Multithreading in JavaScript: A Guide to Web Workers

About this talk

JavaScript is single-threaded — until it isn't. In this talk at WeAreDevelopers World Congress 2025, we explored the Web Workers API as a practical tool for moving heavy computation off the main thread, keeping UIs responsive under load. From the basics of thread communication through `postMessage` to real-world patterns like offloading data processing, the session walked through live demos showing the before-and-after impact on frame rate and interaction latency.

Key takeaways

• Why the single-threaded model breaks down under CPU-intensive workloads
• How Web Workers communicate with the main thread via structured cloning and Transferable objects
• Practical patterns: offloading parsing, encryption, and heavy data transformations
• Measuring the real impact on responsiveness with DevTools and performance profiles