hphp/hack/manual/apis/Interfaces/ConstMap/mapWithKey.md
:::info[Note] This is a point-in-time snapshot of the API documentation from January 2026. Going forward, we will not be maintaining a public copy of these references, and recommend users to refer to the built-in signature helpers available in the Hack LSP instead for complete and up-to-date information. :::
Returns a ConstMap after an operation has been applied to each key and
value in the current ConstMap
public function mapWithKey<Tu>(
(function(Tk, Tv): Tu) $fn,
): ConstMap<Tk, Tu>;
Every key and value in the current ConstMap 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 ConstMap to the returned
ConstMap. 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
ConstMap keys and values.ConstMap<Tk,Tu> - a ConstMap containing the values after a user-specified
operation on the current ConstMap's keys and values is applied.