First Key

:::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 the first key of the given KeyedTraversable, or null if the KeyedTraversable is empty

namespace HH\Lib\C;

function first_key<Tk, Tv>(
  KeyedTraversable<Tk, Tv> $traversable,
): ?Tk;

For non-empty Traversables, see C\first_keyx.

Time complexity: O(1) Space complexity: O(1)

Parameters

• KeyedTraversable<Tk,Tv> $traversable

Returns

• ?Tk

Examples

$dict = dict["key_1" => "a", "key_2" => "b", "key_3" => "c"];
$first_key_result_1 = C\first_key($dict);
echo "First first key result: $first_key_result_1\n";
//Output: First first key result: key_1

$empty_dict = dict[];
$first_key_result_2 = C\first_key($empty_dict);
$first_key_result_2_as_string = $first_key_result_2 ?? "null";
echo "Second first key result: $first_key_result_2_as_string\n";
//Output: Second first key result: null