How Bun’s Native fetch Works Internally And Why It’s Faster Than Node.js or Deno for Backend Development

Introduction

Bun is a modern JavaScript runtime built from scratch to be fast, efficient, and developer-friendly. Unlike Node.js or Deno, which rely on Google's V8 engine, Bun is powered by JavaScriptCore the engine behind Safari and is written in Zig, a low-level systems programming language that helps squeeze out high performance from the runtime.

What sets Bun apart is how deeply integrated it is. It doesn’t just run JavaScript it also includes a built-in bundler, test runner, package manager, and native support for common APIs like fetch. For backend developers, this means fewer dependencies, less configuration, and better performance out of the box.

One of the most important features in any modern JavaScript environment is the fetch API. This article focuses on how Bun implements fetch, how it compares to Node.js and Deno, and why Bun might be a better choice for backend development in many cases.

What Is Fetch, Really?

Before we dive into Bun, it’s worth understanding what fetch actually is. Originally introduced in browsers, fetch is a modern way to make HTTP requests in JavaScript. It replaced older approaches like XMLHttpRequest and is now the standard for making network requests in frontend code.

The fetch API is defined by a community group called WHATWG short for the Web Hypertext Application Technology Working Group. This group is responsible for maintaining and evolving core web platform features, including the HTML and DOM standards. In simple terms, they write the spec that defines how things like fetch, Request, Response, and Headers should behave across environments.

The idea behind having fetch in runtimes like Bun, Node.js, or Deno is to make the backend environment feel more like the browser. That way, code becomes more portable and familiar, especially for full-stack or frontend developers who are transitioning to backend work.

Bun’s Fetch - Built In and Optimized

Bun implements fetch as a first-class citizen. It’s built into the runtime, just like in browsers, and doesn’t require any setup, imports, or third-party libraries. Under the hood, Bun uses its own HTTP client written in Zig and integrates it tightly with JavaScriptCore’s event loop.

This tight integration avoids the overhead seen in environments like Node.js, which have to wrap older low-level modules like http or https. Bun’s implementation focuses on being:

• Fast: Thanks to zero-copy internals and no unnecessary intermediate buffers.

• Spec-compliant: Behaves like browser fetch, supporting features like streaming, redirect policies, and custom headers.

• Minimal: Smaller memory footprint and faster startup time compared to polyfill-based approaches.

Here’s how simple it is to use:

const response = await fetch("https://api.example.com/data");
const json = await response.json();
console.log(json);

It just works no flags, no extra packages, no boilerplate.

Comparing Bun, Node.js, and Deno

Node.js

Node.js didn’t originally have fetch. It was added in version 18 and is now stable, but under the hood, it still depends on its legacy http module and internal stream handling. While this approach works, it adds complexity and overhead.

Performance-wise, Node’s fetch is slower, and its behavior sometimes deviates from the browser spec especially around streaming responses, error handling, and timeouts. You might also encounter memory bloat if you’re handling many concurrent requests.

Deno

Deno, built by the original creator of Node.js, was designed with fetch in mind from the beginning. It’s more web-compatible than Node and uses Rust under the hood, paired with V8. Deno enforces security defaults like blocking network access unless you explicitly allow it which is great for sandboxed environments but can slow down local development.

Deno’s fetch is closer to the browser, but its performance tends to lag behind Bun in real-world benchmarks.

Bun

Bun takes the best parts of both and improves on them:

• It supports fetch natively, no polyfills needed.

• It uses a faster engine (JavaScriptCore) and a custom HTTP client written in Zig.

• It handles concurrency efficiently with minimal memory usage.

• It supports streaming, native compression, redirects, and more all out of the box.

As of version 1.2.15, Bun’s fetch supports HTTP/1.1 with pipelining, handles large concurrent request loads with lower CPU usage, and offers better DNS performance with its internal resolver. It’s built to scale.

Using Bun for Backend Development

Bun isn’t just about fetch. It’s a full backend toolkit. Let’s look at what makes it powerful for building APIs and services.

1. Native HTTP Server API

You can create an HTTP server without Express, Koa, or Fastify:

Bun.serve({
  port: 3000,
  fetch(request) {
    return new Response("Hello from Bun!");
  },
});

This is ideal for microservices, edge deployments, or any API where you want minimal overhead and fast cold starts.

2. Built-in TypeScript Support

No transpilers, no ts-node. Bun runs .ts and .tsx files directly with excellent performance. It also supports JSX and modern ES module syntax.

3. Database Access with Bun.sql

Bun v1.2.15 introduced experimental support for native PostgreSQL queries with Bun.sql. You can make raw SQL queries without third-party drivers or heavy ORMs:

const db = Bun.sql.connect("postgres://user:pass@localhost:5432/db");
const rows = await db.query("SELECT * FROM users");

MySQL and SQLite are planned, and the goal is to offer native bindings for optimal performance.

4. Tooling Included

You get a fast bundler, test runner, and package manager (bun install) all in one. It’s designed to reduce friction during development while maintaining performance.

5. Compatible with npm

Bun supports most npm packages. While some Node-specific modules with deep V8 or native binding dependencies may fail, the compatibility layer is solid for most libraries and continues to improve.

Real-World Applications

If you're building a backend service today, Bun is ready for:

• REST and GraphQL APIs

• Edge-deployed apps and serverless functions

• SSR (Server-Side Rendering) applications

• Static asset delivery

• Lightweight microservices and backends

Its speed and simplicity make it ideal for latency-sensitive applications, or when you want to get something into production fast with minimal complexity.

Conclusion

Bun’s fetch is not just a small feature it’s a gateway into how a modern runtime can rethink backend development. By making fetch native, fast, and browser-compatible, Bun simplifies the developer experience while improving performance.

Combined with its built-in server, native database access, fast TypeScript support, and included tooling, Bun offers a compelling full-stack development experience. It feels like the backend runtime that frontend developers always wanted and now backend engineers can benefit too.

Bun 1.2.15 continues to show that speed, simplicity, and standards-compliance don’t have to be mutually exclusive. If you haven’t already tried Bun for backend development, now’s a great time to experiment.

- Jagadhiswaran Devaraj

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