解決callback hell的新方法Promise

//方法一: 使用Promise的then方法 let fakeAjax = (url, sec) => new Promise((resolve,reject)=>{ setTimeout(()=>{ resolve(`Ajax ${url} OK!`) },sec); }); fakeAjax('api/1',5000).then(value=>{ console.log(value); return fakeAjax('api/2',3000); }).then(value=>{ console.log(value) return fakeAjax('api/3',1000); }).then(value=>{ console.log(value) })

//方法二: 使用async/await方法 let fakeAjax = (url, sec) => new Promise((resolve,reject)=>{ setTimeout(()=>{ resolve(`Ajax ${url} OK!`) },sec); }); const res1 = await fakeAjax('api/1',5000); console.log(res1); const res2 = await fakeAjax('api/2',3000); console.log(res2); const res3 = await fakeAjax('api/3',1000); console.log(res3);

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PS. 當Pomise的實例物件狀態改變(這裡指的是fakeAjax)，then方法裡的成功/失敗的回調函式才會被調用
狀態改變的方式有三種： 再Promise的回調函式裡調用了， 1. reslove() //[[PromiseState]]:fulfilled 2. reject() //[[PromiseState]]:fulfilled 3. throw '出現問題!' //[[PromiseState]]:rejected