Combine 'through' streams

combine-through.js

// combine an existing pipeline:
//   entry -> s1 -> s2 -> s3 -> s4 -> exit
// into:
//   P
// where sX -> P => sX -> entry
//  and  P -> sY => exit -> sY

var combine = function(entry, exit) {
  //simple proxy stream
  var combination = through();
  // -> exit -> combination ->
  exit.pipe(combination);

  // combination -> entry ->
  //  further attempts to pipe to combination
  //  will actually pipe to entry
  combination.on('pipe', function(s) {
    s.unpipe(combination);
    s.pipe(entry);
  });

  return combination;
};

//where 'through' is a basic transform stream helper
var Transform = require('stream').Transform;
var through = function through(transform, flush) {
  //always allow objects
  var t = new Transform({ objectMode: true });
  t._transform = transform || function(data, enc, next) {
    next(null, data);
  };
  t._flush = flush;
  return t;
};