function sign(x) {
    if (typeof x === "number") {
        if (x < 0) {
            return -1;
        } else if (x === 0) {
            return 0;
        } else {
            return 1;
        }
    } else {
        throw new TypeError("not a number");
    }
}
[Sign](https://en.wikipedia.org/wiki/Sign_function) is a function
that extracts the sign of a real number

@param {number} x input value
@returns {number} sign value either 1, 0 or -1
@throws {TypeError} if the input argument x is not a number
@private

@example
sign(2); // => 1