In Typescript, how to use a function before it's declared?

Question

I want to declare a function, pass it as a handler, then define it afterward, my code is like this:

type authenticateHandler = (req: Restify.Request, res: Restify.Response) => any;
server.post("/authentication", authenticateHandler);
server.post("/v1/authentication", authenticateHandler);

const authenticateHandler: authenticateHandler = (req: Restify.Request, res: Restify.Response) => { ... };

I am getting this error when passing the function on line 2, what am I doing wrong?

TS2448: Block-scoped variable 'authenticateHandler' used before its declaration

Answer 1

Either declare your constant function before it's used (as the warning suggests).

type AuthenticateHandler = (req: Restify.Request, res: Restify.Response) => any;

const authenticateHandler: AuthenticateHandler = (req: Restify.Request, res: Restify.Response) => { ... };

server.post("/authentication", authenticateHandler);
server.post("/v1/authentication", authenticateHandler);

Or define it as a function:

function authenticateHandler(req: Restify.Request, res: Restify.Response) => { ... };