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 or modifying 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}`);
      },
    },
  ],
});

NOTE: Plugin lifcycle callbacks (onStart(), onResolve(), etc.) do not have the ability to modify the build.config object in the setup() function. If you want to mutate build.config, you must do so directly in the setup() function:

Bun.build({
  entrypoints: ["./app.ts"],
  outdir: "./dist",
  sourcemap: "external",
  plugins: [
    {
      name: "demo",
      setup(build) {
        // ✅ good! modifying it directly in the setup() function
        build.config.minify = true;

        build.onStart(() => {
          // 🚫 uh-oh! this won't work!
          build.config.minify = false;
        });
      },
    },
  ],
});

Lifecycle callbacks

Plugins can register callbacks to be run at various points in the lifecycle of a bundle:

onStart(): Run once the bundler has started a bundle
onResolve(): Run before a module is resolved
onLoad(): Run before a module is loaded.

A rough overview of the types (please refer to Bun's bun.d.ts for the full type definitions):

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

type PluginBuilder = {
  onStart(callback: () => void): void;
  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" | "css" | "json" | "toml" | "object";

Namespaces

onLoad and onResolve accept an optional namespace string. What is a namespaace?

Every module has a namespace. Namespaces are 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.

The default namespace is "file" and it is not necessary to specify it, for instance: import myModule from "./my-module.ts" is the same as import myModule from "file:./my-module.ts".

Other common namespaces are:

"bun": for Bun-specific modules (e.g. "bun:test", "bun:sqlite")
"node": for Node.js modules (e.g. "node:fs", "node:path")

`onStart`

onStart(callback: () => void): Promise<void> | void;

Registers a callback to be run when the bundler starts a new bundle.

import { plugin } from "bun";

plugin({
  name: "onStart example",

  setup(build) {
    build.onStart(() => {
      console.log("Bundle started!");
    });
  },
});

The callback can return a Promise. After the bundle process has initialized, the bundler waits until all onStart() callbacks have completed before continuing.

For example:

const result = await Bun.build({
  entrypoints: ["./app.ts"],
  outdir: "./dist",
  sourcemap: "external",
  plugins: [
    {
      name: "Sleep for 10 seconds",
      setup(build) {
        build.onStart(async () => {
          await Bunlog.sleep(10_000);
        });
      },
    },
    {
      name: "Log bundle time to a file",
      setup(build) {
        build.onStart(async () => {
          const now = Date.now();
          await Bun.$`echo ${now} > bundle-time.txt`;
        });
      },
    },
  ],
});

In the above example, Bun will wait until the first onStart() (sleeping for 10 seconds) has completed, as well as the second onStart() (writing the bundle time to a file).

Note that onStart() callbacks (like every other lifecycle callback) do not have the ability to modify the build.config object. If you want to mutate build.config, you must do so directly in the setup() function.

`onResolve`

onResolve(
  args: { filter: RegExp; namespace?: string },
  callback: (args: { path: string; importer: string }) => {
    path: string;
    namespace?: string;
  } | void,
): void;

To bundle your project, Bun walks down the dependency tree of all modules in your project. For each imported module, Bun actually has to find and read that module. The "finding" part is known as "resolving" a module.

The onResolve() plugin lifecycle callback allows you to configure how a module is resolved.

The first argument to onResolve() is an object with a filter and namespace property. The filter is a regular expression which is run on the import string. Effectively, these allow you to filter which modules your custom resolution logic will apply to.

The second argument to onResolve() is a callback which is run for each module import Bun finds that matches the filter and namespace defined in the first argument.

The callback receives as input the path to the matching module. The callback can return a new path for the module. Bun will read the contents of the new path and parse it as a module.

For example, redirecting all imports to images/ to ./public/images/:

import { plugin } from "bun";

plugin({
  name: "onResolve example",
  setup(build) {
    build.onResolve({ filter: /.*/, namespace: "file" }, args => {
      if (args.path.startsWith("images/")) {
        return {
          path: args.path.replace("images/", "./public/images/"),
        };
      }
    });
  },
});

`onLoad`

onLoad(
  args: { filter: RegExp; namespace?: string },
  callback: (args: { path: string, importer: string, namespace: string, kind: ImportKind  }) => {
    loader?: Loader;
    contents?: string;
    exports?: Record<string, any>;
  },
): void;

After Bun's bundler has resolved a module, it needs to read the contents of the module and parse it.

The onLoad() plugin lifecycle callback allows you to modify the contents of a module before it is read and parsed by Bun.

Like onResolve(), the first argument to onLoad() allows you to filter which modules this invocation of onLoad() will apply to.

The second argument to onLoad() is a callback which is run for each matching module before Bun loads the contents of the module into memory.

This callback receives as input the path to the matching module, the importer of the module (the module that imported the module), the namespace of the module, and the kind of the module.

The callback can return a new contents string for the module as well as a new loader.

For example:

import { plugin } from "bun";

plugin({
  name: "env plugin",
  setup(build) {
    build.onLoad({ filter: /env/, namespace: "file" }, args => {
      return {
        contents: `export default ${JSON.stringify(process.env)}`,
        loader: "js",
      };
    });
  },
});

This plugin will transform all imports of the form import env from "env" into a JavaScript module that exports the current environment variables.

`.defer()`

One of the arguments passed to the onLoad callback is a defer function. This function returns a Promise that is resolved when all other modules have been loaded.

This allows you to delay execution of the onLoad callback until all other modules have been loaded.

This is useful for returning contens of a module that depends on other modules.

Example: tracking and reporting unused exports

import { plugin } from "bun";

plugin({
  name: "track imports",
  setup(build) {
    const transpiler = new Bun.Transpiler();

    let trackedImports: Record<string, number> = {};

    // Each module that goes through this onLoad callback
    // will record its imports in `trackedImports`
    build.onLoad({ filter: /\.ts/ }, async ({ path }) => {
      const contents = await Bun.file(path).arrayBuffer();

      const imports = transpiler.scanImports(contents);

      for (const i of imports) {
        trackedImports[i.path] = (trackedImports[i.path] || 0) + 1;
      }

      return undefined;
    });

    build.onLoad({ filter: /stats\.json/ }, async ({ defer }) => {
      // Wait for all files to be loaded, ensuring
      // that every file goes through the above `onLoad()` function
      // and their imports tracked
      await defer();

      // Emit JSON containing the stats of each import
      return {
        contents: `export default ${JSON.stringify(trackedImports)}`,
        loader: "json",
      };
    });
  },
});

Note that the .defer() function currently has the limitation that it can only be called once per onLoad callback.