Promise入门/面试必看

//(1)创建一个新的Promise对象
    const p = new Promise((resolve,reject)=>{//执行器函数  同步回调
    //(2)执行异步操作任务
    setTimeout(()=>{
      const time = Date.now()//获取当前时间
      if(time %2 ==0){
        //偶数代表成功
        resolve("成功了，time = "+ time)
      }else{
        //否则失败
        reject("失败了，time = "+ time)
      }
    },1000);
    })
    p.then(
    value =>{//接收成功的数据
      console.log("成功的回调：" + value)
    },
    reason =>{//接受失败的数据
      console.log("失败的回调：" + reason)
      }
    )

const p1 = new Promise((resolve,reject)=>{
      resolve(1)
    })
const p2 = Promise.resolve(2)
const p4 = Promise.reject(4)
const pAll = Promise.all([p1,p2,p3,p4])
const pAll2 = Promise.all([p1,p2])
pAll.then(
    values => {
        console.log('all onResolved()',values)
    },
    reason => {
        console.log('all onrejected()',reason)
    }
)
pAll2.then(
    values => {
        console.log('all2 onResolved()',values)
    },
    reason => {
        console.log('all2 onrejected()',reason)
    }
)
//运行结果
//all onReasoned() 3 返回第一个失败的reason
//all2 onrejected() [1, 2] 全部成功时返回成功的value集合

const pRace = Promise.race([p1,p2,p3])
const pRace2 = Promise.race([p3,p1,p2])
pRace.then(
    value => {
        console.log('race onResolved()',value)
    },
    reason => {
        console.log('race onrejected()',reason)
    }
)
pRace2.then(
    value => {
        console.log('race2 onResolved()',value)
    },
    reason => {
        console.log('race2 onrejected()',reason)
    }
)
//运行结果
//返回第一个执行的结果，根据实际执行时间
//race onResolved() 1  
//race2 onrejected() 3