Map

:::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 value in the current MutableMap

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

Every value in the current Map is affected by a call to map(), unlike filter() where only values that meet a certain criteria are affected.

The keys will remain unchanged from the current MutableMap to the returned MutableMap.

Guide

• Examples

Parameters

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

Returns

• MutableMap<Tk,Tu> - a MutableMap containing key/value pairs after a user-specified operation is applied.