TL;DR: JavaScript is single-threaded, until it isn't. Web Workers, SharedArrayBuffer , and Atomics let you break out of the main thread and do real parallel work. This article covers everything from spawning your first worker to coordinating shared memory safely with atomic operations. The Myth of the Single Thread Every JavaScript developer has heard it: "JS is single-threaded." This is true for your application code on the main thread, but it's never been the full story. The browser itself is heavily multi-threaded. The compositor, the network stack, the GPU process, all parallel. What was missing was a first-class API to let your JavaScript code participate in that parallelism. If you're evaluating how concurrency fits into your broader web application architecture , understanding browser threading primitives is foundational, especially for performance-sensitive products.…