fun (n) {
return new Promise((resolve, reject) => {
setTimeout(() => {
resolve(n * 2)
}, 2000)
})
},
step1(n) {
console.log(step1 with ${n});
return this.fun(n);
},

  step2(n) {
    console.log(`step2 with ${n}`);
    return this.fun(n);
  },
  async zhixing () {
    console.log(2222)
    let result = await this.step1(20)
    console.log(result)
    let result2 = await this.step2(result)
    console.log(result2)
  }