promise

狀態 pending 變成 resolved success 、 rejected  fail

只有這兩種狀態 每個promise對象只能改變一次 成功的結果一般稱為 value  失敗稱為reason

promise 的基本使用

`

1.創建promise函數

  let p = new Promise((resolve, reject) => {
            //   2.執行異步操作的任務
            setTimeout(() => {
                const time = Date.now()
                if (time % 2 == 0) {
                    resolve('成功了')
                } else {
                    reject('失敗了')
                }
            }, 500);
            //3.1   如果成功  resolve(value)
            //3.2   如果失敗 執行 reject(reason)
        })
        p.then(value => {
            //成功得到value onResolved
            console.log('success', value)
        }, reason => {
            //失敗得到reason onRejected
            console.log('fail', reason)
        })

// 回調地獄 回調函數的嵌套調用 外部回調函數異步執行的結果是嵌套的回調函數執行的條件

   dosomething(function(result){
            dosomethingElse(result,function(newResult){
                doThirdThing(newResult,function(finalResult){
                    console.log('go to third'+finalResult)
                },failCallback)
            },failCallback)
        },failCallback)

**  promise 指定回調函數的方式更加靈活,支持鏈式調用，可以解決回調地獄問題  回調函數的方法必須在在使用前就要調用 不方便處理錯誤 promise 則是異步調用 **

        //解決方法  promise 的鏈式調用  
        dosomething().then((result)=>{
          return dosomethingElse(result)
        }).then((newResult)=>{
            return doThirdThing(newResult)
        }).then((finalResult)=>{
            console.log('go to third'+finalResult)
        }).catch(failCallback)
        //終極解決方法 async await 
        async function request(){
            try{
                const result=await dosomething(result);
                const newResult=await dosomethingElse(newResult);
                const finalResult=await doThirdThing(newResult);
            console.log('go to third'+finalResult)
                
            }catch(e){
                failCallback(e)
            }
        }

終極解決方法 async await

   async function request(){
            try{
                const result=await dosomething(result);
                const newResult=await dosomethingElse(newResult);
                const finalResult=await doThirdThing(newResult);
            console.log('go to third'+finalResult)
                
            }catch(e){
                failCallback(e)
            }
        }

`