Node.js stream transform example to fitler the data and having delay in transform function.

node-js-stream-transform-with-delay.js

const fs = require("fs");
const {Transform} = require('stream');
const split = require('split'); 

const readStream = fs.createReadStream('./numbers');
const writeStream = fs.createWriteStream('./evens');
 
function test (chunk, cb) {
  const val = parseInt(chunk.toString());
  if (val % 2 === 0) cb(null, val.toString() + '\n');
  else cb();
}

class Filter extends Transform {
  _transform(chunk, encoding, cb) {
    setTimeout(() => {
      test(chunk, cb);
    }, Math.random() * 10);
  }
}

const filter = new Filter();

readStream.pipe(split()).pipe(filter).pipe(writeStream);