异步async

async也是处理异步的，是promise的扩展

语法：

let p1 = new Promise((resolve,reject) => {
    setTimeout(()=>{
        resolve('1.异步')
    },3000)
})

let p2 = new Promise((resolve,reject) => {
    setTimeout(()=>{
        resolve('2.异步')
    },800)
})

let p3 = new Promise((resolve,reject) => {
    setTimeout(()=>{
        resolve('3.异步')
    },1500)
})
async function as()
{
    let result = await p2  // 等待p2执行
    console.log(result)
}
as() // 输出 2.异步

let as = async()=>{
    let result = [await p1, await p2, await p3]
    console.log(result)
}
as() // 输出 [ '1.异步', '2.异步', '3.异步' ]

注意： 返回的是一个promise对象