How to return a array with objects from a nested async callback function inside a for of loop?

Question

I'm trying to write a small program that will ping internal addresses to check if a server is down. The problem is that I cant get the timing right. If I put a console.log() inside the for of loop it returns the objects I want but several times of cource. If I write a console.log after the for of loop, I only get an empty array, probably because the loop is not finished yet.I have tried to fix this in various ways but keep running into problems. I use node-cmd to send an http ping as normal pings only work against domains.

You can ignore my nightmare regex :) It should just split the response into an array which is then pushed to object

var cmd = require('node-cmd');

const hosts = [
  'https://adressone.com',
  'https://adresstwo.com',
  'https://adressthree.com',
  'https://adressfour.com'
];
let results = [];
for (let host of hosts) {
  cmd.run(`http-ping.exe ${host}`,
    async function(err, data, stderr) {
      const result = await data.split('>');
      const resultDig = result[1].replace(/[^0-9\.]+/g, " ").replace(/2$/g, "").trim().split(' ')

      results.push({
        host: host,
        status: resultDig[0],
        bytes: resultDig[1],
        time: resultDig[2],
      });

    }
  );
}
console.log(results);

The result I would like in console.log(results):

[{
    host: 'https://adressone.com',
    status: '200',
    bytes: '703',
    time: '640'
  },
  {
    host: 'https://adresstwo.com',
    status: '200',
    bytes: '703',
    time: '640'
  },
  {
    host: 'https://adressthree.com',
    status: '200',
    bytes: '703',
    time: '640'
  },
  {
    host: 'https://adressfour.com',
    status: '200',
    bytes: '703',
    time: '640'
  }
]

node-cmd package does not appear to have a promise variant, you either use the sync variant not ideal, or promisify the callback variant, you could then await each iteration. The problem at the moment is your console logging before it's even done. — Keith
– Keith, Commented Sep 5, 2022 at 14:10

Nitheesh · Accepted Answer · 2022-09-06 08:27:46Z

1

The best approach for this question will be by using Promises in JavaScript.

An example for this implementation.

async function parseResult() {
  const hosts = [
    "https://adressone.com",
    "https://adresstwo.com",
    "https://adressthree.com",
    "https://adressfour.com",
  ];
  const promises = [];
  for (let host of hosts) {
    const response = new Promise(function (resolve, reject) {
      setInterval(async () => {
        resolve({
          status: "Success",
        });
      }, 1000);
    });
    promises.push(response);
  }
  const finalResult = await Promise.all(promises);
  return finalResult;
}
parseResult().then((res) => console.log(res));

OR

async function parseResult() {
  const hosts = [
    "https://adressone.com",
    "https://adresstwo.com",
    "https://adressthree.com",
    "https://adressfour.com",
  ];
  const promises = hosts.map(
    (host) =>
      new Promise(function (resolve, reject) {
        setInterval(async () => {
          resolve({
            status: "Success",
          });
        }, 1000);
      })
  );
  const finalResult = await Promise.all(promises);
  return finalResult;
}
parseResult().then((res) => console.log(res));

A Pseudo code for your question will be like below

var cmd = require("node-cmd");
async function parseResult() {
  const hosts = [
    "https://adressone.com",
    "https://adresstwo.com",
    "https://adressthree.com",
    "https://adressfour.com",
  ];
  let promises = [];
  for (let host of hosts) {
    const response = cmd.run(
      `http-ping.exe ${host}`,
      async function (err, data, stderr) {
        const result = await data.split(">");
        const resultDig = result[1]
          .replace(/[^0-9\.]+/g, " ")
          .replace(/2$/g, "")
          .trim()
          .split(" ");

          // Returning a promise here
        return new Promise(function (resolve, reject) {
          resolve({
            host: host,
            status: resultDig[0],
            bytes: resultDig[1],
            time: resultDig[2],
          });
        });
      }
    );
    promises.push(response);
  }
  // promises array will hold a list of promises now
  // Await till all the promises are resolved and that will be your final result
  const finalResult = await Promise.all(promises);
  return finalResult;
}
parseResult().then((res) => console.log(res));

Explanation: Here the parseResult function loops through an array hosts and will run an async operation against each nodes in hosts array. The result of this operation is a Promise, so your final promises array will be holding a list of promises. Once all this promises are resolved, you will get your final result. You can detect when all your promises are resolved by using await Promise.all(promises)

edited Sep 6, 2022 at 8:27

answered Sep 6, 2022 at 6:56

Nitheesh

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3 Comments

Nitheesh Over a year ago

@Amadan Agreed and updated.

ValleP Over a year ago

Thank you for taking the time to answer my question. I feel a bit stupid but I'm pretty new to this. I understood after your explanation how it works with promises and that it waits to return the response until all promises have been resolved. But I'm still figuring out how to push the result to an array that I access globally? Previously I used result.push({status: resultDig[0],}) and so on. Later in the program I want to be able to do if(result.status == '200'){ // do something }

Nitheesh Over a year ago

@ValleP that's correct you have to resolve the promise when the response from the server is 200

some-user · Accepted Answer · 2022-09-05 14:13:42Z

0

I'd suggest to use node-cmd-promise. I removed the splitting:

const cmd = require('node-cmd-promise')

const hosts = [
  'https://adressone.com',
  'https://adresstwo.com',
  'https://adressthree.com',
  'https://adressfour.com',
]

async function main() {
  const results = []
  for (const host of hosts) {
    const result = await cmd(`echo ${host}`)

    // Insert logic here
    const resultDig = [result.stdout, 'foo', 'bar']

    results.push({
      host,
      status: resultDig[0],
      bytes: resultDig[1],
      time: resultDig[2],
    })
  }
  console.log(results)
}

main()

answered Sep 5, 2022 at 14:13

some-user

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1 Comment

Thomas Over a year ago

Is that package still active? Because the github repo returns 404. Besides, node-cmd is literally just forwarding the call to child_process.exec and in the docs you'll find this here to make it async: const util = require('node:util'); const exec = util.promisify(require('node:child_process').exec);

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