🛒 The Event Loop Explained: Why Your Shopify Store’s Speed Depends on It

And what wishlist clicks have to do with coffee orders

Imagine this — you're at a busy café. There's one barista. You walk up, place your order, and wait. Behind you, a long queue forms. Now imagine the barista does everything — takes orders, brews coffee, bakes cookies, even washes dishes.

This, friends, is JavaScript's event loop in action.

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☕ The JavaScript Barista

JavaScript is single-threaded — meaning it has just one worker handling all requests. So how does it manage hundreds of tasks (browser events, API calls, button clicks) without grinding to a halt?

Here’s the simplified flow:

• Call Stack → Executes synchronous tasks

• Web APIs → Browser helpers (setTimeout, fetch, DOM events)

• Callback Queue → Holds asynchronous callbacks

• Event Loop → Continuously checks: “Is the call stack free? If yes, dispatch the next callback!”

Think of it like a solo barista: they process one order, finish it, then move on — unless they get stuck making a 2-minute latte while someone’s trying to buy some cookies.

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🛍️ Why It Matters for Shopify

Shopify storefronts and apps are JavaScript-heavy, especially for:

• Wishlist clicks

• Real-time cart syncs

• Debounced product searches

• Analytics and personalization tracking

A single missed async deal — like a blocking loop or non-deferred script — can kill your speed, hurt LCP, and break your Buy Button responsiveness.

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🍩 Analogy: Shopify Store = JS Café

During peak times (BFCM rush?), your JS code absolutely must not clog the order line.

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🧠 Tips for Shopify Devs

1. Avoid blocking the call stack

Bad

 for (let i = 0; i < 1e9; i++) { // heavy work } 

Good:

setTimeout(() => { // break work into chunks }, 0);

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2. Debounce high-frequency events

const debounce = (fn, delay) => { let timeout; return (...args) => { clearTimeout(timeout); timeout = setTimeout(() => fn(...args), delay); }; };

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3. Parallelize with Promise.all

const [product, recs] = await Promise.all([ fetchProduct(), fetchRecs() ]);

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4. Use requestIdleCallback

For non-essential tasks like analytics or syncing:

requestIdleCallback(() => { // background work });

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5. Measure performance

Use:

• Chrome DevTools → Performance tab

• Shopify’s Theme Inspector for Chrome

• PerformanceObserver and User Timing APIs

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🔁 Wrapping It Up: The Barista Never Sleeps

Every line of JavaScript you write for Shopify adds orders to the queue. The event loop will process them — but only if you code respectfully.

Ask yourself:

“Am I helping the barista, or making them sprint under pressure?”

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✅ TL;DR

• JavaScript = single-threaded barista

• The event loop keeps tasks moving one at a time

• Don’t block the main thread with heavy sync work

• Use async/defer wisely

• Optimize your code path for responsiveness

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If you’re new to how the event loop actually works under the hood, start here:

🎬 Watch:
What the heck is the event loop anyway? – Philip Roberts @ JSConf EU
Still one of the best beginner-friendly talks out there.

🛠️ Try:
JS Event loop Visualiser
A free interactive visualizer where you can type something like:

console.log("Barista starts the day");

setTimeout(() => {
  console.log("Brews coffee");
}, 0);

Promise.resolve().then(() => {
  console.log("Marks loyalty card");
});

console.log("Takes next order");

...and actually see how the call stack, Web APIs, and callback queue work in real time.

🔍 What You'll See in When you run it:

• console.log("Barista starts the day") runs synchronously → printed first.

• setTimeout(..., 0) is asynchronous → gets sent to the Web APIs → queued.

• Promise.then() is a microtask → queued in the microtask queue → runs before setTimeout.

• console.log("Takes next order") is still synchronous, so it prints second.

• Finally, the microtask ("Marks loyalty card") runs.

• And then the macrotask ("Brews coffee") runs.

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☕ This is just the first pour.

Coming up next:

• How to visually inspect event loop performance using DevTools