NodeJS file readwrite line by line

const fs = require('fs');
const readline = require('readline');

// Define input and output file paths
const inputFile = 'input.txt';
const outputFile = 'output.txt';

// Create a readable stream for the input file
const readStream = fs.createReadStream(inputFile);

// Create a writable stream for the output file
// The 'w' flag will create the file if it doesn't exist or overwrite it if it does
const writeStream = fs.createWriteStream(outputFile, { flags: 'w' });

// Create a readline interface to process the input stream line by line
const rl = readline.createInterface({
input: readStream,
crlfDelay: Infinity // Treats all instances of CR LF as a single line break
});

// Listen for the 'line' event on the readline interface
rl.on('line', (line) => {
// Process the line (optional, you can modify 'line' here)
const processedLine = line.toUpperCase(); // Example processing: convert to uppercase

// Write the processed line to the output file, followed by a newline character
writeStream.write(processedLine + '\n');
});

// Listen for the 'close' event on the readline interface, indicating the input file is fully read
rl.on('close', () => {
console.log('Finished reading and writing files line by line.');
// Close the writable stream when done
writeStream.end();
});

// Handle potential errors on the streams
readStream.on('error', (err) => {
console.error('Error reading file:', err);
});

writeStream.on('error', (err) => {
console.error('Error writing file:', err);
});