Reasonable System for JavaScript Structure

This document is a collection of guidelines on how to structure your JavaScript in a standard non-SPA web application. Also see RSCSS, a document on CSS conventions that follows a similar line of thinking.


For a typical non-SPA website, it will eventually be apparent that there needs to be conventions enforced for consistency. While styleguides cover coding styles, conventions on namespacing and file structures are usually absent.

The jQuery soup anti-pattern

You will typically see Rails projects with behaviors randomly attached to classes, such as the problematic example below.

<script src='blogpost.js'></script>
<div class='author footnote'>
  This article was written by
  <a class='profile-link' href="/user/rstacruz">Rico Sta. Cruz</a>.
$(function () {
  $('.author a').on('hover', function () {
    var username = $(this).attr('href')

    showTooltipProfile(username, { below: $(this) })

What’s wrong?

This anti-pattern leads to many issues, which rsjs attempts to address.

  • Ambiguious sources: It’s not obvious where to look for the JS behavior. (Is the handler attached to .author, .footnote, or .profile-link?)

  • Non-reusable: It’s not obvious how to reuse the behavior in another page. (Should blogpost.js be included in the other pages that need it? What if that file contains other behaviors you don’t need for that page?)

  • Lack of organization: Making new behaviors gets confusing. (Do you make a new .js file for each page? Do you add them to the global application.js? How do you load them?)

In a nutshell

RSJS makes JavaScript easy to maintain in a typical web application. All recommendations here have these goals in mind:

  • Keep your HTML declarative (get rid of inline scripts).
  • Put all your imperative code in JS files.
  • Reduce ambiguity by having straight-forward conventions.
  • HTML elements can be components that have behaviors.
  • Behaviors apply JavaScript to a [data-js-behavior-name] selector.


Think in component behaviors

Think that a piece of JavaScript code to will only affect 1 “component”, that is, a section in the DOM.

There files are “behaviors”: code to describe dynamic JS behavior to affect a block of static HTML. In this example, the JS behavior collapsible-nav only affects a certain DOM subtree, and is placed on its own file.

<!-- Component -->
<div class='main-navbar' data-js-collapsible-nav>
  <button class='expand' data-js-expand>Expand</button>

  <a href='/'>Home</a>
/* Behavior - behaviors/collapsible-nav.js */

$(function () {
  var $nav = $('[data-js-collapsible-nav]')
  if (!$nav.length) return

    .on('click', '[data-js-expand]', function () {
    .on('mouseout', function () {

One component per file

Each file should a self-contained piece of code that only affects a single element type.

Keep them in your project’s behaviors/ path. Name these files according to the data-js-___ names (see below) or class names they affect.

└── javascripts/
    └── behaviors/
        ├── collapsible-nav.js
        ├── avatar-hover.js
        ├── popup-dialog.js
        └── notification.js

Load components in all pages

Your main .js file should be a concatenation of all your behaviors.

It should be safe to load all behaviors for all pages. Since your behaviors are localized to their respective components, they will not have any effect unless the element it applies to is on the page.

See loading component files for guides on how to do this for your project.

Use a data attribute

It’s preferred to mark your component with a data-js-___ attribute.

You can use ID’s and classes, but this can be confusing since it isn’t obvious which class names are for styles and which have JS behaviors bound to them. This applies to elements inside the component too, such as buttons (see collapsible-nav example).

<!-- ✗ Avoid using class names -->
<div class='user-info'>...</div>
$('.user-info').on('hover', function () { ... })
<!-- ✓ Better to use data-js attributes -->
<div class='user-info' data-js-avatar-popup>...</div>
$('[data-js-avatar-popup]').on('hover', function () { ... })

You can also provide values for these attributes.

<!-- ✓ Attributes with values -->
<button data-js-tooltip='Close'>...</button>
$('[data-js-tooltip]').on('hover', function () { ... })

Don’t overload class names

If you don’t like data-js-* attributes and prefer classes, don’t add styles to the classes that your JS uses. For instance, if you’re styling a .user-info class, don’t attach an event to it; instead, add another class name (eg, .js-user-info) to use in your JS.

This will also make it easier to restyle components as needed.

✗ Bad: is the JavaScript behavior attached to .user-info? or to
.centered? This can be confusing developers unfamiliar with your code.
  <div class='user-info centered'>...</div>
  $('.user-info').on('hover', function () { ... })

✓ Better: this makes it more obvious to find the source of
the behavior.
  <div class='user-info js-avatar-popup'>...</div>
  $('.js-avatar-popup').on('hover', function () { ... })

No inline scripts

Avoid adding JavaScript thats inlined inside your HTML markup. This includes <script>...</script> blocks and onclick='...' event handlers.

