Collect data from a readable stream into a variable

I'm trying to implement a writable stream that will save the data that is written to it into a variable. This is the implementation of the writable stream:

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

function Collector()
{ 
  Writable.call(this, {objectMode: true});    
  this.entities = [];
};

util.inherits(Collector, Writable);

Collector.prototype._write = function (chunk, encoding, callback)
{
  this.entities.push(chunk);    
  callback();
};


module.exports = Collector;

and this is how I'm trying to test it it out:

var fs = require('fs');
var Tokenizer = require('./tokenizer');
var Processor = require('../parser');
var Collector = require('./collector.js');

var tokenizer = new Tokenizer();
var processor = new Processor();
var collector = new Collector();

var readable = fs.createReadStream('./test/fixtures/test.dxf');

readable.pipe(tokenizer)
        .pipe(parser)
        .pipe(processor); // if this is piped to stdout, lots of data

console.log(collector.entities); // logs an empty array

I'm not sure why, but the entities property is empty after all it has been piped. If I console log this.entities within the _write function, the data is available.

Ultimately I want to be to call a function that returns an array whose elements are made up of data chunks from Processor. Collector was some hacking to see how I could do it, but I haven't gotten very far.

How can I store chunks from a readable stream into a variable and return them from a function?