What is the Bun bundler?

The Bun bundler is an integral part of the Bun toolkit. As a tool designed for JavaScript and TypeScript apps, it is responsible for bundling and minifying code for the browser, Node.js, and other platforms. However, before we dive into the specifics of the Bun bundler, we need to fully understand what a bundler actually is.

What is a bundler?

A bundler is a tool commonly used in web development to optimize and manage the distribution of code and other assets in a web application. Bundlers take various input files, such as JavaScript, CSS, HTML, images, and more, and package them together into optimized bundles that can be efficiently served to a web browser. There are a number of advantages to this approach:

Reduced HTTP requests: The more requests, the slower the page load time. Bundling combines multiple resources into a single file, reducing the number of HTTP requests and improving page load performance.
Optimization of file size: Smaller file sizes mean faster downloads, especially for users on slower internet connections.
Better caching: The browser caches a bundled resource, and when the user revisits your site, it only needs to download the changes. This reduces the amount of data that needs to be transferred and speeds up subsequent page loads.
Optimized loading order: Bundlers often take care of specifying the correct order for loading resources. For example, JavaScript dependencies are loaded before the main application code, ensuring that everything works as expected. This can help avoid issues related to race conditions and dependencies.
Compatibility: Bundlers can transform code to make it compatible with older browsers or different environments. They can transpile modern JavaScript features into older versions for broader browser support.

How is the Bun bundler different from its competitors?

The Bun bundler is engineered for speed and user-friendliness, boasting full compatibility with the feature set of bundlers like Webpack while offering a streamlined configuration process. Bun outpaces Webpack, particularly on larger projects, as demonstrated by benchmark tests revealing it to be a remarkable 220 times faster than the widely prevalent Webpack. We can find a detailed comparison between the performance of Bun and other bundlers in the table below as run on a M1 Macbook Pro.

A good question to ask at this point is what makes Bun faster than it’s competitors? The answer lies in it’s implementation.

Bun’s bundler is built with a fast JavaScript runtime designed as a drop-in replacement for Node.js and is powered by JavaScriptCore, which significantly reduces startup times and memory usage. Also, instead of running plugins in the Node.js process, which can be slow to startup, Bun uses lightweight processes to provide fast plugins. However, what makes Bun bundler different from it’s counterparts is the simplicity of its API. According to the Bun team, their goal is to implement a minimal feature set that fast, stable, and accommodates most modern use cases without sacrificing performance, which they believe hasn’t been done by their competitors in their pursuit of feature-completeness.

interface Bun {
  build(options: BuildOptions): Promise<BuildOutput>;
}
interface BuildOptions {
  entrypoints: string[]; // required
  outdir?: string; // default: no write (in-memory only)
  target?: "browser" | "bun" | "node"; // "browser"
  format?: "esm"; // later: "cjs" | "iife"
  splitting?: boolean; // default false
  plugins?: BunPlugin[]; // [] // see https://bun.sh/docs/bundler/plugins
  loader?: { [k in string]: string }; // see https://bun.sh/docs/bundler/loaders
  external?: string[]; // default []
  sourcemap?: "none" | "inline" | "external"; // default "none"
  root?: string; // default: computed from entrypoints
  publicPath?: string; // e.g. http://mydomain.com/
  naming?:
    | string // equivalent to naming.entry
    | { entry?: string; chunk?: string; asset?: string };
  minify?:
    | boolean // default false
    | { identifiers?: boolean; whitespace?: boolean; syntax?: boolean };
}

There are a couple of standouts here that we need to focus on to fully utilize Bun's bundler:

Tree-Shaking and dead code elimination: Bun's bundler enforces tree-shaking—elimination of unused code. We can add the "sideEffects": false option in the package.json file, which tells the the bundler that the package has no side effects and enables more aggressive dead code elimination.
Flexible targets: Bun's API supports three "targets": "browser", "bun", and "node". Depending on the target, Bun applies different module resolution rules and optimizations, catering to different bundle execution environments
Optimized minification options: As we can see on line 22 of the code, instead of just a boolean flag, Bun's API allows minification options to be specified in a more granular manner.
Plugins: As with the runtime itself, the bundler is designed to be extensible via plugins. In fact, there's no difference at all between a runtime plugin and a bundler plugin.

Note: Currently, the Bun bundler exclusively supports the "esm" (ECMAScript Modules) format. However, the development roadmap includes the expansion of support to encompass other module systems and targets, notably "iife" (Immediately Invoked Function Expression) and "cjs" (CommonJS).

Also, it's important to note that Bun's plugin API is intentionally designed to maintain compatibility with esbuild. This compatibility means that many third-party esbuild plugins can seamlessly integrate with Bun without requiring significant adjustments, simplifying the process of extending Bun's functionality through the use of well-established esbuild plugins.

A benchmark comparing Bun with it's counterparts

This is a performance benchmark of the following bundlers:

Bun
esbuild
Parcel 2
Rollup + Terser
Webpack

To run the benchmark, just click on the the following terminal and observe the output. Each bundler will generate different console outputs. Examine the results for lines resembling the following pattern below each bundler's output:

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Bundler	Time
Bun	0.17s
esbuild	0.33s
Rollup	18.82s
Webpack	26.21s
Parcel	17.95s

What is the Bun bundler?

What is a bundler?

How is the Bun bundler different from its competitors?

Examining Bun's bundler API

A benchmark comparing Bun with it's counterparts