Use FeedParser as a transform stream

headlines5.js

let Transform = require('stream').Transform
let FeedParser = require('feedparser')
let pump = require('pump')

class Filter extends Transform {
    constructor(articles_max, feedparser) {
	super()
	this._writableState.objectMode = true // we can eat objects
	this.articles_max = articles_max
	this.articles_count = 0

	if (feedparser) {
	    this.once('unpipe', () => {
		this.end()	// ensure 'finish' event gets emited
	    })
	}
	this.feedparser = feedparser
    }
    _transform(input, encoding, done) {
	if (this.articles_count++ < this.articles_max) {
	    this.push(input.title + '\n')
	} else {
	    console.error('stop on', this.articles_count)
	    if (this.feedparser) this.feedparser.unpipe(this)
	}
	done()
    }
}

let articles_max = Number(process.argv.slice(2)) || 1
let fp = new FeedParser()
pump(process.stdin, fp, new Filter(articles_max, fp), process.stdout,
     err => err && err.code !== 'EPIPE' && console.error(err))