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MapWithKey

hphp/hack/manual/apis/Classes/HH/ImmMap/mapWithKey.md

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:::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 an ImmMap after an operation has been applied to each key and value in current ImmMap

Hack
public function mapWithKey<Tu>(
  (function(Tk, Tv): Tu) $callback,
): ImmMap<Tk, Tu>;

Every key and value in the current ImmMap 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 the current ImmMap to the returned ImmMap. The keys are only used to help in the operation.

Parameters

  • (function(Tk, Tv): Tu) $callback

Returns

  • ImmMap<Tk,Tu> - an ImmMap containing the values after a user-specified operation on the current ImmMap's keys and values is applied.

Examples

See Map::mapWithKey for usage examples.

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