First Keyx

:::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 throws if the KeyedTraversable is empty

namespace HH\Lib\C;

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

For possibly empty Traversables, see C\first_key.

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"];
$first_keyx_result_1 = C\first_keyx($dict);
echo "First first keyx result: $first_keyx_result_1\n";
//Output: First first keyx result: key_1

$empty_dict = dict[];
$first_keyx_result_2 = C\first_keyx($empty_dict);
//Output: Hit a php exception : exception 'InvariantViolationException' with message 
//'HH\Lib\C\first_keyx: Expected at least one element.'