Tsdown

tsdown — Library Bundler

Blazing-fast bundler for TypeScript/JavaScript libraries powered by Rolldown and Oxc. A modern replacement for tsup.

Setup

npm install -D tsdown

# Or scaffold a new project
npm create tsdown@latest

Requires Node.js 20.19+.

Basic Configuration

// tsdown.config.ts
import { defineConfig } from "tsdown";

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

Defaults (Different from tsup)

Entry Points

// Single entry
entry: ["src/index.ts"],

// Named entries
entry: {
  index: "src/index.ts",
  utils: "src/utils.ts",
  cli: "src/cli.ts",
},

// Glob with exclusions
entry: ["src/**/*.ts", "!**/*.test.ts"],

Output Formats

// Dual ESM + CJS (most common for libraries)
format: ["esm", "cjs"],

// Browser global
format: ["iife"],
globalName: "MyLib",

// UMD for CDN/script tag compatibility
format: ["umd"],
globalName: "MyLib",

Package.json Exports

Auto-generate the exports field:

export default defineConfig({
    entry: ["src/index.ts"],
    format: ["esm", "cjs"],
    dts: true,
    exports: true, // auto-generates package.json exports field
});

Type Declarations

// Basic — generates .d.ts alongside output
dts: true,

// With options
dts: {
  sourcemap: true,
  compilerOptions: {
    composite: false,
  },
},

Always enable dts for published TypeScript libraries.

Dependency Handling

tsdown automatically externalizes dependencies and peerDependencies from package.json.

// Manually external
external: ["react", "react-dom", /^@myorg\//],

// Force-bundle a dependency (inline it)
noExternal: ["tiny-utility-to-inline"],

• External by default: dependencies, peerDependencies.
• Bundled by default: devDependencies.

Platform and Target

// Node.js library (default)
platform: "node",
target: "es2022",

// Browser library
platform: "browser",
target: "es2020",

// Isomorphic
platform: "neutral",

Minification

// Full minification
minify: true,

// Dead code elimination only (smaller output, readable)
minify: "dce-only",

Shims

Add ESM/CJS interop shims (__dirname, __filename, import.meta.url):

shims: true,

Enable when publishing dual ESM/CJS packages that reference Node.js globals.

Source Maps

sourcemap: true,          // external .map files
sourcemap: "inline",      // embedded in output
sourcemap: "hidden",      // generated but not referenced

Unbundle Mode

Preserve the source directory structure instead of bundling into a single file:

export default defineConfig({
    entry: ["src/**/*.ts", "!**/*.test.ts"],
    unbundle: true,
    format: ["esm"],
    dts: true,
});

Useful for utility libraries where consumers may want to import individual modules.

Plugins

tsdown supports Rolldown, Rollup, and unplugin plugins:

import { wasm } from "rolldown-plugin-wasm";

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

Multiple Configurations

Export an array for different build targets:

export default defineConfig([
    {
        entry: ["src/index.ts"],
        format: ["esm", "cjs"],
        dts: true,
    },
    {
        entry: ["src/cli.ts"],
        format: ["esm"],
        platform: "node",
        banner: { js: "#!/usr/bin/env node" },
    },
]);

Conditional Configuration

export default defineConfig((options) => {
    const isDev = options.watch;
    return {
        entry: ["src/index.ts"],
        format: ["esm", "cjs"],
        minify: !isDev,
        sourcemap: isDev,
    };
});

Monorepo / Workspace Builds

Build all packages in a monorepo from a single config:

export default defineConfig({
    workspace: "packages/*",
    entry: ["src/index.ts"],
    format: ["esm", "cjs"],
    dts: true,
});

Lifecycle Hooks

export default defineConfig({
    entry: ["src/index.ts"],
    hooks: {
        "build:before": async (context) => {
            // Pre-build tasks (codegen, validation)
        },
        "build:done": async (context) => {
            // Post-build tasks (copy files, notifications)
        },
    },
});

CI Configuration

Use 'ci-only' and 'local-only' to vary behavior by environment:

export default defineConfig({
    entry: ["src/index.ts"],
    format: ["esm", "cjs"],
    dts: true,
    failOnWarn: "ci-only", // fail on warnings in CI only
    publint: "ci-only", // validate package structure in CI
    attw: "ci-only", // check "are the types wrong" in CI
});

Common Recipes

React Component Library

export default defineConfig({
    entry: ["src/index.tsx"],
    format: ["esm", "cjs"],
    dts: true,
    external: ["react", "react-dom"],
});

Node.js CLI Tool

export default defineConfig({
    entry: ["src/cli.ts"],
    format: ["esm"],
    platform: "node",
    target: "es2022",
    banner: { js: "#!/usr/bin/env node" },
    minify: true,
});

Dual Package with Shims

export default defineConfig({
    entry: ["src/index.ts"],
    format: ["esm", "cjs"],
    dts: true,
    shims: true,
    exports: true,
    clean: true,
});

Migration from tsup

npx tsdown migrate

This handles ~90% of config conversion automatically. Key differences to review:

• format defaults to esm (tsup defaults to cjs).
• clean is enabled by default.
• dts auto-enables if types field exists in package.json.
• onSuccess → use hooks['build:done'] instead.
• splitting → enabled by default, use splitting: false to disable.

CLI Reference

tsdown                           # Build with defaults
tsdown --watch                   # Watch mode
tsdown --format esm,cjs          # Specify formats
tsdown --dts                     # Generate declarations
tsdown --minify                  # Minify output
tsdown --outDir lib              # Custom output directory
tsdown --sourcemap               # Generate source maps
tsdown --config custom.ts        # Custom config file
tsdown src/a.ts src/b.ts         # Multiple entries