for-in-loop.js

var obj = { a: 1, b: 2 }; // a, b are both enumerables properties

// setting {c: 3} as the prototype of 'obj',
// and as we know for-in loop also iterates over the properties obj inherits
// from its prototype, 'c' will also be visited.
Object.setPrototypeOf(obj, { c: 3 });

// we are defining one more property 'd' into our 'obj',
// but we are setting the 'enumerable' to false. It means 'd' will be ignored.
Object.defineProperty(obj, "d", { value: 4, enumerable: false });

for (let prop in obj) {
  console.log(prop);
}
// it will print
// a
// b
// c