Plugins – Bundler

Note — Introduced in Bun v0.1.11.

Bun provides a universal plugin API that can be used to extend both the runtime and bundler.

Plugins intercept imports and perform custom loading logic: reading files, transpiling code, etc. They can be used to add support for additional file types, like .scss or .yaml. In the context of Bun's bundler, plugins can be used to implement framework-level features like CSS extraction, macros, and client-server code co-location.

Usage

A plugin is defined as simple JavaScript object containing a name property and a setup function. Register a plugin with Bun using the plugin function.

yamlPlugin.ts
import type { BunPlugin } from "bun";

const myPlugin: BunPlugin = {
  name: "YAML loader",
  setup(build) {
    // implementation
  },
};

This plugin can be passed into the plugins array when calling Bun.build.

Bun.build({
  entrypoints: ["./app.ts"],
  outdir: "./out",
  plugins: [myPlugin],
});

`--preload`

To consume this plugin, add this file to the preload option in your bunfig.toml. Bun automatically loads the files/modules specified in preload before running a file.

preload = ["./yamlPlugin.ts"]

To preload files during bun test:

[test]
preload = ["./loader.ts"]

Usage without preload

Alternatively, you can import this file manually at the top of your project's entrypoint, before any application code is imported.

app.ts
import "./yamlPlugin.ts";
import { config } from "./config.yml";

console.log(config);

Third-party plugins

By convention, third-party plugins intended for consumption should export a factory function that accepts some configuration and returns a plugin object.

import { plugin } from "bun";
import fooPlugin from "bun-plugin-foo";

plugin(
  fooPlugin({
    // configuration
  }),
);

// application code

Bun's plugin API is based on esbuild. Only a subset of the esbuild API is implemented, but some esbuild plugins "just work" in Bun, like the official MDX loader:

import { plugin } from "bun";
import mdx from "@mdx-js/esbuild";

plugin(mdx());

import { renderToStaticMarkup } from "react-dom/server";
import Foo from "./bar.mdx";
console.log(renderToStaticMarkup(<Foo />));

Loaders

Plugins are primarily used to extend Bun with loaders for additional file types. Let's look at a simple plugin that implements a loader for .yaml files.

yamlPlugin.ts
import { plugin } from "bun";

plugin({
  name: "YAML",
  async setup(build) {
    const { load } = await import("js-yaml");
    const { readFileSync } = await import("fs");

    // when a .yaml file is imported...
    build.onLoad({ filter: /\.(yaml|yml)$/ }, (args) => {

      // read and parse the file
      const text = readFileSync(args.path, "utf8");
      const exports = load(text) as Record<string, any>;

      // and returns it as a module
      return {
        exports,
        loader: "object", // special loader for JS objects
      };
    });
  },
});

With this plugin, data can be directly imported from .yaml files.

index.ts

data.yml

index.ts
import "./yamlPlugin.ts"
import {name, releaseYear} from "./data.yml"

console.log(name, releaseYear);

data.yml
name: Fast X
releaseYear: 2023

Note that the returned object has a loader property. This tells Bun which of its internal loaders should be used to handle the result. Even though we're implementing a loader for .yaml, the result must still be understandable by one of Bun's built-in loaders. It's loaders all the way down.

In this case we're using "object"—a built-in loader (intended for use by plugins) that converts a plain JavaScript object to an equivalent ES module. Any of Bun's built-in loaders are supported; these same loaders are used by Bun internally for handling files of various kinds. The table below is a quick reference; refer to Bundler > Loaders for complete documentation.

Loader	Extensions	Output
`js`	`.mjs` `.cjs`	Transpile to JavaScript files
`jsx`	`.js` `.jsx`	Transform JSX then transpile
`ts`	`.ts` `.mts` `cts`	Transform TypeScript then transpile
`tsx`	`.tsx`	Transform TypeScript, JSX, then transpile
`toml`	`.toml`	Parse using Bun's built-in TOML parser
`json`	`.json`	Parse using Bun's built-in JSON parser
`napi`	`.node`	Import a native Node.js addon
`wasm`	`.wasm`	Import a native Node.js addon
`object`	none	A special loader intended for plugins that converts a plain JavaScript object to an equivalent ES module. Each key in the object corresponds to a named export.

Loading a YAML file is useful, but plugins support more than just data loading. Let's look at a plugin that lets Bun import *.svelte files.

sveltePlugin.ts
import { plugin } from "bun";

await plugin({
  name: "svelte loader",
  async setup(build) {
    const { compile } = await import("svelte/compiler");
    const { readFileSync } = await import("fs");

    // when a .svelte file is imported...
    build.onLoad({ filter: /\.svelte$/ }, ({ path }) => {

      // read and compile it with the Svelte compiler
      const file = readFileSync(path, "utf8");
      const contents = compile(file, {
        filename: path,
        generate: "ssr",
      }).js.code;

      // and return the compiled source code as "js"
      return {
        contents,
        loader: "js",
      };
    });
  },
});

Note: in a production implementation, you'd want to cache the compiled output and include additional error handling.

The object returned from build.onLoad contains the compiled source code in contents and specifies "js" as its loader. That tells Bun to consider the returned contents to be a JavaScript module and transpile it using Bun's built-in js loader.

With this plugin, Svelte components can now be directly imported and consumed.

import "./sveltePlugin.ts";
import MySvelteComponent from "./component.svelte";

console.log(mySvelteComponent.render());

Reading `Bun.build`'s config

Plugins can read and write to the build config with build.config.

Bun.build({
  entrypoints: ["./app.ts"],
  outdir: "./dist",
  sourcemap: "external",
  plugins: [
    {
      name: "demo",
      setup(build) {
        console.log(build.config.sourcemap); // "external"

        build.config.minify = true; // enable minification

        // `plugins` is readonly
        console.log(`Number of plugins: ${build.config.plugins.length}`);
      },
    },
  ],
});

Reference

namespace Bun {
  function plugin(plugin: { name: string; setup: (build: PluginBuilder) => void }): void;
}

type PluginBuilder = {
  onResolve: (
    args: { filter: RegExp; namespace?: string },
    callback: (args: { path: string; importer: string }) => {
      path: string;
      namespace?: string;
    } | void,
  ) => void;
  onLoad: (
    args: { filter: RegExp; namespace?: string },
    callback: (args: { path: string }) => {
      loader?: Loader;
      contents?: string;
      exports?: Record<string, any>;
    },
  ) => void;
  config: BuildConfig;
};

type Loader = "js" | "jsx" | "ts" | "tsx" | "json" | "toml" | "object";

The onLoad method optionally accepts a namespace in addition to the filter regex. This namespace will be be used to prefix the import in transpiled code; for instance, a loader with a filter: /\.yaml$/ and namespace: "yaml:" will transform an import from ./myfile.yaml into yaml:./myfile.yaml.

Usage

--preload

Third-party plugins

Loaders

Reading Bun.build's config

Reference

`--preload`

Reading `Bun.build`'s config