function forIn(object, iteratee, thisArg) {
      if (typeof iteratee != 'function' || typeof thisArg != 'undefined') {
        iteratee = baseCallback(iteratee, thisArg, 3);
      }
      return baseFor(object, iteratee, keysIn);
    }
Iterates over own and inherited enumerable properties of an object invoking
`iteratee` for each property. The `iteratee` is bound to `thisArg` and invoked
with three arguments; (value, key, object). Iterator functions may exit
iteration early by explicitly returning `false`.

@static
@memberOf _
@category Object
@param {Object} object The object to iterate over.
@param {Function} [iteratee=_.identity] The function invoked per iteration.
@param {*} [thisArg] The `this` binding of `iteratee`.
@returns {Object} Returns `object`.
@example

function Shape() {
  this.x = 0;
  this.y = 0;
}

Shape.prototype.z = 0;

_.forIn(new Shape, function(value, key) {
  console.log(key);
});
// => logs 'x', 'y', and 'z' (iteration order is not guaranteed)