Map With Key

:::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 value is the result of calling the given function on the original value and key

namespace HH\Lib\Dict;

function map_with_key<Tk as arraykey, Tv1, Tv2>(
  KeyedTraversable<Tk, Tv1> $traversable,
  (function(Tk, Tv1): Tv2) $value_func,
): dict<Tk, Tv2>;

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

Parameters

• KeyedTraversable<Tk,Tv1> $traversable
• (function(Tk, Tv1): Tv2) $value_func

Returns

• dict<Tk, Tv2>