An Introduction to the Bun JavaScript Runtime

Dec 06, 2023

JavaScript continues to stand as the cornerstone of modern programming languages. Amidst this ever-evolving landscape, a new player has emerged — the Bun JavaScript Runtime. This groundbreaking runtime environment promises to redefine the way developers conceive and execute their JavaScript applications.

The History: Where Bun Fits In with Node and Deno

In 2009, Node.js burst onto the scene, courtesy of Ryan Dahl. While not the pioneer in server-side JavaScript runtimes, Node.js quickly became a force to be reckoned with. Fast forward to 2023, and Node.js is still at the forefront, boasting the most extensive development ecosystem with a staggering 3.2 million modules, racking up almost 500 billion downloads weekly (as reported by

Deno, unveiled by Ryan Dahl in 2020, is a JavaScript development remix that addresses legacy concerns in Node.js. It spans security, API compatibility, tooling, and module management. Deno received positive feedback but has yet to dethrone Node's dominance.

In 2022, Jarred Sumner introduced Bun, fueled by his frustration with Node.js's sluggishness during a Next.js project. Unlike its counterparts using the V8 engine, Bun opted for the JavaScriptCore engine, known for powering WebKit browsers like Safari.

Bun zeroes in on performance and developer experience, aiming to streamline JavaScript without sacrificing its core strengths. What sets it apart is the ability to evolve swiftly compared to Node.js, which grapples with backward compatibility.

Like Deno, Bun seamlessly supports JavaScript and TypeScript, eliminating the need for third-party transpilers or configurations. It's not just another tool. It's a versatile replacement for Node.js, Deno, serverless runtimes, and an entire toolkit—from npm, npx, yarn to TypeScript compilers, dotenv, nodemon, pm2, Webpack, Babel, and Jest.

The initial Bun runtime was stable, but the collective efforts of nearly 300 developers culminated in the release of Bun version 1.0 in September 2023.

Tasty Bun Benefits

Node.js and Deno have long relied on Chrome's V8 JavaScript engine. But Bun takes a distinctive route by embracing the JavaScriptCore engine, the powerhouse behind WebKit browsers like Safari. What sets Bun apart further is its choice of language - Zig, a low-level programming language featuring manual memory management and native threading for efficient concurrency handling. The outcome? A lightweight runtime that boasts a smaller memory footprint, faster start-up times, and a performance potential. It can outpace Node.js and Deno up to 4 times under specific benchmarking conditions.

Like Deno, Bun seamlessly supports JavaScript and TypeScript without needing third-party transpilers or configurations. It extends its support to .jsx and .tsx files, converting HTML-like markup into native JavaScript. Additionally, it ventures into experimental territory by offering support for running WebAssembly-compiled .wasm files.

Internally, Bun leverages ES Modules, embraces top-level await, translates CommonJS, and implements Node's node_modules resolution algorithm. Bun's caching strategy is noteworthy. It caches modules in ~/.bun/install/cache/ and employs hardlinks to copy them into a project's node_modules directory. This unique approach ensures that all projects on your system reference a single instance of the same library, optimizing disk space and installation performance.

Bun's versatility extends to bundling, where it can import all dependencies into a single file and target Bun, Node.js, and client-side JavaScript. This feature reduces reliance on external tools like esbuild or Rollup. The command-line interface options are also available through a JavaScript API, enabling the creation of sophisticated build scripts without needing a dedicated task runner.

Bun also introduces a hot mode, monitoring for changes and facilitating soft reloads of modules. All files are re-evaluated while preserving the global state. Project .env files are automatically loaded and parsed, simplifying the handling of environment variables without needing external packages like dotenv.

In addition to its proprietary APIs for networking, file access, and child processes, Bun supports a range of Web APIs (fetch, URL, blob, WebSocket, JSON, setTimeout, and events) and Node.js compatibility APIs (console, assert, dns, http, path, stream, util, __dirname, and __filename). Bun asserts that 90% of the most-used APIs are fully implemented, with room for verification based on project-specific requirements.

To cap it off, Bun introduces a native SQLite3 client, potentially streamlining dependencies in certain projects.

