递归执行模型

function fn1 () {
  console.log(111)
  fn2()
  console.log('fn1')
}
function fn2 () {
  console.log(222)
  fn3()
  console.log('fn2')
}
function fn3 () {
  console.log(333)
  fn4()
  console.log('fn3')
}
function fn4 () {
  console.log(444)
  console.log('fn4')
}
fn1()