Say Goodbye to Frozen Interfaces!

Have you ever clicked a button on a website and watched it freeze? It’s a common experience. Frontend developers aim to build fast and responsive websites, but when the browser handles heavy tasks, the interface can slow down or stop working smoothly.

This is where Web Workers come in. They handle those heavy tasks in the background, helping the website stay smooth and responsive no matter what’s going on behind the scenes.

What Are Web Workers?

Web Workers are a browser feature that allows JavaScript to run in background threads, separate from the main UI thread. This means offloading expensive computations or data processing tasks—without blocking the interface.

In simple terms: Web Workers let your site multitask.

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Why Do We Need Web Workers?

JavaScript runs on a single thread by default. That means everything—animations, event listeners, data fetching, rendering—all happens in the same place. If one task takes too long (say, parsing a large file or crunching numbers), it can block everything else. That leads to:

• Frozen UI

• Unresponsive buttons

• Janky animations

• Poor user experience

Web Workers solve this by moving heavy lifting to a different thread, so UI remains fast and interactive.

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What Can be done with Web Workers?

Here are some real-world use cases where Web Workers shine:

1. Heavy Computation

Running CPU-intensive tasks like image processing, complex math, or data transformations? Use a Web Worker to avoid locking the UI.

2. Data Parsing and JSON Processing

Got a massive JSON response from your backend? Instead of parsing it in the main thread, pass it to a Worker.

3. Real-time Data Handling

Workers can listen to WebSockets or process live data streams (e.g., stock tickers, game engines) without interrupting the user interface.

4. Image and Video Processing

Need to filter or manipulate images in the browser? Offload those expensive operations to a Web Worker.

5. Code Compilation or Linting in Browser

Online editors like CodePen or StackBlitz use Web Workers to compile or lint code without freezing the editor.

6. Background Synchronization

Syncing data in the background (especially in PWA scenarios) without annoying the user? A perfect job for Web Workers.

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A Quick Example

Below are two examples demonstrating the impact of running heavy computations in the main thread versus offloading them to a Web Worker.

Without a worker, the loop would block your UI. With a worker? The main thread keeps running while the heavy task completes in the background.

In the Blocker example, a canvas is provided where users can draw, along with a "Run Heavy Task" button that initiates a computationally intensive loop (summing from 0 to 1e9). When this button is clicked, the main thread gets blocked, causing the UI to freeze — users are unable to draw on the canvas until the task completes.

In the Non-blocker example, the same heavy computation is offloaded to a Web Worker. This allows the main thread to remain responsive, so users can continue drawing on the canvas without any lag or freezing. Once the computation finishes, its result is displayed — without affecting the UI performance.

The complete implementation can be found in the CodeSandbox linked below

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Few things to consider

• No DOM Access: Workers can't access the DOM directly. They're for computation, not UI.

• Separate Files: Workers usually live in separate JS files (though there are tricks to inline them with blobs or bundlers like Webpack).

• iOS has limited support for Web Workers in PWAs, unlike other modern browsers. As a result, PWAs on iPhones or iPads may struggle to offload heavy tasks to background threads.

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Libraries to Use Web Workers in React/Next.js

Setting up Web Workers manually in a React or Next.js project can get tricky due to module bundlers (like Webpack or Vite). Fortunately, some excellent libraries make this process painless:

comlink

• Created by Google Chrome Labs.

• Simplifies the process of communicating with Web Workers using proxy-like syntax.

useWorker

• A React hook based specific library to run functions in background threads.

• Very clean API for React developers

With the right tools and setup, Web Workers can help developers keep their frontend smooth and responsive, even during heavy processing.

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So glad you made it here! 🙌

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