Exercise For Creating a Type Check Polyfill

object-is.js

// TODO: define polyfill for `Object.is(..)`
if(!Object.is || true) {
  Object.is = function ObjectIs(x,y) {
    let xNegZero = isItNegZero(x);
    let yNegZero = isItNegZero(y);
    
    if(xNegZero || yNegZero) {
      return xNegZero && yNegZero;
    }
    else if(isItNaN(x) && isItNaN(y)){
      return true;
    }
    else {
     return x === y; 
    }
    
    function isItNegZero(v) {
     return v == 0 && (1/v) == -Infinity; 
    }
    
    function isItNaN(v) {
      //NaN is the only value that not equal to itself in JavaScript
      return v !== v;
    }
  }
}


// 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);