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)
????????????}
????????}

`

promise 的基本使用

原文：https://www.cnblogs.com/duanyiwen/p/13281584.html