Returns a MutableMap after an operation has been applied to each key and value in the current MutableMap

public function mapWithKey<Tu>(
  (function(Tk, Tv): Tu) $fn,
): MutableMap<Tk, Tu>;

Every key and value in the current MutableMap is affected by a call to mapWithKey(), unlike filterWithKey() where only values that meet a certain criteria are affected.

The keys will remain unchanged from this MutableMap to the returned MutableMap. The keys are only used to help in the mapping operation.


  • (function(Tk, Tv): Tu) $fn - The callback containing the operation to apply to the current MutableMap keys and values.