Bun has been making noise in the JavaScript community for a while now, but 2025 was the year it started appearing in production stacks I actually interact with. After using it across several projects, I want to give a grounded overview of what Bun does well, where it’s still rough, and whether you should consider it for your next project.

What Is Bun, Exactly?

Bun is a JavaScript runtime, package manager, bundler, and test runner rolled into one binary. It’s written in Zig and uses JavaScriptCore (the engine behind Safari) instead of V8 (the engine behind Node.js and Chrome). The single-binary approach means you don’t need separate tools for running code, installing packages, bundling assets, and running tests.

Installing Bun is straightforward:

curl -fsSL https://bun.sh/install | bash

On macOS, brew install oven-sh/bun/bun works too.

Speed: The Headline Feature

Bun’s marketing centres on performance, and the benchmarks are real. Package installation is dramatically faster than npm. On a project with 400 dependencies, bun install completes in under 3 seconds on my MacBook Pro. The equivalent npm install takes around 25 seconds.

Script execution is also faster. A “Hello World” Node.js script takes roughly 30ms to start. Bun manages it in about 7ms. This matters less for long-running servers but is noticeable for CLI tools and scripts that start and stop frequently.

The HTTP server performance is impressive too. Bun’s built-in Bun.serve() handles roughly 2-3x more requests per second than Node.js with Express in synthetic benchmarks. Real-world applications with database calls and business logic see smaller but still meaningful gains.

Node.js Compatibility

This is where things get interesting. Bun aims for full Node.js compatibility, and it’s close but not quite there. The common APIs work: fs, path, http, crypto, child_process, Buffer, streams. Most npm packages install and run without modification.

The gaps appear in less common APIs and edge cases. Some native Node.js addons don’t compile with Bun. Certain stream behaviour differences can cause subtle bugs. A few popular packages have Bun-specific issues tracked in their repositories.

For a practical test, I tried running three of my existing Node.js projects with Bun without any code changes:

1. Express API with Prisma: Worked perfectly. All routes functional, database queries correct.
2. Next.js application: Partially worked. The dev server ran but some middleware behaved differently.
3. CLI tool using Commander.js: Worked without issues.

Your mileage will vary depending on your dependency tree, but straightforward applications have a good chance of running unmodified.

The Built-in Bundler

Bun includes a bundler that replaces tools like webpack, esbuild, or Rollup for many use cases. It supports TypeScript and JSX out of the box, tree shaking, code splitting, and source maps.

bun build ./src/index.ts --outdir ./dist --minify

For simple bundling tasks, this works well and is extremely fast. For complex configurations — custom plugins, advanced code splitting strategies, CSS processing — you might still need a dedicated bundler. The Bun bundler’s plugin API is growing but isn’t as mature as webpack’s or Rollup’s.

Bun’s Test Runner

The built-in test runner uses a Jest-compatible API, which means existing tests often work with minimal changes:

import { expect, test, describe } from "bun:test";

describe("math", () => {
  test("adds numbers", () => {
    expect(2 + 2).toBe(4);
  });
});

Test execution is fast — noticeably faster than Jest, roughly comparable to Vitest. The main limitation is the mocking API, which is more limited than Jest’s. Complex mocking scenarios might require workarounds.

SQLite Built In

One of my favourite Bun features is the built-in SQLite driver. No npm package needed:

import { Database } from "bun:sqlite";

const db = new Database("myapp.db");
const users = db.query("SELECT * FROM users WHERE active = 1").all();

This is significantly faster than the better-sqlite3 npm package because there’s no N-API bridge overhead. For applications that use SQLite as their database, Bun is an excellent runtime choice.

Should You Adopt Bun?

For new projects where you control the entire stack, Bun is worth trying. The speed improvements are real, and the all-in-one approach reduces tooling complexity. Start with a side project or internal tool to get familiar with the differences.

For existing production Node.js applications, I’d recommend a cautious approach. Run your test suite with Bun first. If everything passes, try it in a staging environment. Don’t switch production runtimes based on benchmark charts alone.

Bun is a legitimate alternative to Node.js, not a replacement. The JavaScript ecosystem is better for having competition, and Bun’s focus on performance is pushing Node.js to improve as well. The choice between them increasingly comes down to specific project needs rather than one being objectively superior.