interface Input {
  inString: string;
  inNumber: number;
  inOptionalString: string | undefined;
}

interface Output {
  outString: string;
}

let mapperSchema: ObjectMapperSchema<Input, Output> = {
  outString: (input: Input) => input.inString,
};

mapperSchema = {
  outString: (input, _context: undefined | void) => input.inString,
};

mapperSchema = {
  outString: "inString", // This is okay, since `Input["inString"]` has the same type as `Output["outString"]`
};

// mapperSchema = {
//   outString: "inNumber", // This will error. `inNumber` has type `number`, so can't be used to populate `outString` which requires a `string`.
// };

// mapperSchema = {
//  outString: "inOptionalString", // This will error. `inOptionalString` can be `undefined`, which isn't compatible with `string` (when strict null checks are on).
// };

interface Context {
  capitalize: boolean;
}

const mapperSchemaWithContext: ObjectMapperSchema<Input, Output, Context> = {
  outString: (input, context) => context.capitalize ? input.inString.toUpperCase() : input.inString,
};