Why ~ operator returns -1 for a function in Javascript?

Question

~(function () {}).toString(); is absolutely valid JavaScript syntax and I saw that it returns -1.

I know that ~ is not operator. For instance ~5=~0101 which means 1010 in base 2 and 10 in decimal.

console.log(~(function () {}).toString());

But what is the explanation in this situation ?

Maybe ~NaN returns -1.

Explanation: https://www.joezimjs.com/javascript/great-mystery-of-the-tilde/ — Praveen Kumar Purushothaman, Oct 18 '17 at 13:44
This the same if you'll try to do this with `NaN` like so `~(NaN)` it will give the same result. — Krusader, Oct 18 '17 at 13:45

score 2 · Accepted Answer · answered Oct 18 '17 at 13:49

2

As per spec

Let oldValue be ToInt32(GetValue(expr)).

Number((function () {}).toString();) -> Number("function () {}") -> NaN

Again as per spec

If number is NaN, +0, −0, +∞, or −∞, return +0.

so ~NaN amounts to ~0 which is -1

answered Oct 18 '17 at 13:49

gurvinder372

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score 1 · Answer 2 · answered Oct 18 '17 at 13:45

Taken from this blog: The Great Mystery of the Tilde(~):

The tilde is an operator that does something that you’d normally think wouldn’t have any purpose. It is a unary operator that takes the expression to its right performs this small algorithm on it (where N is the expression to the right of the tilde): -(N+1). See below for some samples.

console.log(~-2); //  1
console.log(~-1); //  0
console.log(~0);  // -1
console.log(~1);  // -2
console.log(~2);  // -3

Why ~ operator returns -1 for a function in Javascript?

2 Answers2