在具有回调的函数中返回数据

Question

我刚刚开始使用 Node.js，我已经习惯了回调。有时它只是让我度过了一段糟糕的时光。我有一个函数 dwnloadData() ，它下载数据并将数据附加到一个空数组中并发送该数组。

在这里下载我正在使用回调，我得到 arrData 空有人可以让我知道更正。

要求：arrData，数组应该有使用for循环下载的所有数据，并且应该使用promise返回。

downloadData(url) {

        return new Promise((resolve, reject) => {

            var arrData = [];

            for (var i = 0; i < url.length; i++) {

                request.get(url[i], function (error, response, body) {

                    if (!error && response.statusCode == 200) {

                        var content = body

                        var jsonArray = JSON.parse(content);

                    }

                });

               arrData.push(jsonArray)

            }

        resolve(arrData)

        });

    }

Answer 1

您正在遍历 url 并获取jsonData仅在回调函数内有效的 。因为你不知道什么时候request.get会解决。所以你基本上在做什么，循环遍历 url，启动 GET 请求。但不等待结果并返回函数。（同样，您需要.push在回调内部。您可以做的是，制作一个承诺数组并运行它，通过Promise.all它自动返回一个数组。而且，您正在并行运行 GET 请求。function downloadData(url) {&nbsp; &nbsp; const promises = [];&nbsp; &nbsp; for (let i = 0; i < url.length; i++) {&nbsp; &nbsp; &nbsp; &nbsp; promises.push(&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; new Promise((resolve, reject) => {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; request.get(url[i], function (error, response, body) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (!error && response.statusCode == 200) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; const content = body&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; const jsonArray = JSON.parse(content);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; resolve(jsonArray)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }else{&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; resolve()&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; });&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; })&nbsp; &nbsp; &nbsp; &nbsp; );&nbsp; &nbsp; }&nbsp; &nbsp; return Promise.all(promises);}但是使用基于承诺的 HTTP 库就像axios自然地进行承诺链接或异步等待一样。

Answer 2

这是因为您在 promise 中使用了回调函数，当您调用 api 时，它不会因为异步而在那里等待，它会更进一步并调用 resolve。var rp = require('request-promise');downloadData(url) {&nbsp; &nbsp; return new Promise(async (resolve, reject) => {&nbsp; &nbsp; &nbsp; &nbsp; var arrData = [];&nbsp; &nbsp; &nbsp; &nbsp; let jsonArray;&nbsp; &nbsp; &nbsp; &nbsp; for (var i = 0; i < url.length; i++) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; try{&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; let res = await rp.get(url[i]);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; if (res.statusCode === 200) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; let content = body&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; jsonArray = JSON.parse(content);&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }catch (e) {&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; reject(e)&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; &nbsp; &nbsp; arrData.push(jsonArray)&nbsp; &nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; &nbsp; &nbsp; resolve(arrData)&nbsp; &nbsp; });}

Answer 3

将 async/await 与 request-promise 模块一起使用。例如const request = require('request-promise');app.get('/', async (req, res, next) => {&nbsp;&nbsp;// ...&nbsp;const result = await downloadData(url);&nbsp;});async function downloadData(url) {&nbsp; const arrData = [];&nbsp; for (var i = 0; i < url.length; i++) {&nbsp; &nbsp; try {&nbsp; &nbsp; &nbsp; let data = await request.get(url[i]);&nbsp; &nbsp; &nbsp; if (data) {&nbsp; &nbsp; &nbsp; &nbsp; arrData.push(JSON.parse(data))&nbsp; &nbsp; &nbsp; }&nbsp; &nbsp; }&nbsp; &nbsp; catch (err) {&nbsp; &nbsp; &nbsp; console.log(err)&nbsp; &nbsp; }&nbsp; }&nbsp;return arrData;}