I think it's worth writing about motivation and possible use cases.
I think this sentence is very suitable for the case when you need to compare the signs of two numbers in proper way:

const sameSigns = Math.signbit(x) === Math.signbit(y);
const diffSigns = Math.signbit(x) !== Math.signbit(y);

Why Math.sign is not works in this case?

1. if x == +0 and y == -0 Math.sign(x) === Math.sign(y) will return true. See why this important.
2. if one of variables x or y is NaN Math.sign(x) === Math.sign(y) will always return false.

When we need compare signs? It's pretty often required in computational geometry:
https://github.com/search?q=SAME_SIGNS&type=Code