Using 'this' in Hack Language Facebook

Using this in Hack Language is shown below:

As we see in the example below that 'this' can also be used as return type of a function and also to set the values of member variables of a class.
Example:

<?hh
class Base {
  private int $x = 0;
  public function setValue(int $x):this {
   $this->x = $x;
    // $this has type "this"
  return $this;
}