A simple demo of Koa in TypeScript

koa_ts.ts

import Session from "koa-session";
import Router from "koa-router";
import Koa from "koa";

interface IContext {
  dummy: string;
}

interface JContext {
  dummy: number;
}

type RContext = Koa.ParameterizedContext<
  Koa.DefaultState,
  Router.IRouterParamContext<Koa.DefaultState, Koa.Context> 
  & Koa.Context
  & IContext | JContext
>;

// Must pass the generic
const app = new Koa<Koa.DefaultState, RContext>();
// Must pass the generic
const router = new Router<Koa.DefaultState, RContext>();

// Must have type
function yumy(ctx: RContext, next: Koa.Next) {
  ctx.dummy = "dummy 1";
  ctx.dummy = 123;
  return next();
}

app.use(yumy);

// No need type
app.use((ctx, next: Koa.Next) => {
  console.log(ctx.dummy);
  ctx.dummy = "dummy 2";
  ctx.dummy = 456;
  return next();
});

// Must have type
function ctrl(ctx: RContext) {
  ctx.body = ctx.dummy;
}

router.get("/dummy/:id", ctrl);

app.use(router.routes());
app.listen(4000, () => {
  console.log('4000...');
});