Understanding how Koa cascading works

In a nutshell, the cascading in Koa works by invoking middleware downstream sequentially and then controlling them to flow back upstream sequentially. It is best to create a simple Koa app to demonstrate this great feature in Koa:

1. Create an index.js file in the /src/ directory, just like we did in the previous section:

// src/index.js
const Koa = require('koa')
const app = new Koa()

app.use(async ctx => {
  console.log('Hello World')
  ctx.body = 'Hello World'
})
app.listen(3000)

2. Create three pieces of middleware just before the Hello World middleware as follows, so that we can run them first:

app.use(async (ctx, next) => {
  console.log('Time started at: ', Date.now())
  await next()
})

app.use(async (ctx, next) => {
  console.log('I am the first')
  await next()
  console.log('I am the last')
})

app.use(async (ctx, next) => {
  console.log('I am the second')
  await...