How does the Boolean return value in this JavaScript work?

Question

I'm pretty new to programming and have been working hard to teach myself. I came across the following problem online and tried to solve it, but hit a dead end:

"Write a function that accepts three arguments and returns true if only one of those arguments is truthy, and false if not. Do not use && or || operators or if statements."

This stumped me for the past two days, so I moved on to the solution, which I'm having trouble figuring out:

function onlyOne(x, y, z) {
    return (!!x + !!y + !!z === 1);
}

I understand the syntax, but I don't understand the logic or why this works. Would someone be able to help me out? I want to learn why the code works, and not just memorize syntax.

The addition operator treats the boolean values as integers, 0 for false, 1 for true. !! is a way to convert just about any value to its boolean equivalent — Phil
– Phil, Commented Sep 28, 2017 at 0:13
FYI, this sort of boolean comparison is called an "exclusive OR" or XOR if you're interested in doing some more research. — Phil
– Phil, Commented Sep 28, 2017 at 0:19
It might be worth pointing out that !! isn't a special operator but just two repeated !. The first one converts the argument to a Boolean and inverts the result, and the second one inverts it again to arrive back at the original object's truthiness value. — ApproachingDarknessFish
– ApproachingDarknessFish, Commented Nov 14, 2017 at 19:46

Alexander O'Mara · Accepted Answer · 2017-09-28 00:17:35Z

4

!! casts the value to an actual boolean. When you perform addition on boolean values, JavaScript coerces them into number, false being 0 and true being 1.

Basically, (!!x + !!y + !!z === 1) casts x, y, and z to true or false, then adds them all together. If only 1 of them is truthy, the value will be 1, and === 1 will be true, else it will be false.

Here is an example showing how addition with booleans works:

console.log(false + false); // 0
console.log(false + true);  // 1
console.log(true + true);   // 2

answered Sep 28, 2017 at 0:17

Alexander O'Mara

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Derek 朕會功夫 · Accepted Answer · 2017-09-28 00:19:03Z

1

!! "casts" your variable into a boolean which can only has the value 0 or 1 when casted back into a number via the + operator. If you think about it, you can only get 1 after adding them together if only one variable is true.

edited Sep 28, 2017 at 0:19

answered Sep 28, 2017 at 0:13

Derek 朕會功夫

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1 Comment

Phil Over a year ago

"which can only has the value 0 or 1" <- this isn't quite accurate. The boolean primitives true and false are type-cast to integers 1 and 0 by using them in a mathematical expression

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