How can track write progress when piping with Node.js?

Question

I am trying to track the progress of a pipe from a read stream to write stream so I can display the progress to the user.

My original idea was to track progress when the data event is emitted as shown here:

const fs = require('fs');
let final = fs.createWriteStream('output');

fs.createReadStream('file')
    .on('close', () => {
        console.log('done');
    })
    .on('error', (err) => {
        console.error(err);
    })
    .on('data', (data) => {
        console.log("data");
        /* Calculate progress */
    })
    .pipe(final);

However I realized just cause it was read, doesn't mean it was actually written. This can be seen if the pipe is removed, as the data event still emits.

How can track write progress when piping with Node.js?

Answer 1

You can use a dummy Transform stream like this:

const stream = require('stream');

let totalBytes = 0;
stream.pipeline(
    fs.createReadStream(from_file),
    new stream.Transform({
        transform(chunk, encoding, callback) {
            totalBytes += chunk.length;
            console.log(totalBytes);
            this.push(chunk);
            callback();
        }
    }),
    fs.createWriteStream(to_file),
    err => {
        if (err)
            ...
    }
);