This API is part of the Hack Standard Library, not HHVM itself.

Returns a new dict where each value is the result of calling the given function on the original value

namespace HH\Lib\Dict;

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

To use an async function, see Dict\map_async().