Reading and sending data to user in sequence in node.js

Question

I'm coding website in node.js using express and socket.io. The server is sending to user photo and data from every file in directory using socket.emit function.

The problem with this code: The server is sending data from 2 files but image in second emit is same as first beacouse before sends first img is changing data of first file to second data.

I found this solution but it's not working. Some of the server side code:

function send(file) {

        return new Promise((resolve, reject) => {
                fs.readFile(dir + file, 'utf8', (err, jsonString) => {

                    jsonString = JSON.parse(jsonString);

                    buffer = ""
                    img = false

                    img_path = dir + jsonString.number

                    if (fs.existsSync(img_path + '.jpg')) {

                        fs.readFile(img_path + '.jpg', function (err, buf) {
                            buffer = buf
                            img = true
                        });
                    }
                    if (fs.existsSync(img_path + '.png')) {

                        fs.readFile(img_path + '.png', function (err, buf) {
                            buffer = buf
                            img = true
                        });
                    }

                    setTimeout(() => { // <---- Without wait buffer will be blank

                        console.log(img_path) //both times is same
                        data = {
                            number: jsonString.number,
                            image: img,
                            buffer: buffer.toString('base64')
                        }

                        socket.emit('data', data)
                        console.log("sended!")
                        resolve()
                    }, 100)

                    if (err) {
                        console.error(err);
                        return;
                    }
                });
    })
}

async function files() {

  fs.readdir(dir, (err, files) => {
      if (err) {
          throw err
      }

      files.forEach(async file => {
      
          if (file.split('.').pop() == 'json') {
             await send(file)
          }
       })
   })
}

files()

But this is not working. Thanks for help!

`fs.readFile()` is async, you are using it like `fs.readFileSync()`. You could change the readFile to readFileSync but you actually have to restucture the code to handle async properly. — A1rPun, Oct 15 '22 at 15:24

score 0 · Accepted Answer · answered Oct 15 '22 at 16:16

There are a few misconceptions that have to addressed:

async and sync flows used in same context, that's why you needed a setTimeout
async function in forEach doesn't do what you expect
variable management, use let or const

Some small things:

argument passing, dir wasn't available in some contexts
async function definition for send

For example your code could be rewritten like this:
Do note that I did not run this program

const { promisify } = require("util");
const readFile = promisify(fs.readFile);
const exists = promisify(fs.exists);
const readdir = promisify(fs.readdir);

async function getBuffer(filePath) {
  const isFile = await exists(filePath);
  if (!isFile) return;
  return readFile(filePath);
}

async function send(dir, file) {
  const content = await readFile(file, 'utf8');

  if (!content) return;

  const json = JSON.parse(content);
  const img_path = dir + json.number;

  const buf1 = await getBuffer(`${img_path}.jpg`);
  const buf2 = await getBuffer(`${img_path}.png`);

  console.log(img_path);
  const data = {
    number: json.number,
    image: buf1 || buf2,
    buffer: (buf1 || buf2).toString('base64'),
  }
  socket.emit('data', data);
  console.log("sended!");
}

async function files(dir) {
  const files = await readdir(dir);
  for (const file in files) {
    if (file.split('.').pop() === 'json') {
      await send(dir, file);
    }
  }
}

files("<directory>");

Thanks! I know my code is not perfect but I'm still learning js. When i run this there is any errors, but it isn't sending. It's stopping on `for (const file in files) {` line. `console.log(files)` returns `Promise { }`. I provided appropriate directory but I don't know how to solve this — PioterSky, Oct 15 '22 at 17:06
@PioterSky Great that you're learning! The only help I can give is break down the larger problem into smaller, more manageable problems. — A1rPun, Oct 15 '22 at 17:41
Help with these smaller, more manageable problems can be asked on StackOverflow! — A1rPun, Oct 15 '22 at 18:03

Reading and sending data to user in sequence in node.js

1 Answers1