nodejs之async异步编程

async/await 一般和 Prmoise 一起使用。下面的代码是一个生成 Promise 对象的工具方法。

function doTask({id, success=true,time=100}){
  return new Promise(function (resolve, reject) {
    setTimeout(function(){// 模拟异步
      var msg
      if(success) {
        msg = `thing ${id} is done`
        console.log(msg)
        resolve(msg);
      } else {
        msg = `thing ${id} failed`
        console.error(msg)
        reject(msg)
      }
    }, time)
  })
}

1.多个串行操作

async function doSerialTing() {
  await doTask({id:1})
  console.log('sth')
  await doTask({id:2, time: 1000})
  await doTask({id:3})
}

doSerialTing()
//thing 1 is done
//sth
//thing 2 is done
//thing 3 is done

2.多个并行操作

async function doParallelTing() {
  var [res1, res2, res3] = await Promise.all([
      doTask( {id:1}),
      doTask( {id:2, time: 1000 }), 
      doTask( {id:3})
  ])
  console.log(res1,res2,res3)
}

doParallelTing()

第一个例子: await只能用在async函数

• 错误示例1：程序不能运行

let funPromise = function (time) {
    return new Promise(function (resolve, reject) {
        //Pending 进行中
        setTimeout(function () {
            resolve(); // 从 pending 变为 resolved
        }, time);
    })
};

let funAsync = async function () {
    let numArr = [1003, 1002, 1001];
    // 会报错 await没有在async函数中
    numArr.forEach(function(value, index){
        await funPromise(value);
    })
}

funAsync();

• 错误示例2：程序能运行，结果不是我们想要的

let funPromise = function (time) {
    return new Promise(function (resolve, reject) {
        //Pending 进行中
        setTimeout(function () {
            // 从 pending 变为 resolved
            resolve(time + ' : ' + new Date()); 
        }, time);
    })
};

let funAsync = async function () {
    let numArr = [1003, 1002, 1001];
    numArr.forEach(async function(value, index){
        //三个funPromise()操作将是并发执行，也就是同时执行，而不是继发执行
        let result = await funPromise(value);
        console.log(result);
    })
}

funAsync();

• 正确示例

let funPromise = function (time) {
    return new Promise(function (resolve, reject) {
        //Pending 进行中
        setTimeout(function () {
            // 从 pending 变为 resolved
            resolve(time + ' : ' + new Date());  
        }, time);
    })
};

let funAsync = async function () {
    let numArr = [1003, 1002, 1001];
    // 三个funPromise()操作将是继发执行
    for (let value of number){
        let result = await funPromise(value);
        console.log(result);
    }
}

funAsync();