Map Keys

:::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 new dict where each key is the result of calling the given function on the original key

namespace HH\Lib\Dict;

function map_keys<Tk1, Tk2 as arraykey, Tv>(
  KeyedTraversable<Tk1, Tv> $traversable,
  (function(Tk1): Tk2) $key_func,
): dict<Tk2, Tv>;

In the case of duplicate keys, later values will overwrite the previous ones.

Time complexity: O(n * f), where f is the complexity of $key_func Space complexity: O(n)

Parameters

• KeyedTraversable<Tk1,Tv> $traversable
• (function(Tk1): Tk2) $key_func

Returns

• dict<Tk2, Tv>