Koa: Passing data from middleware to route handler

router-1.ts

/*
 * Description:
 *
 * There are times when we want to re-use some data created/derived in a middleware later in a route handler.
 *
 * The default Koa way of doing so would be using `context.state`. Unfortunately `context.state` is hard to type and
 * quite implicit, since it's an object shared and potentially mutated by all middlewares and the route handler.
 */

/*
 * Option 1: Default approach
 */

import createError from "http-errors"
import JWT from "jsonwebtoken"
import Koa from "koa"
import Router from "koa-router"
import logger from "./logger"
import { findProductsByUserId } from "./models/product"

function authenticate() {
  return (context: Koa.Context, next: (ctx: Koa.Context) => Promise<any>) => {
    try {
      const user = JWT.verify(context.request.getHeader("Authorization"), "secret")

      // Put user data into context state
      context.state.user = user
    } catch (error) {
      logger.error(error)
      throw createError(401)
    }

    await next(context)
  }
}

const app = new Koa()
const router = new Router()

router.get("/products", authenticate(), async ({ response, state }) => {
  response.body = await findProductsByUserId(state.user.id)
})

app.use(router.routes())
app.use(router.allowedMethods())

router-2.ts

/*
 * Option 2: Don't re-use, parse auth token a 2nd time
 * 
 */

import createError from "http-errors"
import JWT from "jsonwebtoken"
import Koa from "koa"
import Router from "koa-router"
import logger from "./logger"
import { findProductsByUserId } from "./models/product"

function authenticate() {
  return (context: Koa.Context, next: (ctx: Koa.Context) => Promise<any>) => {
    try {
      JWT.verify(context.request.getHeader("Authorization"), "secret")
      // Do not even put user data into context state
    } catch (error) {
      logger.error(error)
      throw createError(401)
    }

    await next(context)
  }
}
    
function getUserFromRequest (request: Koa.Request) {
  try {
    const user = JWT.verify(request.getHeader("Authorization"), "secret")
    return user
  } catch (error) {
    logger.error(error)
    throw createError(401)
  }
}

const app = new Koa()
const router = new Router()

router.get("/products", authenticate(), async ({ request, response }) => {
  // Parse JWT again
  const user = getUserFromRequest(request)
  response.body = await findProductsByUserId(user.id)
})

app.use(router.routes())
app.use(router.allowedMethods())