Node.js Async/Promise explanation with JIMP?

Question

I'm working on a Discord bot that takes an uploaded picture attachment, saves it to a temporary file after editing, and then uploads it to the Discord server. Everything works, except for the timing. The sendImage function gets called after the generateImagePixel function, but tries to upload the temp image before jimp.write is finished, causing an ENOENT error. How would I fix my timing issue?

client.on('message', message => {
    if (message.content.includes(config.prefix + 'pixel')) {
        var tempname = d.getTime();
        console.log(tempname);
        generateImagePixel(message.attachments, tempname).then(() => sendImage(tempname, message))
    }
});

function generateImagePixel(msg, name) {
    return new Promise((resolve, reject) => {
        msg.forEach(a => {
            try {
                jimp.read(a.url, function (err, image) {
                    const clone = image.clone();
                    clone.pixelate(8)
                    .rgba(true)
                    .filterType(0)         
                    .write('./temp/' + name + '.png');
                });
            } catch(err) {
                 console.log(err);
            }
        });
        resolve(name)
    })
}

function sendImage(tempname, msg) {
    msg.channel.send({
        file: './temp/'+ tempname + '.png' // Or replace with FileOptions object
    });
}

Answer 1

This is a typical example of executing asynchronous functions in a synchronous loop.

jimp.read is asycnhronous, so each call returns immediately before the actual stuff it does finishes. Therefore msg.forEach also finishes before jimp is done.

Once you are using something asynchronous, all you do has to be in asynchronous style:

function generateImagePixel(msg, name) {
  const promises = msg.map(attachment => {
    return jimp.read(attachment.url)
      .then(image => {
        return image
          .pixelate(8)
          .rgba(true)
          .filterType(0)         
          .write('./temp/' + name + '.png');
      })
      .catch(console.error);
  });

  return Promise.all(promises)
    .then(() => name);
}