LastKey

:::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 last key in the current Vector

public function lastKey(): ?int;

Returns

• ?int - The last key (an integer) in the current Vector, or null if the Vector is empty.

Examples

This example shows how lastKey() can be used even when a Vector may be empty:

function echoLastKey(Vector<string> $v): void {
  $last_key = $v->lastKey();
  if ($last_key !== null) {
    echo 'Last key: '.$last_key."\n";
  } else {
    echo 'No last key (Vector is empty)'."\n";
  }
}

<<__EntryPoint>>
function basic_usage_main(): void {
  // Will print "Last key: 3"
  echoLastKey(Vector {'red', 'green', 'blue', 'yellow'});

  // Will print "No last key (Vector is empty)"
  echoLastKey(Vector {});
}