[Home](./index.md) > [kibana-plugin-plugins-expressions-public](./kibana-plugin-plugins-expressions-public.md) > [ExpressionFunctionDefinition](./kibana-plugin-plugins-expressions-public.expressionfunctiondefinition.md) > [fn](./kibana-plugin-plugins-expressions-public.expressionfunctiondefinition.fn.md) ## ExpressionFunctionDefinition.fn() method The actual implementation of the function. Signature: ```typescript fn(input: Input, args: Arguments, context: Context): Output; ``` ## Parameters | Parameter | Type | Description | | --- | --- | --- | | input | Input | | | args | Arguments | | | context | Context | | Returns: `Output`