async

async

1. async函数是对generator函数的改进
   • 内置执行器，generator函数的执行需要执行器
   • 更好的语义
   • 更广的适用性，await命令后面可以跟Promise(异步)和原始类型(同步)
   • 返回值是Promise而不是Iterator对象，方便后续操作
2. 错误处理
   • async函数内部未捕获的错误会使，async返回Promise的状态变为rejected
   • await命令后跟的Promise对象状态为rejected，会导致await命令报错，阻塞async函数后续执行
3. 异步迭代器next()方法返回Promise对象,Promise对象fulfilled态后返回{value,done}对象
   • async generator*生成异步遍历器
   • for await...of遍历异步遍历器
   • yield*展开异步遍历器
   • async iterable

      interface AsyncIterable {
          [Symbol.asyncIterator]() : AsyncIterator,
      }
     
      interface AsyncIterator {
          next(value?: any) : Promise,
      }
      interface IterationResult {
          value: any,
          done: boolean,
      }
     

tips

1. async函数内部是继发的执行，但是async函数外部不是

    // 按序请求并打印
    var urlArr = ['https://www.baidu.com','https://www.vip.com','https://www.qq.com']
    var func = async ()=>{
        for(let url of urlArr){
            let response = await fetch(url)
            await new Promise(res=>setTimeout(()=>res(),2000))
            let text = await response.text()
            console.log('===='+text.substr(0,100))
        }
    }
    func()
    // 并发请求，按序打印
    var func2 = async ()=>{
        for (let textPromise of urlArr.map(async url=>{
            let response = await fetch(url)
            await new Promise(res=>setTimeout(()=>res(),2000))
            return response.text()
        })){
            let text = await textPromise
            console.log('===='+text.substr(0,100))
        }
    }
    func2()