對於`Promise`部分屬性的理解

對於Promise.then()和Promise.catch()的理解

今天碰到問題是這樣子的:
調試bug的時候發現axios走了then也走了有catch,在我印象裏是走了then就不應走catch(後來發現是我理解錯了)
代碼是這樣的

this.axios.post('/user/login', params)
    .then(res => {
        console.log('response', res)
    })
    .catch(err => {
      // 這個catch catch的是then裏的異常，then裏若是有任何異常都會被catch捕獲
        console.log('catch')
        console.error(err.message)
    })

仔細看了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)).

簡單來說調用Promise.catch()等於調用Promise.prototype.then(undefined, onRejected)
因爲Promise.then()返回的是一個Promise對象，返回值解釋以下：

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

若是拋出異常返回一個執行rejected的Promise對象即至關於調用返回Promise的Promise.then(undefined, onRejected)

對於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.

若是Promise.catch()又拋出異常則至關於又調用Promise.then(undefined, onRejected)
若是未拋出異常則至關於調用Promise.then(onResolved,undefined)

promise.all()是順序開始,異步執行,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);
});

詳見這裏