CODE EXAMPLE FOR JAVASCRIPT

Iterate Through the Keys of an Object with a for...in Statement

let users = {
  Alan: {
    age: 27,
    online: true
  },
  Jeff: {
    age: 32,
    online: true
  },
  Sarah: {
    age: 48,
    online: true
  },
  Ryan: {
    age: 19,
    online: true
  }
};

function isEveryoneHere(userObj) {
  return ["Alan", "Jeff", "Sarah", "Ryan"].every(name =>
    userObj.hasOwnProperty(name)
  );
}