Efficient isNegative(), isPositive(), sameSign() and copySign() functions.

copySign.js

function copySign(toUpdate, signedNum) {
    return toUpdate + 1 / toUpdate < 0 === signedNum + 1 / signedNum < 0
        ? toUpdate
        : -toUpdate;
}

isNegative.js

function isNegative(x, opt_includeZero) {
    return (opt_includeZero ? x + 1 / x : x) < 0;
}

isPositive.js

function isPositive(x, opt_includeZero) {
    return (opt_includeZero ? x + 1 / x : x) > 0;
}

sameSign.js

function sameSign() {
    for (
      // NOTE: Using var here for code optimization in Google Closure Compiler
      var isPositive, number, numbers = arguments, i = numbers.length;
      number = numbers[--i], i >= 0 && 'number' === typeof number && number === number;
    ) {
        let newIsPositive = number + 1 / number > 0;
        if (isPositive == null) isPositive = newIsPositive;
        else if (isPositive !== newIsPositive) return false;
    }
    return i < 0;
}

~example-run.js

// Test isNegative() and isPositive()
let numbers = [0, 0.123456789, 1, 123456789, Infinity];
numbers = ['A'].concat([...numbers].reverse().map(x => -x), numbers, ['Z']);
for (const [name, fn] of [['isNegative', isNegative], ['isPositive', isPositive]]) {
    for (const bool of [false, true]) {
        for (const x of numbers) {
            console.log(`${name}(${(x === 0 && 1/x < 0) ? '-0' : x}, ${bool}) => ${fn(x, bool)}`);
        }
    }
}

// Test sameSign()
for (const {args, result} of [
    {args: ['-2'], result: false},
    {args: ['2'], result: false},
    {args: [-2], result: true},
    {args: [2], result: true},
    {args: [0, 0], result: true},
    {args: [0, -0], result: false},
    {args: [-0, 0], result: false},
    {args: [-0, -0], result: true},
    {args: [0, 0, {}], result: false},
    {args: [{}, 0, 0], result: false},
    {args: [-0, -0, {}], result: false},
    {args: [{}, -0, -0], result: false},
]) {
    console.assert(sameSign(...args) === result, 'Failed for', args);
}

This Gist is being referenced by https://cwestblog.com/2023/06/28/github-gists-hack-pibb-extension/