var concatFiles = require('stream-concat-files');

// Concatenate them into a file
concatFiles(['a.txt', 'b.txt', 'c.txt'], 'concat.txt', function (err) {
  if (err) throw err;

  // concat.txt is now saved
});

// Or concatenate them into an existing write stream, and don't end it
concatFiles(['a.txt', 'b.txt'], someWritableStream, {end: false}, function (err) {
  if (err) throw err;
  
  // someWritableStream has now been written to (but not ended)
});