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 keyset where the value is the result of calling the given async function on the original values in the given traversable

namespace HH\Lib\Keyset;

function map_async<Tv, Tk as arraykey>(
  Traversable<Tv> $traversable,
  (function(Tv): Awaitable<Tk>) $async_func,
): Awaitable<keyset<Tk>>;

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

• Traversable<Tv>$traversable
• (function(Tv): Awaitable<Tk>) $async_func

Returns

• Awaitable<keyset<Tk>>