By putting imperative logic outside your .js files (eg, JavaScript in your .html), it makes your application harder to test and they pose a significant maintenance burden. Keep all your imperative code in your JS files, and your declarative code in your HTML.

<!-- ✗ Avoid -->
  $('button').on('click', function () { ... })
<!-- ✗ Avoid -->
<body onload="loadup()">

Prefer to use JS behaviors instead of inline scripts.

Bootstrap data with meta tags

A common pattern is to use inline <script> tags to leave data that scripts will pick up later on. In the spirit of avoiding inline scripts, these patterns should also be avoided.

<!-- ✗ Avoid -->
window.UserData = { email: '[email protected]', id: 9283 }

If they’re going to be used in a component, put that data inside the HTML element and let the behavior pick it up.

<!-- ✓ Used by the user-info behavior -->
<div class='user-info' data-js-user-info='{"email":"[email protected]","id":9283}'>

If multiple components are going to use this data, put it in a meta tag in the <head> section.

  <!-- option 1 -->
  <meta property="app:user_data" content='{"email":"[email protected]","id":9283}'>

  <!-- option 2 -->
  <meta property="app:user_data:email" content="[email protected]">
  <meta property="app:user_data:id" content="9283">

This keeps your HTML files declarative, and your JS files imperative. (Also see: Imperative vs. Declarative from

function getMeta (name) {
  return $('meta[property=' + JSON.stringify(name) + ']').attr('content')

getMeta('app:user_data:email')  // => '[email protected]'

Writing code

These are conventions that can be handled by other libraries. For straight jQuery however, here are some guidelines on how to write behaviors.

Consider using onmount

onmount is a library that allows you to write safe, idempotent, reusable and testable behaviors. It makes your JavaScript code compatible with Turbolinks, and it’d allow you to write better unit tests.

$.onmount('[data-js-push-button]', function () {
  $(this).on('click', function () {

Writing code without onmount

The onmount library solves many pitfalls when writing component behaviors. If you choose not to use it, however, be prepared to deal with those caveats on your own. This next section describes these cases in detail.

Use document.ready

Add your events and initialization code under the document.ready handler. This assures you that the element you’re binding behaviors to already exists. (There is an exception to this–see the next section).

$(function() {
  $('[data-js-expanding-menu]').on('hover', function () {
    // ...

If you’re using onmount, just run onmount() on document.ready as its documentation describes it.

Use event delegation

The only time document.ready is not necessary is when attaching event delegations to document. These are typically best done before DOM ready, which would allow them to work even when the entire document hasn’t loaded completely.

This allows you to listen for those events whether the element is on the page or not, unlike doing $('[data-js-menu]').on(...) which needs to be done when the element is on the page.

$(document).on('click', '[data-js-menu]', function () {
  // ...

However, this technique comes at a runtime performance cost. If you bind events for click for 50 components, every click on the page will check for 50 possible event handlers. Instead, consider using onmount and bind your events directly to elements as needed.

Also see extras for more info on event delegation.

Use each() when needed

When your behavior needs to either initialize first and/or keep a state, consider using jQuery.each. See extras for a more detailed example.

$(function() {
  $('[data-js-expanding-menu]').each(function() {
    var $menu = $(this)
    var state = {}

    // - do some initialization code on $menu
    // - bind events to $menu
    // - use `state` to manage state

If you are using onmount, this is not necessary.

Avoid side effects

Make sure that each of your JavaScript files will not throw errors or have side effects when the element is not present on the page.

$(function () {
  var $nav = $("[data-js-hiding-nav]")

  // Don't bind the `scroll` event handler if the element isn't present.
  // This will avoid making the page sluggish unnecessarily. This also
  // avoids the error that $nav[0] will be undefined.
  if (!$nav.length) return

  $('html, body').on('scroll', function () {
    if ($nav[0].disabled) return
    var isScrolled = $(window).scrollTop() > $nav.height()
    $nav.toggleClass('-hidden', !isScrolled)

If you’re using onmount, things like the $nav.length check is not necessary.

Dynamic content

There are times when you may wish to apply behaviors to content that may appear on the page later on. This is the case for AJAX-loaded content such as modal dialogs.

You can do three approaches for this:

  • Use onmount (recommended). This is the easiest and most scalable solution.
  • Use event delegation on document. This only works if you’re only binding events.
  • Wrap your initialization code in a function and run in when the modal appears (described below).

If you’re not using onmount or event delegation, you can wrap your initialization code in a function. Run that function on document.ready and when the DOM changes (eg, in the case of Bootstrap modal dialogs).

