Returns a ConstMap containing the values of the current ConstMap that meet a supplied condition applied to its keys and values

public function filterWithKey(
  (function(Tk, Tv): bool) $fn,
): ConstMap<Tk, Tv>;

Only keys and values that meet a certain criteria are affected by a call to filterWithKey(), while all values are affected by a call to mapWithKey().

The keys associated with the current ConstMap remain unchanged in the returned ConstMap; the keys will be used in the filtering process only.


  • (function(Tk, Tv): bool) $fn - The callback containing the condition to apply to the current ConstMap keys and values.


  • ConstMap<Tk,Tv> - a ConstMap containing the values after a user-specified condition is applied to the keys and values of the current ConstMap.