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 MutableMap

public function firstKey(): ?Tk;

Returns

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