-bang) operator as described here in conjunction with HTML5 local storage (I'm storing 0 and 1 values and testing for truthiness, but I also need a missing key to be false, hence the undefined at the start.

Although it echos to the console as false when typecast, it doesn't in the 'if' statement.

var foo = undefined;

// outputs undefined
console.log(foo)

// typecast to non-inverted boolean 
console.log(!!foo);

if (!!foo) {
    console.log("If was false before, why won't this execute?");
} else {
    console.log("It didn't work");
}

Produces:

undefined
false
It didn't work

http://jsfiddle.net/YAAA7/
(Chrome v 23.0.1271.97 & Firefox 16.0.1, OS X 10.8.2)

Edit - corrected code:

(The previous 'if' statement was just evaluating as false, so that branch would never run.)

var foo = false;

// outputs undefined
console.log(foo)

// typecast to non-inverted boolean 
console.log(!!foo);

if (!!foo == false) {
    console.log("Matches for foo undefined, foo = 0 and foo = false");
} else {
    console.log("Matches for foo = 1 and foo = true");
}

http://jsfiddle.net/YAAA7/1/

Well, `!!foo` is `false`, as you say, so the `else` branch will be taken. This is how it is supposed to work. — Frédéric Hamidi, Dec 17 '12 at 19:08
In one of the answer's comments, someone noted, correctly, that '!!' is not an operator--it's two '!' operators. — AlexMA, Dec 17 '12 at 19:10
It's the oposite of the oposite of what the value of the variable. — Elias Van Ootegem, Dec 17 '12 at 19:17
@AlexMA In that case would say it's better practice to write `Boolean(foo)` instead of `!!foo`? — William Turrell, Dec 17 '12 at 19:26
@williamt Yes, it's probably better practice if the code is likely to be edited by those who aren't familiar with the notation. But some programmers enjoy throwing in a stylistic flourish once in a while, especially if it reduces the number of characters. — AlexMA, Dec 18 '12 at 03:18

score 5 · Answer 1 · 2013-01-20T04:05:49.013

5

That is expected behavior. If you double not false, you get false. This ,!l is a not operator. This ,!!, is two not operators. The double not operator is sometimes used to cast to the Boolean type.

! false === true
!! false === false

edited Jan 20 '13 at 04:05

answered Dec 17 '12 at 19:08

score 1 · Accepted Answer · edited Dec 20 '12 at 22:13

var foo = undefined;

foo will return true. Because, in JavaScript "", undefined, 0, NaN, false and null are considered falsey values.

http://www.mapbender.org/JavaScript_pitfalls:_null,_false,_undefined,_NaN .

From your code:

var foo = undefined; //false

console.log(!!foo); // !!foo = false;

if(!!foo) { //false
  console.log("It will not come because the condition fails");
}else{
    console.log("Else Part");
 }

Boolean operators in if statement not working

2 Answers2