sh
npm install bufferstreams --save

js
var BufferStreams = require('bufferstreams');

Fs.createReadStream('input.txt')
  .pipe(new BufferStreams(function(err, buf, cb) {

    // err will be filled with an error if the piped in stream emits one.
    if(err) {
      throw err;
    }

    // buf will contain the whole piped in stream contents
    buf = Buffer(buf.toString('utf-8').replace('foo', 'bar'));

    // cb is a callback to pass the result back to the piped out stream
    // first argument is an error that will be emitted if any
    // the second argument is the modified buffer
    cb(null, buf);

  }))
  .pipe(Fs.createWriteStream('output.txt'));