function running that's not explicitly called - node.js

Question

Below is virtually all of the code for a node.js app that lets you get playlists for artists if you run the command simply followed by an artists name
simplay the Beatles
From the output in the terminal, I know that the code in the ._flush method (added to the prototype of UrlsForNamesTransform) is getting run but it's never explicitly called. UrlsForNamesTransform extends the Transform stream in node.js, which I mention because I've seen it in other code before where a function is running without explicitly getting called (at least that I can see). Is it something about Transform or what is happening to make the code in ._flush run?
This is the github repo for the code https://github.com/thlorenz/simplay
'use strict';
var urlschema = 'http://ws.audioscrobbler.com/2.0/?method=artist.getsimilar&artist={{artist}}&api_key={{apikey}}&format=json';
var hyperquest = require('hyperquest')
, table = require('text-table')
, colors = require('ansicolors')
, styles = require('ansistyles')
var stream = require('stream');
var util = require('util');
var Transform = stream.Transform;
util.inherits(UrlsForNamesTransform, Transform);
function UrlsForNamesTransform (opts) {
if (!(this instanceof UrlsForNamesTransform)) return new UrlsForNamesTransform(opts);
opts = opts || {};
Transform.call(this, opts);
this._writableState.decodeStrings = false;
this.artist = opts.artist;
this.json = '';
}
UrlsForNamesTransform.prototype._transform = function (chunk, encoding, cb) {
this.json += chunk.toString();
cb();
};
UrlsForNamesTransform.prototype._flush = function (cb) {
var records = [];
try {
var o = JSON.parse(this.json);
var artists = o.similarartists.artist;
if (!Array.isArray(artists)) {
this.push('Sorry, no records for "' + this.artist + '" where found, please correct your spelling and/or try another artist.');
return cb();
}
artists.forEach(function (node) {
var youtubeurl = 'http://www.youtube.com/results?search_query={{artist}},playlist'.replace('{{artist}}', node.name);
var rdiourl = 'http://www.rdio.com/search/{{artist}}/artists/'.replace('{{artist}}', node.name);
var lastfmurl = 'http://www.last.fm/music/{{artist}}'.replace('{{artist}}', node.name);
var lastfmRadioUrl = 'http://www.last.fm/listen/artist/{{artist}}'.replace('{{artist}}', node.name);
var urls = [
''
, colors.white(' youtube: ') + styles.underline(colors.brightBlue(encodeURI(youtubeurl)))
, colors.blue (' rdio: ') + styles.underline(colors.brightBlue(encodeURI(rdiourl)))
, colors.brightRed (' last.fm: ') + styles.underline(colors.brightBlue(encodeURI(lastfmurl)))
, colors.red (' last.fm radio: ') + styles.underline(colors.brightBlue(encodeURI(lastfmRadioUrl)))
, ''
, ''].join('\n');
records.push([ '\n' + colors.brightYellow(node.name), colors.cyan(node.match), urls ]);
})
this.push(table(records.reverse()));
cb();
} catch (err) {
cb(err);
}
}
var go = module.exports =
/**
* Retrieves similar artists for the given artist from last.fm using the apikey.
* Then it converts the information to display youtube.com, last.fm, rdio playlist/artist urls for each artist.
*
* #name simplay
* #function
* #param {String} artist the artist to find similar artists for
* #param {String} apikey the api key to be used with last.fm
* #return {ReadableStream} that will push the url information
*/
function simplay(artist, apikey) {
if (!artist) throw new Error('Please provid the artist that you like to get similar artist links for');
if (!apikey) throw new Error('Please set LASTFM_API env variable to your API key: http://www.last.fm/api/account/create');
var url = urlschema
.replace('{{artist}}', artist)
.replace('{{apikey}}', apikey);
return hyperquest(url)
.on('error', console.error)
.pipe(new UrlsForNamesTransform({ artist: artist }))
.on('error', console.error)
};

Answer 1

The important line is this one:
util.inherits(UrlsForNamesTransform, Transform);
What this means is that UrlsForNamesTransform is a subclass of Transform. There is very good documentation on subclassing Transform, which can be found on the node.js api site.
Essentially, a subclass of Transform must implement _transform and can implement _flush, but is expected to never call either of those functions. Methods in Transform will call them based on events on the incoming stream.