Plugin Development Guide

This guide walks you through creating custom Bunup plugins with lifecycle hooks. For an overview of the plugin system and using Bun plugins, see the Plugins guide.

Creating a Bunup Plugin

Bunup plugins provide additional hooks into the build process beyond what Bun's native plugin system offers. These plugins can be used to extend Bunup's functionality with custom build steps, reporting, and more.

import type { BunupPlugin } from "bunup";

export function myBunupPlugin(): BunupPlugin {
	return {
		name: "my-bunup-plugin",
		hooks: {
			onBuildStart: async (ctx) => {
				console.log("Starting build with options:", ctx.options);
				ctx.options.banner = "/* Built with my plugin */";
			},

			onBuildDone: async (ctx) => {
				const { files, options, meta } = ctx;
				console.log("Build completed with files:", files.length);
				console.log("Package name:", meta.packageJson.data?.name);

				for (const file of files) {
					console.log(`Generated: ${file.pathRelativeToOutdir} (${file.kind})`);
				}
			},
		},
	};
}

This example demonstrates both available hooks:

Hook	Purpose	Capabilities
onBuildStart	Runs before the build starts	Setup tasks, modify build options
onBuildDone	Runs after the build completes	Access build output, post-processing, reporting

To use this plugin:

import { defineConfig } from "bunup";
import { myBunupPlugin } from "./my-bunup-plugin";

export default defineConfig({
	entry: "src/index.ts",
	format: ["esm", "cjs"],
	plugins: [myBunupPlugin()],
});

Available Hooks

Bunup plugins support the following hooks:

onBuildStart

Called before a build starts, allowing you to perform setup or preprocessing tasks.

onBuildStart: (ctx: OnBuildStartCtx) => Promise<void> | void

• ctx.options: The build options that will be used for this build. You can modify these options to affect the build process.

onBuildDone

Called after a build is successfully completed, providing access to the build result.

onBuildDone: (ctx: OnBuildDoneCtx) => Promise<void> | void

Where OnBuildDoneCtx contains:

• files: Array of generated output files
• options: The build configuration options that were used
• meta: Build execution metadata (package.json and root directory)

OnBuildDoneCtx Details

The OnBuildDoneCtx provides comprehensive information about the build with a flattened structure for easy access:

files

The files property is an array of BuildOutputFile objects representing all generated output files:

type BuildOutputFile = {
	entrypoint: string | undefined;
	kind: "entry-point" | "chunk" | "asset" | "sourcemap" | "bytecode";
	fullPath: string;
	pathRelativeToRootDir: string;
	pathRelativeToOutdir: string;
	dts: boolean;
	format: Format;
	size: number;
};

Property	Type	Description
entrypoint	string | undefined	Entry point that generated this file (undefined for chunks/assets)
kind	'entry-point' | 'chunk' | 'asset' | 'sourcemap'	Type of generated file
fullPath	string	Absolute path to the generated file
pathRelativeToRootDir	string	Path relative to project root
pathRelativeToOutdir	string	Path relative to output directory
dts	boolean	Whether this is a TypeScript declaration file
format	Format	Output format (esm, cjs, etc.)
size	number	The size of the file in bytes

meta

The meta object contains build metadata:

type BuildMeta = {
	packageJson: PackageJson;
	rootDir: string;
};

Property	Type	Description
packageJson	PackageJson	Parsed package.json content that is used for the build
rootDir	string	Root directory of the project

Error Handling

Plugins should handle errors gracefully. If a plugin hook throws an error, the build will fail:

export function robustPlugin(): BunupPlugin {
	return {
		name: "robust-plugin",
		hooks: {
			onBuildDone: async ({ files }) => {
				try {
					await processFiles(files);
				} catch (error) {
					console.error("Plugin failed:", error);
					throw new Error(`robust-plugin failed: ${error.message}`);
				}
			},
		},
	};
}

Example: Bundle Size Reporter Plugin

export function bundleSizeReporter(maxSize?: number): BunupPlugin {
	return {
		name: "bundle-size-reporter",
		hooks: {
			onBuildDone: async ({ files }) => {
				const sizes = await Promise.all(
					files
						.filter((f) => f.kind === "entry-point")
						.map(async (file) => {
							const buffer = await Bun.file(file.fullPath).arrayBuffer();
							return {
								path: file.pathRelativeToOutdir,
								size: buffer.byteLength,
								format: file.format,
							};
						}),
				);

				console.log("\n📦 Bundle Sizes:");
				for (const { path, size, format } of sizes) {
					const sizeKB = (size / 1024).toFixed(2);
					console.log(`  ${path} (${format}): ${sizeKB} KB`);

					if (maxSize && size > maxSize) {
						throw new Error(`Bundle ${path} exceeds maximum size of ${maxSize} bytes`);
					}
				}
			},
		},
	};
}