Since your initializer may be called multiple times in a page, you will need to make them idempotent by bypassing elements that it has already been applied to. This can be done with an include guard pattern.

void (function() {
function init() {
  $('[data-js-key-value-pair]').each(function () {
    var $parent = $(this)

    // an include guard to keep it idempotent
    if ($'key-value-pair-loaded')) return
    $'key-value-pair-loaded', true)

    // init code here

$(document).on('', init)

If you’re using onmount, simply bind onmount() to these events (such as Idempotency will be taken care of for you.


Keep the global namespace clean

Place your publically-accessible classes and functions in an object like App.

if (!window.App) window.App = {}

App.Editor = function() {
  // ...

Organize your helpers

If there are functions that will be reusable across multiple behaviors, put them in a namespace. Place these files in helpers/.

/* helpers/format_error.js */
if (!window.Helpers) window.Helpers = {}

Helpers.formatError = function (err) {
  return "" + err.project_id + " error: " + err.message

Third party libraries

If you want to integrate 3rd-party scripts into your app, consider them as component behaviors as well. For instance, you can integrate select2.js by affecting only .js-select2 classes.

└── javascripts/
    └── behaviors/
        ├── colorpicker.js
        ├── select2.js
        └── wow.js
// select2.js -- affects `[data-js-select2]`
$(function () {
// wow.js -- affects `.wow`
$(function () {
  new WOW().init()

Separate your vendor libs

Keep your 3rd-party libraries in something like vendor.js.

This will have browsers cache the 3rd-party libs separately so users won’t need to re-fetch them on your next deployment.

It also makes it easier to create new app packages should you need more than one (eg, one for your public pages and another for private dashboards).

/* vendor.js */
//= require jquery
//= require jquery_ujs
//= require bootstrap
/* application.js */
//= require_tree ./helpers
//= require_tree ./behaviors

Load 3rd-party resources asynchronously

For third-party resources that are loaded externally, consider loading them with an asynchronous loading mechanism such as script.js.

Some 3rd party libraries are loaded via <script> tags, such as Google Maps’s API. These vendors typically expect you to embed them like so:

<script src='//;libraries=geometry'></script>
$.onmount('.map-box', function () {
  Gmaps.buildMap({ ... })

If your website doesn’t use them everywhere, it’d be wasteful to include them on every page. You can selectively include them only on pages that need them, but that would hamper reusability: if you want to reuse the .map-box component in another page, you’ll need to re-include the script in that other page as well.

Instead, consider using script.js wrapped in a helper function to load them on an as-needed basis.

Helpers.useGoogleMaps = function useGoogleMaps (fn) {
  var url = ''

  $script(url, function () {
    // pass the global `Gmaps` variable to the callback function.

This useGoogleMaps helper can be used like so:

var useGoogleMaps = Helpers.useGoogleMaps

$.onmount('.map-box', function () {
  useGoogleMaps(function (Gmaps) {
    Gmaps.buildMap({ ... }) // use Gmaps here


Loading component files

Rails: this can be accomplished with the asset pipeline’s built-in require_tree.

// js/application.js
/*= require_tree ./behaviors
/*= require_tree ./initializers

Browserify: you can use require-globify.

require('./behaviors/**/*.js', { mode: 'expand' })
require('./initializers/**/*.js', { mode: 'expand' })

Webpack: you can use require.context to load multiple CSS files. See this StackOverflow answer for details.

function requireAll (r) { r.keys().forEach(r) }

requireAll(require.context('./behaviors/', true, /\.js$/))
requireAll(require.context('./initializers/', true, /\.js$/))

Brunch: you can use glob-brunch.

glob('./behaviors/**/*.js', (e, files) => files.forEach(require))
glob('./initializers/**/*.js', (e, files) => files.forEach(require))

/* brunch-config.js */
  plugins: {
    glob: {
      appDir: '...'


This document is a result of my own trial-and-error across many projects, finding out what patterns are worth adhering to on the next project.

This document prioritizes developer sanity first. The approaches here may not have the most performant, especially given its preference for event delegations and running document.ready code for pages that don’t need it. Regardless, the pros and cons were weighed and ultimately favored approaches that would be maintainable in the long run.

The guidelines outlined here are not a one-size-fits-all approach. For one, it’s not suited for single page applications, or any other website relying on very heavy client-side behavior. It’s written for the typical conventional web app in mind: a collection of HTML pages that occasionally need a bit of JavaScript to make things work.

As with every other guideline document out there, try out and find out what works for you and what doesn’t, and adapt accordingly.

Further reading

  • - reasonable system for CSS stylesheet structure
  • onmount - for safe, idempotent JavaScript behaviors and easy Turbolinks support
  • script.js - for loading external scripts