node_writable_stream.js

var stream = require('stream');
var util = require('util');

function MyStream() { stream.Writable.call(this); }
util.inherits(MyStream, stream.Writable);
MyStream.prototype._write = function (chunk, encoding, callback) {
  console.log("_write " + chunk.length);
  callback();
}

var aStream = new MyStream();
aStream.on('finish', function () { console.log('finish emitted'); })
aStream.write('x');
aStream.end();

this logs

_write 1
finish emitted

i need to do extra processing (asynchronous) after all writes are done and before finish is emitted.