Plugins – Runtime

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.

myPlugin.ts
import { plugin, type BunPlugin } from "bun";

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

plugin(myPlugin);

Plugins have to be loaded before any other code runs! To achieve this, use the preload option in your bunfig.toml. Bun automatically loads the files/modules specified in preload before running a file.

preload = ["./myPlugin.ts"]

To preload files before bun test:

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

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
  }),
);

Bun's plugin API is loosely 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());

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";

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

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

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

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

bunfig.toml
preload = ["./yamlPlugin.ts"]

Once the plugin is registered, .yaml and .yml files can be directly imported.

index.ts

data.yml

index.ts
import data from "./data.yml"

console.log(data);

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");

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

      // read and compile it with the Svelte compiler
      const file = await Bun.file(path).text();
      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());

Virtual Modules

This feature is currently only available at runtime with Bun.plugin and not yet supported in the bundler, but you can mimic the behavior using onResolve and onLoad.

To create virtual modules at runtime, use builder.module(specifier, callback) in the setup function of a Bun.plugin.

For example:

import { plugin } from "bun";

plugin({
  name: "my-virtual-module",

  setup(build) {
    build.module(
      // The specifier, which can be any string - except a built-in, such as "buffer"
      "my-transpiled-virtual-module",
      // The callback to run when the module is imported or required for the first time
      () => {
        return {
          contents: "console.log('hello world!')",
          loader: "js",
        };
      },
    );

    build.module("my-object-virtual-module", () => {
      return {
        exports: {
          foo: "bar",
        },
        loader: "object",
      };
    });
  },
});

// Sometime later
// All of these work
import "my-transpiled-virtual-module";
require("my-transpiled-virtual-module");
await import("my-transpiled-virtual-module");
require.resolve("my-transpiled-virtual-module");

import { foo } from "my-object-virtual-module";
const object = require("my-object-virtual-module");
await import("my-object-virtual-module");
require.resolve("my-object-virtual-module");

Overriding existing modules

You can also override existing modules with build.module.

import { plugin } from "bun";
build.module("my-object-virtual-module", () => {
  return {
    exports: {
      foo: "bar",
    },
    loader: "object",
  };
});

require("my-object-virtual-module"); // { foo: "bar" }
await import("my-object-virtual-module"); // { foo: "bar" }

build.module("my-object-virtual-module", () => {
  return {
    exports: {
      baz: "quix",
    },
    loader: "object",
  };
});
require("my-object-virtual-module"); // { baz: "quix" }
await import("my-object-virtual-module"); // { baz: "quix" }

Reading the 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.