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max_by

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 the largest element of the given Traversable, or null if the Traversable is empty

namespace HH\Lib\Math;

function max_by<T>(
  Traversable<T> $traversable,
  (function(T): num) $num_func,
): ?T;

The value for comparison is determined by the given function. In the case of duplicate numeric keys, later values overwrite previous ones.

For numeric elements, see Math\max().

Parameters​

Returns​

  • ?T

Examples​

$items = vec[tuple('foo', 9), tuple('bar', 8), tuple('baz', 7)];
$maxItem = Math\max_by($items, $item ==> $item[1]);
echo "The max item is " . $maxItem[0];

Ouput

The max item is foo