An example of using Kefir streams with a recursive function.

fetchRecursively.js

'use strict';
const Kefir = require('kefir');

// fetch takes a url string, pretends to fetch it over the network, and returns
// a Kefir stream that emits a Page object when it's done, and then the stream
// ends.
function fetch(url) {
  let links;
  switch (url) {
    case 'foo.com': links = ['bar.com', 'baz.com']; break;
    case 'bar.com': links = ['zz.com', 'zzz.com']; break;
    case 'baz.com': links = ['foo.com', 'z.com']; break;
    case 'z.com': links = ['zz.com']; break;
    default: links = [];
  }
  const page = {url, links};
  return Kefir.later(10, page);
}

// getLinks takes a Page object, pretends to spend time parsing it, and returns
// a Kefir stream of all of the links in the page as strings, and then the
// stream ends when the page is done being parsed for links.
function getLinks(page) {
  return Kefir.sequentially(1000, page.links);
}

// fetchRecursively takes a url string and returns a Kefir stream of Page
// objects (the same kind fetch returns). fetchRecursively fetches the given
// url, emits the Page object received into the returned stream, gets the links
// from the page object, and repeats the process for all link urls found, but
// skips urls that have already been fetched. The returned stream ends when all
// of the fetching is done.
function fetchRecursively(url, alreadyFetchedUrls = new Set()) {
  if (alreadyFetchedUrls.has(url)) return Kefir.never();
  alreadyFetchedUrls.add(url);

  const pageStream = fetch(url);
  return Kefir.merge([
    pageStream,
    pageStream
      .flatMap(getLinks)
      .flatMap(url => fetchRecursively(url, alreadyFetchedUrls)),
  ]);
}

fetchRecursively('foo.com').log('fetchRecursively');
// fetchRecursively <value> { url: 'foo.com', links: [ 'bar.com', 'baz.com' ] }
// fetchRecursively <value> { url: 'bar.com', links: [ 'zz.com', 'zzz.com' ] }
// fetchRecursively <value> { url: 'baz.com', links: [ 'foo.com', 'z.com' ] }
// fetchRecursively <value> { url: 'zz.com', links: [] }
// fetchRecursively <value> { url: 'zzz.com', links: [] }
// fetchRecursively <value> { url: 'z.com', links: [ 'zz.com' ] }
// fetchRecursively <end>