Bun, Why Are We Trying to Replace Node.js?

If you’ve spent any time in the JavaScript ecosystem over the last year, you’ve heard the name Bun thrown around, usually with a benchmark chart attached, showing it running circles around Node.js. But speed alone doesn’t explain why a runtime gets attention. To actually understand what Bun is doing, you have to ask a more basic question first:

Why do we even have this many separate tools in the first place?

1. The Node.js Era: How We Got Here

Node.js solved an important problem: it allowed JavaScript to run outside the browser, on servers. It uses the V8 engine along with an event-driven, non-blocking I/O model.

This idea of JavaScript everywhere is one of the main reasons frontend developers started using JavaScript for backend tools as well. The same language could now be used to build UIs, run development servers, create build scripts, and even build APIs.

But Node.js itself was only a runtime. It could run JavaScript, but it did not provide all the tools developers needed.

So, over time, different tools were created for different needs:

• Share and manage packages: npm was created as a package registry and CLI.
• Run packages without installing them globally: npx was introduced later.
• Test your code: Node.js did not initially have a built-in testing solution, so tools like Jest, Mocha, and Vitest became popular.
• Bundle and transform code: Node.js did not handle browser bundling, tree-shaking, or transpilation by itself. Tools like Webpack, Rollup and later Vite filled this gap.

The important thing is that these tools were not created as one complete system. They were added over time as new problems appeared.

As a result, modern frontend projects often depend on many different tools, each with its own configuration and way of working. That is why you might see files like package.json, jest.config.js and vite.config.js, along with a large node_modulesfolder.

So, Node.js solved the JavaScript execution problem, but it did not solve the tooling fragmentation problem.

And that is exactly where Bun comes in.

2. What Exactly Is Bun?

Bun has a simple idea: instead of using separate tools for different parts of frontend development, why not combine them into one?

Bun brings the runtime, package manager, test runner and bundler together in a single tool.

Here’s what Bun provides:

• Runtime: Runs JavaScript and TypeScript directly. In many cases, you don’t need a separate tool like ts-node.
• Package manager: bun install can replace npm install, yarn, or pnpm. It also works with the existing package.json.
• Test runner: bun test is built into Bun and has a Jest-compatible API, so existing tests can often be moved with little change.
• Bundler: bun build can bundle JavaScript, TypeScript, and JSX for production, similar to tools like Webpack and esbuild.

The main idea is not that every feature in Bun is completely new. Tools like Jest, Vite, and Webpack are already mature and widely used.

The advantage of Bun is simplicity.

Instead of managing multiple tools, configurations and dependencies, you can use one tool for most of these tasks. This can mean fewer config files, fewer dependencies and fewer compatibility issues between different tools.

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Bun is trying to bring the frontend tooling ecosystem under one roof.

3. Why Is Bun Fast?

It’s useful to understand why Bun is fast, without going too deep into compiler-level details. There are a few main reasons:

1. JavaScriptCore instead of V8

• Node.js uses V8, the JavaScript engine used by Chrome. Bun uses JavaScriptCore (JSC), the engine behind Safari.
• JSC has traditionally focused more on fast startup, while V8 is highly optimized for long-running applications.
• This can make a difference in development tools where applications are started and stopped frequently.

2. Native implementation

• Bun was originally built using Zig, which played a big role in its early performance.
• In 2026, Bun started moving parts of its runtime to Rust. This includes the layer responsible for things like Node.js APIs, the package manager, and the bundler.
• The JavaScript engine itself is still JavaScriptCore. Rust is simply replacing parts of the native layer around it.
• Whether using Zig or Rust, the main idea is the same: Bun handles many operations natively instead of relying on JavaScript running on top of Node.js.
• This reduces overhead for tasks like file operations, hashing and dependency resolution.

3. Faster startup

• A lot of the time spent in frontend development isn’t actually spent executing JavaScript. It can be spent starting different tools.
• For example, a workflow might need to start Node.js, then npm, then a transpiler, and then a test runner.
• Each tool has its own startup time. Bun combines many of these tools into a single process, which means fewer processes need to start separately.

