Enhancing The Efficiency Of Shopify Themes (Case Research)

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The dreaded refactor of outdated code will be difficult. Code evolves over time with extra options, new or altering dependencies, or perhaps a objective of efficiency enhancements. When tackling an enormous refactor, what are the issues it’s best to give attention to and what efficiency enhancements are you able to count on?

I’ve been constructing Shopify themes for the higher a part of a decade. Once I labored in-house at Shopify in 2013, themes have been pretty easy by way of code complexity. The toughest half was that Shopify required themes to help IE8, and up till late 2020, IE11. That meant there was quite a lot of trendy JavaScript we couldn’t make the most of with out generally sizable polyfills.

Eight years later, in 2021, themes are infinitely extra complicated as a result of Shopify has launched a ton of latest options (to associate with our in-house concepts at Archetype Themes). The issue is that constructing new performant options will solely go up to now when a few of your codebase is so outdated that it has outdated IE polyfills or IE10 CSS hacks. Our themes had fairly good velocity scores for a way a lot they provided, however they have been undoubtedly bloated.

Our Objective Was Easy

Higher efficiency throughout the board. Quicker time to first paint. Much less blocking JS. Much less code complexity.

Getting there was the onerous half. It included:

Take away jQuery and rewrite ~6k traces of JS per theme in Vanilla JS
Take away Handlebars.js, as our templating wants have been method too small for such a big bundle
Standardizing code shared between themes (take away duplication)

Transferring away from jQuery was a blessing, however an extended course of. Fortunately, Tobias Ahlin has a implausible information on among the fast conversions away from jQuery. Whereas going by means of these modifications, it was the proper time to rethink some extra fundamental points like how my JS was structured and the way parts have been initialized.

Take away jQuery

Writing Vanilla JS at all times appeared like a pipe dream. We needed to help outdated IE, so it was simply really easy to disregard any try at eradicating it. Then IE 11 help was dropped by Shopify and the clouds parted — it was our time.

Why take away jQuery anyway? I’ve heard plenty of arguments about this, reminiscent of its bundle dimension isn’t that dangerous in comparison with a framework like React. Effectively, jQuery isn’t a framework like React so it’s a little bit of a non-starter comparability. jQuery is a method of utilizing CSS-like selectors and developer-friendly syntax for issues like animations and Ajax requests. Most of all, it helped with cross-browser variations so builders didn’t have to consider it.

We needed to take away it for a couple of causes:

Much less JS is nice for efficiency;
It isn’t wanted in trendy browsers;
Shopify’s CEO made a push for pure JS in themes.

I’m a type of builders who have been caught up to now. I knew jQuery inside and outside and will make it pull off almost something I attempted. Was it excellent? No, after all not. However while you take a look at the lifecycle of some JS frameworks that flamed out, jQuery has at all times been regular and that was acquainted and protected to me. Eradicating our reliance on it and untangling it from ~6k traces of code (for every theme) felt insurmountable — particularly once I couldn’t know for positive my efficiency scores would profit or by how a lot.

Our strategy was to remark out every module we had, take away jQuery, and slowly add in every module or perform separately whereas it was rewritten. We began with the best file, one with a couple of features and some selectors. Good and simple, no errors in dev instruments, time to maneuver on.

We did this one after the other, remembering the simple fixes from the early information once we acquired to the complicated ones like refactoring the entire potential options related to a product and its add-to-cart kind (I counted, it’s 24 distinctive issues). Ultimately, we acquired the product JS from 1600 traces of code to 1000. Alongside the way in which, we discovered higher methods to do some issues and would return and refactor as wanted.

We realized Vanilla JS isn’t scary, it’s only a bit extra of an intentional method of writing code than jQuery. We additionally realized some historical code was a multitude — we wanted to arrange the JS to be extra modular and take away duplicate code (extra on that under). However earlier than that, we needed to play with among the enjoyable JS we’d solely utilized in different tasks.

Intersection Observer API

Shopify themes are highly effective in that they let retailers transfer parts across the web page nevertheless they need. Which means, because the developer, you don’t know the place the component is, whether or not it exists, or what number of exist.

