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firstKey

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 MutableSet

public function firstKey(): ?arraykey;

Since sets do not have keys, it returns the first value.

This method is interchangeable with firstValue().

Returns​