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optional-callback
=============================================

optionalCallback(fn, self) ⇒ function
-------------------------------------

Transform a Promise-returning function into a function that can
optionally take a callback as the last parameter instead.

| **Kind**: global function
| **Returns**: function - a function that can take callbacks as well.

+---------+------------+---------------------------------------------------+
| Param   | Type       | Description                                       |
+=========+============+===================================================+
| fn      | function   | a function that returns a Promise                 |
+---------+------------+---------------------------------------------------+
| self    | Object     | (optional) ``this`` to be used when applying fn   |
+---------+------------+---------------------------------------------------+