Map Async

:::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 async function on the original value

namespace HH\Lib\Dict;

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

For non-async functions, see Dict\map().

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

The IO operations for each of calls to $async_func will happen in parallel.

Parameters

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

Returns

• Awaitable<dict<Tk,Tv2>>