Functional JavaScript for crawling the Web

This post has been published more than 9 years ago, it may be obsolete by now.

I’ve been giving JavaScript & CasperJS training sessions lately, and was amazed how few people are aware of the Functional Programming capabilities of JavaScript. Many couldn’t see obvious usage of these in Web development, which is a bit of a shame if you ask me.

Let’s take things like map and reduce from the Array prototype:

function square(x) {
  return x * x;

function sum(x, y) {
  return x + y;

[1, 2, 3].map(square).reduce(sum)
// 14

I’ve been hearing a few times things like:

Well yeah that’s cool, but I don’t do maths, I’m a Web developer.

And each time it turns me a little sad.


As we’re programming language hipsters, in this article we’ll use ES6 short function syntax which has landed a few weeks ago in Firefox Nightlies and eases a lot writing code in the functional style:

var square = x => x * x;
var sum = (x, y) => x + y;

[1, 2, 3].map(square).reduce(sum)
// 14

We’ll also use other ES6 features as well because, you know, today is our future already.

This article contents will also probably hurt some people feelings, probably because there’s a lot to hate in there when you come from a pure OOP landscape. Please think of this article as an exercise of thought instead of yet another new JavaScript tutorial™.

Crawling the DOM using FP

Take this DOM fragment featuring a good ol’ data table as an example:

      <th>Population (M)</th>
      <th>GNP (B)</th>
    <tr><td>United Kingdom</td><td>64.231</td><td>2290</td></tr>

To map the country names to a regular array of strings:

var rows = document.querySelectorAll("tbody tr");
[], row => row.querySelector("td").textContent);
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

It worked: our map operation transformed a list of DOM table row elements to the text value of their very first cell. Well, it feels like we could probably enhance the code ergonomics a bit here.

Note: If you wonder why do we use [] in lieu of just calling map from the element list prototype, that’s because NodeList doesn’t implement the Array interface… Yeah, I know.

As an illustrative exercise, let’s write our own map function to make a passed iterable always exposing the Array interface; also, let’s invert the order of passed args to ease further composability (more on this later):

const map = (fn, iterable) => [], fn);

Note: we declare map as a constant to avoid any accidental mess. Also, I don’t see obvious reasons for a function to be mutated here.

So we can write:

var rows = document.querySelectorAll("tbody tr");
map(row => row.querySelector("td").textContent, rows);
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

As a side note, this map implementation also works for strings:

map(x => x.toUpperCase(), "foo");
// ["F", "O", "O"]

We can also write a tiny abstraction on top of querySelectorAll, again to ensure further composability:

const nodes = (sel, root) => (root || document).querySelectorAll(sel);

So now we can write:

var rows = nodes("tbody tr");
map(node => nodes("td", node)[0].textContent, rows);
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

Hmm, the operations being performed within the function passed to map (finding a first child node, getting an element property value) sound like things we’re most likely to do many times while extracting information from the DOM. And then we’d probably want better code semantics as well.

For starters, let’s create a first() function for finding the first element out of a collection:

const first = iterable => iterable[0];
// first([1, 2, 3]) => 1

Our example becomes:

map(node => first(nodes("td", node)).textContent, rows);
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

In the same vein, we could use a prop() higher order function — basically a function returning a function — one more time to create a reusable & composable property getter (we’ll get back to this, read on):

const prop = name => object => object[name];
// const getFoo = prop("foo");
// getFoo({foo: "bar"}) => "bar"

If you struggle understanding how this works, this is how we would write prop using current function syntax:

function prop(name) {
  return function(object) {
    return object[name];

Let’s use our new property getter generator:

const getText = prop("textContent");

map(node => getText(first(nodes("td", node))), rows);
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

Now, how about having a generic for finding a node’s child elements from a selector? Let’s do this:

const finder = selector => root => nodes(selector, root);

const findCells = finder("td");
// 30

Don’t panic, again this is how we’d write it using standard function declaration syntax:

function finder(selector) {
  return function(root) {
    return nodes(selector, root);

Let’s use it:

const getText = prop("textContent");
const findCells = finder("td");

map(node => getText(first(findCells(node))), rows);
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

At this point, you may be wondering how this is possibly improving code readability and maintainability… Now is the perfect time to use function composition (you waited for it), to aggregate & chain minimal bits of reusable code.

