Promise 当做状态机使用

---

const fn = (num) =>{
	if(num > 2){
		return '数字大于2'
	}
	if(num === undefined){
		return '数字不存在'
	}
	return num
}

1. Promise 状态机 改写:
   1.

   ```jsx
   const fn = (num) =>{
   if(num > 2){
   return Promise.reject('数字大于2')
   }
   if(num === undefined){
   return Promise.reject('数字不存在')
   }
   return Promise.resolve(num)
   }

   //使用
   fn(num).then(res => {message.success(res)}).catch(err => message.error(err))
   ```

2. Promise Async 状态机 改写:
   1.

   jsx const fn = async(num) =>{ if(num > 2){ return Promise.reject('数字大于2') } if(num === undefined){ return Promise.reject('数字不存在') } return num }

3. Promise Async Throw 状态机 改写:
   1.

   jsx const fn = async(num) =>{ if(num > 2){ throw '数字大于2' } if(num === undefined){ throw '数字不存在' } return num }