Building a CSS selector evaluator using the list monad in Javascript

In this post I will explain how to write an evaluator for a CSS selector. The CSS specification defines selectors, which indicate to which DOM elements a given list of style declarations needs to be applied. For example, the selector:

#test > li

selects the two li elements in the following document:

<body id="body">
  <ul id="test">

Of course, these days most modern web browsers support the Selectors API so if you just need a function with the described functionality, take a look at querySelector and querySelectorAll.

Rather, this post explains how one might implement this feature in a web browser. So, how do we write a function which takes as an input a CSS selector and returns a list of the selected DOM elements? Let’s start by writing a function which selects the child elements of a given element based on a predicate function. I will give an implementation Javascript, but I will write a type signature for each function using Scala notation.

// selectByPredicate: (Element => Bool) => Element => List[Element]
function selectByPredicate (f) {
  return function (e) {
    var result = new Array();
    for (var i = 0; i < e.children.length; i++)
      if (f(e.children[i])) result.push(e.children[i]);
    return result;

By using this general select function we are able to write specialized select functions. We may for example write a function which selects child elements which have a specific id or a specific tagname:

// selectById: String => Element => List[Element]
function selectById (id) {
  return selectByPredicate(function (e) { 
    return == id; 

// selectByTagName: String => Element => List[Element]
function selectByTagName (name) {
  return selectByPredicate(function (e) { 
    return e.tagName == name.toUpperCase(); 

// selectByClassName: String => Element => List[Element]
function selectByClassName (name) {
  return selectByPredicate(function (e) { 
    return e.classList.contains(name); 

Now, we are able to translate the individual parts of the CSS selector to a function by partially applying our select functions:

CSS Selector Part Partial Applied Selector Function Remaining Type
#test selectById(‘test’) Element => List[Element]
li selectByTagName(‘li’) Element => List[Element]
.foo selectByClassName(‘foo’) Element => List[Element]

Our next job is finding a method to chain these partially applied selector functions together. Notice how the remaining type of our partially applied functions lines up. However, one quickly sees that simply using function composition is not going to work here: the result of a select function is a list of elements while the input of a select function is just a single element.

To solve this problem, we are going to define a function called flatMap which allows us to apply a function with the type A => List[B] to an argument of type List[A]. In Javascript we may implement flatMap on lists as following:

// flatMap[A,B]: List[A] => (A => List[B]) => List[B]
Array.prototype.flatMap = function (f) {

// join[A]: List[List[A]] => List[A]
Array.prototype.join = function () {
  return this.reduce(function (left, right) {
    return left.concat(right)
  }, new Array());

Now, we are able to compose the individual select function together and write one function which selects the elements given by the CSS selector #test > li:

// xs contains the result of the CSS selector: #test > li
var body = document.getElementById('body');
var xs = [body].flatMap(

Looks quite elegant right? For those interested, I uploaded a working example. In this post we defined a function flatMap to allow the composition of functions of the type A => List[B]. In a next post I will generalize the flatMap function to work on other types than lists, showing that our approach here is just a specific implementation of a more general concept called a monad.