ES9中Promise是如何“兜底”操作的？

function Gen(time){
    return new Promise((resolve,reject)=>{
        setTimeout(function(){
            if(time<500){
                reject(time);
            }else{
                resolve(time);
            }
        },time)
    })
}

Gen(Math.random()*1000)
.then(val=>console.log('resolve',val))
.catch(err=>console.log('reject',err))
.finally(()=>{console.log('finish')})//关闭数据库，保证数据库是断开的

//resolve 564.8225885780598
//finish

//多刷几次，输出会变
//reject 94.27830818104587
//finish