To initialize these parts, we had been utilizing scroll occasions that constantly checked if a component was seen on the web page with this perform:

theme.isElementVisible = perform($el, threshold) {
var rect = $el[0].getBoundingClientRect();
var windowHeight = window.innerHeight || doc.documentElement.clientHeight;
threshold = threshold ? threshold : 0;

// If offsetParent is null, it means the component is completely hidden
if ($el[0].offsetParent === null) {
return false;

return (
rect.backside >= (0 – (threshold / 1.5)) &&
rect.proper >= 0 &&
rect.prime <= (windowHeight + threshold) &&
rect.left <= (window.innerWidth || doc.documentElement.clientWidth)

Despite the fact that these scroll occasions have been throttled, there was quite a lot of math being performed by the browser on a regular basis. It by no means actually felt too sluggish, but it surely did take up a spot within the name stack which impacted different JS competing for precedence. I want we had performed extra efficiency analysis on this replace particularly as a result of I feel it’s accountable for most of the enhancements in Time to interactive and Whole blocking time that you just’ll see under.

In comes the Intersection Observer API. Now that IE11 help wasn’t required, I used to be so completely happy to have the ability to absolutely make the most of this. In brief, it’s an asynchronous method of figuring out when a component is seen within the window. No extra sluggish measurements and scroll occasions.

To initialize a component when it’s seen, we use one thing so simple as this:

component: doc.querySelector(‘div’),
callback: myCallback

The entire JS required for the component shall be dealt with inside myCallback, stopping it from doing something till it’s seen.

This units up an observer for that component, after which removes the observer as soon as it’s seen. It’s at all times good to scrub up after your self even should you assume there may not be a lot affect with out it. If there’s a callback, we run it and our module is able to go.

theme.initWhenVisible = perform(choices) {
var threshold = choices.threshold ? choices.threshold : 0;

var observer = new IntersectionObserver((entries, observer) => {
entries.forEach(entry => {
if (entry.isIntersecting) {
if (typeof choices.callback === ‘perform’) {
}, {rootMargin: ‘0px 0px ‘+ threshold +’px 0px’});


You possibly can go a threshold to initialize the component earlier than it’s on the display too, which will be useful if you wish to preload one thing like Google’s Map API barely earlier than the component is seen so it’s prepared when it’s.

Layzloading Pictures And object-fit

We use lazysizes for lazy-loading our photographs. It has some useful plugins for additionally loading background photographs, however requires much more markup in your component. Whereas the plugins are fairly small, it’s yet one more factor that’s simply eliminated with pure CSS.

Utilizing object-fit in CSS meant that we might place a picture identical to a background picture, however as an <img> component and get all the advantages of regular lazy-loading with out further JS. The actual profit in that is we’re one step nearer to utilizing native browser lazy-loading (which doesn’t help background photographs). We’ll nonetheless should load in lazysizes as a fallback when the native strategy isn’t supported, but it surely means eradicating a whole dependency.

if (‘loading’ in HTMLImageElement.prototype) {
// Browser helps `loading`
} else {
// Fetch and initialize lazysizes

MatchMedia API

Up to now, we used enquire.js to know when breakpoints modified. That is used when resizing parts, altering a module’s arguments for desktop vs cell, or just to indicate/conceal parts you can’t with CSS.

As an alternative of counting on one other bundle, as soon as once more we are able to go along with a local resolution in matchMedia.

var question = ‘display and (max-width:769px)’;
var isSmall = matchMedia(question).matches;

matchMedia(question).addListener(perform(mql) {
if (mql.matches) {
isSmall = true;
doc.dispatchEvent(new CustomEvent(‘matchSmall’));
else {
isSmall = true;
doc.dispatchEvent(new CustomEvent(‘unmatchSmall’));

With only a few traces of code, we are able to pay attention for breakpoint modifications and alter a useful variable that’s used elsewhere and set off a customized occasion that particular modules can pay attention for.

doc.addEventListener(‘matchSmall’, perform() {
// destroy desktop-only options
// initialize mobile-friendly JS

Searching down duplicate code

As I discussed originally, we had slowly constructed options into our themes for years. It didn’t take lengthy for some parts to be constructed out that have been type of like others, like a full-width homepage video and later movies in your product itemizing or a popup video modal.

YouTube’s API, for instance, initialized in a different way 3 times and had almost an identical callbacks and accessibility options constructed out per-module. It was a bit embarrassing we didn’t construct it smarter within the first place, however that’s how you recognize you’re rising as a developer.

We took this time to consolidate lots of our modules to be standalone helpers. YouTube grew to become its personal technique that each one sections from all of our themes might use. It meant refactoring by breaking it down into probably the most fundamental components:

Default API arguments (overridable by the initializing module)
A div ID to initialize the video onto
ID of the YouTube video to load
Occasions (API is prepared, video state modified, and so forth)
Play/pause when not in view
Deal with iOS low energy mode when autoplay not supported

My strategy was to do that all on paper earlier than coding, which is one thing that at all times helps me type out what’s integral to the module I’m constructing vs what’s customized by the mother or father that’s initializing it — a division of labor if you’ll.