Note: If you’re familiar with the UNIX philosophy, that’s exactly the same approach as when using the pipe operator:

 $ ls -la | awk '{print $2}' | grep pattern | wc -l

Let’s create a sequence function to help composing functions sequentially:

const sequence = function() {
  return [], function(comp, fn) {
    return () => comp(fn.apply(null, arguments));

This one is a bit complicated; it basically takes all functions passed as arguments and returns a new function capable of processing them sequencially, passing to each the result of the previous execution:

const squarePlus2 = sequence(x => 2 + x, x => x * x);
// 4 * 4 + 2 => 18 => Aspirine is in the bathroom

In classic notation without using a sequence, that would be the equivalent of:

function plus2(x) {
    return 2 + x;

function square(x) {
    return x * x;

function squarePlus2(x) {
    return plus2(square(x));

// 18

By the way, sequence is a very good place to use ES6 Rest Arguments which have also landed recently in Gecko; let’s rewrite it accordingly:

const sequence = function(...fns) {
  return fns.reduce(function(comp, fn) {
    return (...args) => comp(fn.apply(null, args));

Let’s use it in our little DOM crawling example:

const getText = prop("textContent");
const findCells = finder("td");

map(sequence(getText, first, findCells), rows)
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

What I like the most about the FP style is that it actually describes fairly well what’s going to happen; you can almost read the code as you’d read plain English (caveat: don’t do this at family dinners).

Also you may want to have the functions passed in the opposite order, ala UNIX pipes, which usually enhances legibility a bit for seasonned functional programmers; let’s create a compose function for doing just that:

const compose = (...fns) => sequence.apply(null, fns.reverse());

map(compose(findCells, first, getText), rows);
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

Wait, is this really better?

As a side note, one may argue that:

map(sequence(getText, first, findCells), rows);

Is not much really better than:

map(row => getText(first(findCells(row))), rows);

Though the composed approach is probably more likely to scale when adding many more functions to the stack:

sequence(a, b, c, d, e, f, g, h)(foo);

Last, a composed function is itself composable by essence, and that’s probably a killer feature:

map(sequence(getText, sequence(first, findCells)), rows);
// ["Belgium", "France", "Germany", "Greece", "Italy", …]

Which something like this:

var crawler = new Crawler("table");
crawler.findCells("tbody tr").first().getText();

Is hardly likely to offer.

A few more examples

To compute the total population of listed countries:

const reduce = (fn, init, iterable) => [], fn, init);
const second = (iterable) => iterable[1];
const sum = (x, y) => x + y;

var populations = map(compose(findCells, second, getText, parseFloat),
reduce(sum, 0, populations);
// 403.31000000000006

To generate a JSON export of the whole table data:

const partial = (fn, ...r) => (...a) => fn.apply(null, r.concat(a))
const nth = n => (iterable) => iterable[n - 1];
const third = nth(3);
const getTexts = partial(map, getText);
const asObject = (data) => ({
  name:       first(data),
  population: parseFloat(second(data)),
  gnp:        parseFloat(third(data))

var countries = map(compose(findCells, getTexts, asObject), rows);
// "[{"name":"Belgium","population":11.162,"gnp":419}, …

To compute the global average GNP per capita for these countries:

const perCapita = c => ({name:, perCapita: c.gnp / c.population});

var gnpPerCapita = map(perCapita, countries);
// "[{"name":"Belgium","perCapita":37.5380756136893}, …

To filter countries having more than n€ of GNP per capita, sort them by descending order and export the result as JSON:

const select = (fn, iterable) => [], fn)
const sort = (fn, iterable) => [], fn);

const sortDesc = partial(sort, (a, b) => a.perCapita > b.perCapita ? -1 : 1);
const healthy = partial(select, c => c.perCapita > 38);

const healthyCountries = compose(healthy, sortDesc);
// "[{"name":"Netherlands","perCapita":42.45311104495385}, …

I could probably go on and on, but you get the picture. This post is not to claim that the FP approach is the best of all in JavaScript, but that it certainly has its advantages. Feel free to play with these concepts for a while to make your mind, eventually :)

If you’re interested in Functional JavaScript, I suggest the following resources:

  • Pure, functional JavaScript, an inspiring talk from Christian Johansen;
  • JavaScript Allongé, an online book which covers most of its aspects in a very comprehensive style (you should buy it);
  • List Out of Lambda, a blog post from Steve Losh where he reinvents lists purely using functions in JavaScript (!);
  • If you’re hooked with FP (yay!), have a look at Clojure and its port targetting the JavaScript platform, ClojureScript.

If you’re interested in ECMAScript 6, here are some good links to read about: