20211014 - Promise 解决xml2js的返回值问题

因为Podcast的description只能在feedUrl里找到，无法直接获取，而feedUrl是xml文件，所以获取之后要先转成json格式。parseString import自xml2js的built-in方法。

修改前：直接接收parseString的返回值

const desc = parseString(data, (err, result) => {
      if (err) {
        return
      }

      // console.log(result)

      // const { rss } = result
      // const { channel } = rss
      // const { description } = channel[0]
      const { description } = result.rss.channel[0]
      // console.log(channel)
      console.log(description)  // 正常值

      return description
    })

输出的结果是一个正常值，但返回到json中却是一个又臭又长的对象。。。

修改后：创建一个Promise，Promise里面有一个函数（executor, which is called automatically），这个executor的参数resolve和reject是两个回调函数（由js本身提供）

const desc = await new Promise((resolve, reject) => parseString(data, (err, result) => {
      if (err) {
        reject(null)
      } else {
        // console.log(result)

        // const { rss } = result
        // const { channel } = rss
        // const { description } = channel[0]
        const { description } = result.rss.channel[0]
        // console.log(channel)
        console.log(description)

        resolve(description)
      }
    })).catch(error => {
      console.log(error)
    })  // 记得处理reject的情况！！！否则会报错！暂时的结论：await只有当resolved后才会返回given的result，如果是rejected的情况，会抛出错误，需要catch处理。

成功！

重点：

When the executor obtains the result, be it soon or late, doesn’t matter, it should call one of these callbacks:

• resolve(value) — if the job is finished successfully, with result value.
• reject(error) — if an error has occurred, error is the error object.

The promise object returned by the new Promise constructor has these internal properties:

• state — initially "pending", then changes to either "fulfilled" when resolve is called or "rejected" when reject is called.
• result — initially undefined, then changes to value when resolve(value) called or error when reject(error) is called.

💡 There can be only a single result or an error

The properties state and result of the Promise object are internal. We can’t directly access them. We can use the methods .then/.catch/.finally for that.

await will wait for a Promise value to resolve, otherwise it’ll just wrap the value it is given in a promise and resolve the promise right away.

参考：

How to get xml2js results out of the parser in ES6?

Promise - JavaScript | MDN

await - JavaScript | MDN

Promise