對于`Promise`部分屬性的理解

琛ｈ。發(fā)布于2019-08-22 15:05 / 1520人閱讀

摘要：對于和的理解今天碰到問題是這樣子的調(diào)試的時(shí)候發(fā)現(xiàn)走了也走了有在我印象里是走了就不該走后來發(fā)現(xiàn)是我理解錯(cuò)了代碼是這樣的這個(gè)的是里的異常，里如果有任何異常都會(huì)被捕獲仔細(xì)看了是這樣解釋的簡單來講調(diào)用等于調(diào)用由于返回的是一個(gè)對象，返回值解釋如下如果

對于Promise.then()和Promise.catch()的理解

今天碰到問題是這樣子的:
調(diào)試bug的時(shí)候發(fā)現(xiàn)axios走了then也走了有catch,在我印象里是走了then就不該走catch(后來發(fā)現(xiàn)是我理解錯(cuò)了)
代碼是這樣的

this.axios.post("/user/login", params)
    .then(res => {
        console.log("response", res)
    })
    .catch(err => {
      // 這個(gè)catch catch的是then里的異常，then里如果有任何異常都會(huì)被catch捕獲
        console.log("catch")
        console.error(err.message)
    })

仔細(xì)看了Promise.catch()MCDN是這樣解釋的

The catch() method returns a Promise and deals with rejected cases only. It behaves the same as calling Promise.prototype.then(undefined, onRejected) (in fact, calling obj.catch(onRejected) internally calls obj.then(undefined, onRejected)).

簡單來講調(diào)用Promise.catch()等于調(diào)用Promise.prototype.then(undefined, onRejected)
由于Promise.then()返回的是一個(gè)Promise對象，返回值解釋如下：

throws an error, the promise returned by then gets rejected with the thrown error as its value;

如果拋出異常返回一個(gè)執(zhí)行rejected的Promise對象即相當(dāng)于調(diào)用返回Promise的Promise.then(undefined, onRejected)

對于Promise.catch()的返回值是這樣解釋的：

The Promise returned by catch() is rejected if onRejected throws an error or returns a Promise which is itself rejected; otherwise, it is resolved.

如果Promise.catch()又拋出異常則相當(dāng)于又調(diào)用Promise.then(undefined, onRejected)
如果未拋出異常則相當(dāng)于調(diào)用Promise.then(onResolved,undefined)

promise.all()是順序開始,異步執(zhí)行,順序返回

// 示例代碼
const getRandom = () => +(Math.random()*1000).toFixed(0);

const asyncTask = taskID => new Promise(resolve => {
    let timeout = getRandom();
    console.log(`taskID=${taskID} start.`);
    setTimeout(function() {
        console.log(`taskID=${taskID} finished in time=${timeout}.`);
        resolve(taskID)
    }, timeout);
});

Promise.all([asyncTask(1),asyncTask(2),asyncTask(3)])
.then(resultList => {
    console.log("results:",resultList);
});

詳見這里