const app = {
  middleware: [],

  callback (ctx) {
    console.log(ctx)
  },

  use (fn) {
    this.middleware.push(fn)
  },

  go (ctx) {
    this.middleware.reduceRight((next, fn, i) => {
      return () => {
        console.log('next: ', next)
        console.log('fn: ', fn)
        fn(ctx, next)
      }
    }, this.callback.bind(this, ctx))()
  }
}

app.use((ctx, next) => {
  ctx.name = 'Lucy'
  next()
})

app.use((ctx, next) => {
  ctx.age = 12
  next()
})

app.use((ctx, next) => {
  console.log(`${ctx.name} is ${ctx.age} years old.`)
  next()
})

app.go({})

上面代碼的執(zhí)行結(jié)果是

next:  () => {
  console.log('next: ', next)
  console.log('fn: ', fn)
  fn(ctx, next)
}
fn:  (ctx, next) => {
  ctx.name = 'Lucy'
  next()
}
next:  () => {
  console.log('next: ', next)
  console.log('fn: ', fn)
  fn(ctx, next)
}
fn:  (ctx, next) => {
  ctx.age = 12
  next()
}
next:  ? callback(ctx) {
  console.log(ctx)
}
fn:  (ctx, next) => {
  console.log(`${ctx.name} is ${ctx.age} years old.`)
  next()
}
Lucy is 12 years old.
{name: "Lucy", age: 12}

有沒有大佬能解釋一下執(zhí)行結(jié)果為啥是這個