Node.js 24 is LTS, Bun ships 1.3.14, and Deno 2.7 adds parallel serving in 2026. Here
What “concurrency” means in each runtime
All three run JavaScript and TypeScript on a single main thread for app logic, then push work off that thread through different escape hatches. Concurrency here is not “run everything in parallel on the CPU.” It is how the runtime schedules I/O, timers, and background work without blocking request handling, and how it lets you opt into real multi-core work when you need it.
Node.js 24, now on LTS, keeps the familiar event loop, libuv thread pool for certain filesystem and crypto operations, and worker threads for CPU-bound jobs. Bun aims for a faster default path on the same mental model—async I/O first, workers when you must. Deno 2.7’s parallel serving is a different bet: scale request handling across isolates or workers more deliberately, so multi-core use is a first-class server feature rather than something you wire only after you hit a wall.
Where each model fits day-to-day work
Most web APIs spend time waiting on databases, caches, and HTTP clients. On that workload, the event loop is enough if you never block it with heavy CPU or synchronous I/O. Choose Node.js 24 when you want LTS stability, a huge ecosystem, and concurrency patterns your team already knows: Promise-based APIs, worker threads for isolation, and cluster or process managers for multi-core HTTP.
Choose Bun when startup time, install speed, and a batteries-included toolchain matter as much as raw concurrency design—still async-first, with workers available when a request path becomes CPU-bound. Choose Deno when you want stronger defaults around permissions and TypeScript, and when parallel serving in 2.7 matches how you want to scale: spread load across cores without inventing your own process topology from scratch.
- I/O-heavy services: Prefer clean async code on one process; add workers only for hot CPU paths.
- CPU-heavy pipelines: Isolate work (workers or separate processes) so the loop stays free for connections.
- Multi-core HTTP: Use Node’s process model, Bun’s equivalent patterns, or Deno’s parallel serving depending on how much you want the runtime to own fan-out.
Practical rules that hold across runtimes
Never block the main thread with large JSON parsing of huge payloads, tight crypto loops, or synchronous disk reads on the request path. Measure before splitting: if latency is wait time, more workers will not help and can hurt under memory pressure. Share nothing mutable between workers; pass messages or use shared memory only with a clear ownership model.
For production, treat concurrency as three layers: (1) non-blocking I/O on the event loop, (2) bounded worker or process pools for CPU, (3) horizontal scale behind a load balancer. Node.js 24 LTS is the safe default for long-lived services. Bun 1.3.14 is a strong fit when developer velocity and a single runtime toolset outweigh ecosystem inertia. Deno 2.7 is worth a serious look when parallel serving reduces the custom glue you would otherwise write to use multiple cores.
How to decide without chasing benchmarks
Write a thin spike that hits your real bottlenecks—DB round-trips, JWT verification, image or PDF work—not a synthetic “hello world” flood. Keep the same application shape across candidates so you compare scheduling and tooling, not rewrite quality. Prefer the runtime whose concurrency model matches how you already operate: process supervisors and worker threads (Node), all-in-one local workflow (Bun), or built-in parallel request serving (Deno).
If the team is Node-native and needs LTS for years, stay on Node.js 24 and invest in non-blocking design. If greenfield tooling friction is the main tax, try Bun. If multi-core HTTP without a custom cluster story is the goal, evaluate Deno 2.7’s parallel serving against your deploy model. Concurrency wins come from not blocking the loop and from clear isolation boundaries—not from picking a logo.