问题：如何让下面的代码执行输出 1 2 3

const app = () => {
  console.log(2)
}

const start = app
  .before(() => {
    console.log(1)
  })
  .after(() => {
    console.log(3)
  })

start()

答案：

Function.prototype.after = function (afterFn) {
  return (...args) => {
    this.apply(this, args)
    afterFn.apply(this, args)
  }
}

Function.prototype.before = function (beforeFn) {
  return (...args) => {
    beforeFn.apply(this, args)
    this.apply(this, args)
  }
}