Object.is() polyfill - A small coding exercise to use JavaScript types and coercion to write my own Object.is().

object-is-polyfill-kata.js

// TODO: define polyfill for `Object.is(..)`
if (!Object.is || true) {
    Object.is = function(firstValue, secondValue) {

        // Take NaN into account
        if (typeof(firstValue) === 'number' && typeof(secondValue) === 'number') {
            if (firstValue.toString() === 'NaN' && secondValue.toString() === 'NaN') {
                return firstValue.toString() === 'NaN' && secondValue.toString() === 'NaN'
            }
        }

        // Take -0 into account - 1/-0 is -Infinity, 1/0 is Infinity.
        if (firstValue === 0 || secondValue === 0) {
            return (1 / firstValue) === (1 / secondValue);
        }

        // Otherwiser test using ===
        return firstValue === secondValue;
    }
}

// tests:
console.log(Object.is(42,42) === true);
console.log(Object.is("foo","foo") === true);
console.log(Object.is(false,false) === true);
console.log(Object.is(null,null) === true);
console.log(Object.is(undefined,undefined) === true);
console.log(Object.is(NaN,NaN) === true);
console.log(Object.is(-0,-0) === true);
console.log(Object.is(0,0) === true);

console.log(Object.is(-0,0) === false);
console.log(Object.is(0,-0) === false);
console.log(Object.is(0,NaN) === false);
console.log(Object.is(NaN,0) === false);
console.log(Object.is(42,"42") === false);
console.log(Object.is("42",42) === false);
console.log(Object.is("foo","bar") === false);
console.log(Object.is(false,true) === false);
console.log(Object.is(null,undefined) === false);
console.log(Object.is(undefined,null) === false);