Iterates over own and inherited enumerable properties of an object invoking iteratee for each property. The iteratee is invoked with three arguments:
(value, key, object). Iteratee functions may exit iteration early by explicitly returning false.

function Foo() {
  this.a = 1;
  this.b = 2;
}

Foo.prototype.c = 3;

_.forIn(new Foo, function(value, key) {
  console.log(key);
});
// => logs 'a', 'b', then 'c' (iteration order is not guaranteed)