checkmail.js

// some helper fns.. maybe in a different file or not
function validateEmail(email){
	return false;
}

export default (db) => {
	// define some route handlers
    function onPostCheckMail (req, res) {
    	if(validateEmail(req.body.email) === true){ //validateEmail is undefined
	  		res.end("available");
	  		return;
	  	}
	  	res.end("taken");
    }
	
	// return them
	return { onPostCheckMail }
}

index.js

//index.ts
import createRegister from './checkmail';

const register = createRegister(db);

app.post('/checkemailp', register.onPostCheckMail);