Objects as arguments

const origNum = 8;
const origObj = {color: 'blue'};
const changeItUp = (num, obj) => {
  num = 7;
  obj.color = 'red';
};
changeItUp(origNum, origObj);
// Will output 8 since integers are passed by value.
console.log(origNum);
// Will output 'red' since objects are passed 
// by reference and are therefore mutable.
console.log(origObj.color);