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
    }
    ```