4. Built-in TypeScript and JSX support

• Bun can run TypeScript and JSX directly without requiring a separate Babel or ts-node setup in many cases.
• The transpiler is built into Bun itself rather than being another JavaScript package installed through node_modules.

Bun isn’t simply “V8 but faster.”

It takes a different approach by combining multiple tools into one system and implementing many of the core operations natively.

Fewer tools + fewer processes + less overhead = faster development workflows.

4. Plot twist: Bun rewrote itself in Rust — using AI agents

Here’s the part of the Bun story that’s more interesting than any benchmark chart.

For its first three years, Bun’s whole identity was tied to Zig. Its creator, Jarred Sumner, had picked Zig deliberately: small language, no garbage collector, close to the metal, good fit for a tiny team trying to move fast. It worked. Zig was the story people told about why Bun was fast.

Then, in 2026, that story flipped. Sumner rewrote the entire runtime, over a million lines of code — from Zig into Rust. The eyebrow-raising part isn’t the language switch itself (plenty of systems software has migrated languages before). It’s how it happened:

• The port was done in about 11 days.
• It was carried out largely by a fleet of Claude agents working in parallel, not a team of engineers hand-translating file by file.
• The estimated API cost was around $165,000.
• The result passed 99.8% of Bun’s existing test suite on Linux x64 — the first supported target, with macOS, Windows and ARM following afterward.

Why rewrite a working system?

If Zig was already working well, why move to Rust?

• Bun was growing. Zig worked well when Bun was smaller, but as the project grew, it needed more contributors.
• Rust has a much bigger developer community. It is easier to find developers who already know Rust than developers with deep Zig experience.
• Rust is widely used for systems programming. Many developers and projects already use it, so moving to Rust could make Bun easier to maintain and contribute to in the long run.
• The goal wasn’t simply to make Bun faster. The main idea was to make the project easier for a larger group of developers to work on.
• What made the story unusual was the speed. Rewriting more than a million lines of code would normally take a long time. Bun’s rewrite happened in around 11 days, with a lot of help from AI agents.
• But this also raised concerns. Zig creator Andrew Kelley criticized the rewrite as “unreviewed slop.” His point was that even if the code passes tests, it doesn’t mean humans have carefully reviewed every part of a huge AI-generated codebase.
• Why this is worth knowing, beyond trivia: it’s a live case study in a question a lot of engineering orgs are quietly asking right now:

If an AI agent can port a mature codebase to a new language in under two weeks and pass 99.8% of your tests, how much of that do you trust without a human having read it?

That’s what makes Bun’s rewrite interesting. It’s not just about Zig vs Rust. It’s also a real-world example of how AI agents could change the way large software projects are built, migrated and maintained.

5. Should You Actually Use Bun?

Bun is fast and increasingly mature, but faster doesn’t automatically mean better for every project.A few practical takeaways:

1. Starting a new project?
   If you want fewer config files, less tooling glue and faster local development, Bun is a strong option.
2. Already using Node.js?
   If your project depends heavily on Node APIs, native modules, framework tooling, or Node-specific infrastructure, migrating only for speed may not be worth the compatibility risk.
3. You can adopt Bun gradually.
   For example, you can try bun install as a package manager while continuing to use Node.js as your runtime. This lets you get some of Bun's benefits without changing the entire stack.
4. The bigger question isn’t just speed.
   The interesting question is whether frameworks, native modules, cloud platforms, and the wider ecosystem will adopt Bun as deeply as they have adopted Node.js.

The Real Takeaway

Instead of having a separate runtime, package manager, test runner and bundler, Bun brings these capabilities together in one integrated tool.

So the real question isn’t just “Is Bun faster?” It’s whether the ecosystem will adopt this integrated approach as widely as it adopted Node.js.

For now, Bun is definitely worth watching — and for new projects, it may already be worth trying.

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