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MapWithKey

hphp/hack/manual/apis/Interfaces/MutableMap/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 a MutableMap after an operation has been applied to each key and value in the current MutableMap

Hack
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.

Parameters

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

Returns

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