Now our three themes that initialize YouTube movies a complete of 9 other ways use a single file. That’s an enormous code complexity win for us, and makes any future updates a lot simpler for me and different builders which may contact the code. By utilizing this similar strategy for different modules whereas changing to Vanilla JS, it allowed us to maneuver almost half of every theme’s JS to a single shared module throughout all of them.

That is one thing that was invaluable to our group and our multi-project setup and may not be helpful to your tasks precisely, however I imagine the method is. Serious about simplicity and avoiding duplication will at all times profit your mission.

We did the identical for slideshow modules (picture slideshows, testimonials, product web page photographs, announcement bars), drawers and modals (cell menus, cart drawers, e-newsletter popups), and plenty of extra. One module has one goal and can share again to the mother or father solely what’s required. This meant much less code shipped, and cleaner code to develop with.

Efficiency Stats

Lastly, the good things. Was this all value it? Most of this was performed blindly with the belief that much less JS, smarter initializing, and extra trendy approaches would end in sooner themes. We weren’t dissatisfied.

We began all of this work with Movement, our first theme. It had probably the most bloated JS and the largest room for enchancment.

52% much less JS shipped
Desktop dwelling web page speeds (with heavy parts like a number of movies, featured merchandise, slideshows with giant photographs)

Desktop dwelling web page
Earlier than

Lighthouse rating

Whole blocking time

Time to interactive

Largest contentful paint

Cell product pages

Cell product web page
Earlier than

Lighthouse rating

Whole blocking time

Time to interactive

Largest contentful paint

Then we moved on to Impulse, our second and most feature-heavy theme.

40% much less JS shipped
28% sooner cell dwelling web page speeds

Desktop dwelling web page
Earlier than

Lighthouse rating

Whole blocking time

Time to interactive

Largest contentful paint

30% sooner cell dwelling web page and product web page speeds

Cell product web page
Earlier than

Lighthouse rating

Whole blocking time

Time to interactive

Largest contentful paint

Whilst you might discover these numbers acquired lots higher, they’re nonetheless not nice. Shopify themes are handcuffed by the platform so our place to begin is already difficult. That may very well be a completely separate article, however right here’s the overview:

Shopify has quite a lot of overhead: function detection, monitoring, and fee buttons (Apple Pay, Google Pay, ShopPay). Should you’re on a product web page with dynamic fee buttons you will be about 187kb of Shopify scripts vs. 24.5kb theme information. Most websites could have Google Analytics, and perhaps a Fb Pixel or different monitoring scripts loaded on prime of all this.

The excellent news is that these scripts are loaded pretty effectively and most don’t block the web page rendering a lot. The dangerous information is that there is nonetheless quite a lot of JavaScript loading on these pages which are out of the theme’s management and trigger some flags on Lighthouse scores.

Apps are an enormous bottleneck and retailer homeowners, typically, do not know. We routinely see outlets with 20+ apps put in, and even a easy app can drop your Shopify velocity rating by 10+ factors. Right here’s the breakdown of our Impulse theme with three apps put in.

Right here’s an ideal case examine on apps and their impact on efficiency.

We’re nonetheless within the means of ending these updates to our third theme, Streamline. Streamline additionally has another efficiency options in-built that we’re exploring including to our different themes, reminiscent of loadCSS by Filament Group to stop the CSS from being a render-blocking useful resource.

These numbers aren’t insignificant. It’s extensively reported that velocity issues and even small modifications could make huge impacts. So whereas we’re pleased with all of this progress, it’s not the top. Efficiency will proceed to be a dominant a part of our builds and we gained’t cease searching for extra methods to simplify code.

What’s Subsequent?

Efficiency is an ongoing problem, one we’re excited to maintain pushing on. Just a few issues on our listing are:

Use resize observer as a substitute of window occasions
Transfer absolutely to native browser picture lazy-loading (with lazysizes fallback for Safari)
Solely load JS that’s used on the present web page so we don’t ship an enormous file the time (an enormous problem on Shopify proper now)
Enhance our efficiency information with Smashing’s 2021 efficiency guidelines
Setup Lighthouse in GitHub actions to see efficiency impacts as we develop

Sources For Shopify Builders

Should you’re constructing on Shopify, or need to get began, listed below are some useful assets for you:

Arrange an area theme growth workflow
Theme Inspector Chrome extension to debug bottlenecks in Liquid
Shopify cheat sheet
What liquid is on the market to you
Liquid templating language docs
Just a few years in the past these have been painful to get by means of, however they’ve since turn into gold for Shopify builders

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