20180305_对于`Promise`部分属性的理解

[openks]

对于Promise.then()和Promise.catch()的理解

今天碰到问题是这样子的: 调试bug的时候发现axios走了then也走了有catch,在我印象里是走了then就不该走catch(后来发现是我理解错了) 代码是这样的

this.axios.post('/user/login', params)
    .then(res => {
        console.log('response', res)
    })
    .catch(err => {
      // 这个catch catch的是then里的异常，then里如果有任何异常都会被catch捕获
        console.log('catch')
        console.error(err.message)
    })

仔细看了Promise.catch()MCDN是这样解释的

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)).

简单来讲调用Promise.catch()等于调用Promise.prototype.then(undefined, onRejected) 由于Promise.then()返回的是一个Promise对象，返回值解释如下：

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

如果抛出异常返回一个执行rejected的Promise对象即相当于调用返回Promise的Promise.then(undefined, onRejected)

对于Promise.catch()的返回值是这样解释的：

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()又抛出异常则相当于又调用Promise.then(undefined, onRejected) 如果未抛出异常则相当于调用Promise.then(onResolved,undefined)

promise.all()是顺序开始,异步执行,顺序返回

// 示例代码
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);
});

详见这里