Bun vs Deno vs Node.js

In the dynamic landscape of JavaScript runtimes, the trio of Bun, Deno, and Node.js each brings its strengths to the table. Deno, in addressing Node's limitations, faced some adoption hurdles.

Third-party module support:

  • Deno initially lacked support for Node's third-party modules, making migration less straightforward for developers heavily reliant on the existing Node.js ecosystem.

Learning curve:

  • Migrating to Deno meant acquiring new techniques, posing a barrier for developers comfortable with Node.js.

Development experience:

  • Despite offering an improved development experience, Deno didn't compel developers to make the switch as Node.js was deemed "good enough" for many.

To bridge the gap and entice developers, Deno introduced compatibility options with Node.js. However, Node.js, not one to be outdone, adopted some of Deno's features, such as ES modules, a native test runner, and a watch mode.

Bun takes a distinct approach by aiming to be a swift, Node-compatible engine infused with Deno's advancements. While showing promise, Bun faces challenges.


  • Bun delivers impressive performance, yet few developers voice concerns about Node.js speed. The question remains: is a speed boost enough to prompt a shift?


  • While compatibility is commendable, supporting all Node.js modules in a different JavaScript engine, particularly with JavaScriptCore, presents a formidable challenge. The ongoing evolution of V8 in Node.js raises the question of whether JavaScriptCore can match the pace with less investment.

Tooling Suite:

  • Bun aspires to replace your tooling suite, but it's a work in progress. While promising, it hasn't reached the comprehensive feature set offered by Deno.

In this evolving landscape, the competition among Bun, Deno, and Node.js is not just about performance but also about the ease of transition, compatibility, and the ability to cater to the diverse needs of developers. Each runtime strives to carve its niche, offering a compelling proposition for the ever-discerning developer community.

Compatibility of Bun with Node.js

Bun's compatibility with Node.js is generally robust, particularly for smaller, simpler projects. In many cases, you might find that launching scripts using bun start instead of npm start requires little to no modification.

Here's a breakdown of Bun's compatibility with Node.js.

Built-in modules and APIs:

  • Bun seamlessly supports a range of built-in Node.js modules and APIs, including but not limited to fs, path, http, console, assert, and more.

Global variables and objects:

  • Global variables and objects, such as __dirname and process, are well-supported in Bun, aligning with Node.js conventions.

Module Resolution Algorithm:

  • Leveraging the Node.js module resolution algorithm, Bun adeptly locates files within the node_modules directory, ensuring smooth integration with existing Node.js projects.

Despite these commendable aspects, it's worth noting the caveat mentioned by Bun 1.0—it claims to run "virtually any Node.js application in the wild." However, the acknowledgment follows, expressing a degree of caution. Complex applications may encounter challenges, especially those with intricate dependencies and structures. Obscure error messages generated deep within third-party modules could pose hurdles for seamless execution.

While Bun strives for compatibility and aims to handle a broad spectrum of Node.js applications, the complexity inherent in certain projects might introduce nuances that require careful consideration. Developers venturing into migration or adopting Bun for larger and more intricate applications may need to navigate potential challenges with a discerning eye and, if necessary, seek tailored solutions for specific cases.

Bun summary

Bun has certainly proven itself as a capable JavaScript runtime, showcasing impressive speed and compatibility. However, in the realm of mission-critical projects or legacy applications, Node.js remains the reigning champion. While running your app with bun start is worth a try, the reality is that the larger your codebase, the more likely it will require modifications for smooth execution.

Deno, with its maturity and feature completeness, emerges as a strong contender, especially for new projects. Bun, while actively developed and promising, is still relatively new. Although stable, its long-term future is uncertain at this early stage. Nevertheless, Bun introduces intriguing concepts, such as CLI APIs and auto-loaded .env files, which could be considered by both the Node.js and Deno teams in future developments.

While the significance of the Bun version 1.0 milestone might be debated, psychologically, it marks a notable shift, making Bun feel more complete and usable. Developers are increasingly considering the runtime and toolset for their projects. Unlike Deno, which initially took a radical and somewhat incompatible path, Bun has prioritized compatibility and speed from the outset. While achieving close to 100% Node.js compatibility is an ongoing pursuit, Bun could be viewed as a viable drop-in replacement for certain tools in legacy projects.

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