Promise的使用场景

1.await && async 关键字等待请求

async function test(){
  var message = await new Promise((resolve,reject)=>{
      setTimeout(function(){
        resolve("{message:'hello'}")
      },3000)
  })
  .then(value=>{ return JSON.parse(value) })
  .then(value=>{
      return value
  })
  console.log(message)
}

2.分步请求

var message = new Promise((resolve,reject)=>{
     //正式请求前先请求验证
      setTimeout(function(){
         resolve('success')
      },3000)
}).then(value=>{
    if(value=='success'){//身份验证成功
        return new Promise((resolve,reject)=>{
            setTimeout(function(){
                resolve({username:'xx'})
            },3000)
        })
    }
})

message.then(res=>{
    console.log(res.username)
})