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map

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 each value is the result of calling the given function on the original value

namespace HH\Lib\Keyset;

function map<Tv1, Tv2 as arraykey>(
  Traversable<Tv1> $traversable,
  (function(Tv1): Tv2) $value_func,
): keyset<Tv2>;

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

Parameters​

Returns​

  • keyset<Tv2>

Examples​

$result = Keyset\map(keyset[1,2,3,4], $val ==> $val*2);
print_r($result);
//result: keyset[2,4,6,8]

$result = Keyset\map(keyset[1,2,3,4], $val ==> $val);
print_r($result);
//result: keyset[1,2,3,4]