FirstKey

:::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 in the current MutableVector

public function firstKey(): ?int;

Returns

• ?int - The first key in the current MutableVector, or null if the current MutableVector is empty.