Returning data in a function having callbacks

Question

I've just started with Node.js and callbacks are something I'm getting used to it. Sometimes it just gives me a bad time. I have a function dwnloadData() which downloads the data and append the data into an empty array and send that array.

Here for downloading I'm using callback and I'm getting arrData empty Can someone pls let me know the correction.

Requirement: arrData, the array should have all the data downloaded using for loop and should return using promise.

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)
        });
    }

Your request.get callback happens later. arrData.push happens immediately. — James
– James, Commented Jun 14, 2019 at 12:13

Aritra Chakraborty · Accepted Answer · 2019-06-14 12:19:27Z

You are looping through the url and getting the jsonData which is only valid inside the callback function. Because you don't know when the request.get will resolve.

So what are you basically doing is, Looping over the urls, initiating the GET request. But not waiting for the result and returning the function. (also again, you need to .push inside the callback.

What you can do is, making a array of promises and run it via Promise.all which automatically returns an array. And bonus, you are running the GET requests in parallel.

function downloadData(url) {
    const promises = [];
    for (let i = 0; i < url.length; i++) {
        promises.push(
            new Promise((resolve, reject) => {
                request.get(url[i], function (error, response, body) {
                    if (!error && response.statusCode == 200) {
                        const content = body
                        const jsonArray = JSON.parse(content);
                        resolve(jsonArray)
                    }else{
                        resolve()
                    }
                });
            })
        );
    }
    return Promise.all(promises);
}

But use a promise based HTTP library like axios to do promise chaining or async-await naturally.

Laakal · Accepted Answer · 2019-06-14 12:26:39Z

0

Use async/await with request-promise module.For example

const request = require('request-promise');

app.get('/', async (req, res, next) => { 

 // ...
 const result = await downloadData(url); 

});

async function downloadData(url) {

  const arrData = [];

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

    try {

      let data = await request.get(url[i]);

      if (data) {
        arrData.push(JSON.parse(data))
      }

    }
    catch (err) {
      console.log(err)
    }

  }

 return arrData;

}

edited Jun 14, 2019 at 12:26

answered Jun 14, 2019 at 12:19

Laakal

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Comments

Syed Afzal · Accepted Answer · 2019-06-14 12:31:35Z

0

This is because you used call back function inside a promise, When you call an api it doesn't wait there because of asynchronous, it go further and called resolve.

var rp = require('request-promise');
downloadData(url) {

    return new Promise(async (resolve, reject) => {
        var arrData = [];
        let jsonArray;

        for (var i = 0; i < url.length; i++) {
            try{
                let res = await rp.get(url[i]);
                if (res.statusCode === 200) {
                    let content = body
                    jsonArray = JSON.parse(content);
                }
            }catch (e) {
                reject(e)
            }
            arrData.push(jsonArray)
        }
        resolve(arrData)
    });
}

edited Jun 14, 2019 at 12:31

answered Jun 14, 2019 at 12:12

Syed Afzal

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3 Comments

Syed Afzal Over a year ago

use request-promise here

Aditya Malviya Over a year ago

I'm already using normal promise, it won't affect if i use request-promise right ?

Aditya Malviya Over a year ago

returning empty data

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