Stream from one file to another one line at a time

textstreaming.js

var fs = require('fs');

var input = fs.createReadStream('input.txt');
var output = fs.createWriteStream('output.txt');
var buffer = '';

input.on('data', function(data) {
  buffer += data;
  var index = buffer.indexOf('\n');
  while (index > -1) {
    var line = buffer.substr(0, index);
    buffer = buffer.substr(index + 1);
    parseLine(line);
    index = buffer.indexOf('\n');
  }
  
  // Show progress on the console
  process.stdout.write('.');
});

input.on('end', function() {
  console.log('Finished reading input');
  output.end(); // Terminate the stream
  output.destroySoon(); // Close the file when the write queue is drained
});

output.on('drain', function() {
  // Output buffer is empty again, continue reading
  input.resume();
});

function parseLine(line) {
  // Apply a simple upper-case transformation. If the kernel buffer is full,
  // stop reading from the input file until the output.drain event is called
  if (!output.write(line.toUpperCase() + '\n', 'ascii'))
    input.pause();
}