Bun Is Written in the Zig Systems Programming Language

When Bun exploded onto the JavaScript scene in 2022, developer Jarred Sumner claimed it was dramatically faster than Node and npm at everything from starting a server to installing packages. Any developer who heard those numbers wondered the same thing: how did a small project achieve that? Part of the answer is that Bun was not written in the language most people expected. Its core is built largely in Zig, a low level systems programming language that is still far from mainstream.

Zig was created by Andrew Kelley as a modern successor to C, focusing on simple syntax, strong compile time behavior, and predictable native performance. It gives a developer the low level control of C without the modern pain points, no hidden allocations, no preprocessor, and no implicit conversions that silently surprise you. For someone writing a JavaScript runtime, that combination is exactly what you want. You need to read every byte, manage every allocation, and still keep the codebase maintainable.

The choice of Zig explains a lot about why Bun feels fast. A JavaScript runtime spends its life at the boundary between your high level code and the operating system, parsing source, resolving imports, reading files, and speaking to the network. Getting those operations right and fast requires tight control over memory and system calls, which is precisely the kind of work at which a low level language excels. Building that plumbing in something high level would have added an unavoidable layer of overhead.

Bun does not stop at Zig. It also leans on JavaScriptCore, the engine from WebKit, rather than the V8 engine used by Node. So the runtime is a blend of careful native code in Zig and a battle tested JavaScript engine. The result is a toolkit that combines a runtime, a bundler, a test runner, and a package manager in one product, all sharing the same fast native foundation instead of calling out to separate tools.

For senior developers, Bun is a compelling example of using the right tool for a performance critical job. Most JavaScript developers will never write the runtime itself, but understanding that a speed focused project chose Zig over C or C++ tells you something about the modern systems landscape. Zig offers the performance of a compiled language with a cleaner developer experience, and projects like Bun act as a real world proof that the approach works at scale.

The practical takeaway is less about which runtime you adopt and more about how technology stacks get their speed. When a tool promises a big performance win, it is worth asking what runs underneath. In Bun's case the answer is a deliberate, focused native core written in Zig. That decision, more than any marketing number, is the real reason the toolkit can be so fast, and it is a good reminder that the foundation of a tool usually determines what it is capable of.