colinhacks/zshy

🐒
zshy

The no-bundler build tool for TypeScript libraries. Powered by tsc.
by @colinhacks

What is zshy?

zshy (zee-shy) is a bundler-free batteries-included build tool for transpiling TypeScript libraries. It was originally created as an internal build tool for Zod but is now available as a general-purpose tool for TypeScript libraries.

• 👑 Powered by tsc — The gold standard for TypeScript transpilation
• 📦 Bundler-free — No bundler or bundler configs involved
• 🟦 No config file — Reads from your package.json and tsconfig.json
• 📝 Declarative entrypoint map — Specify your TypeScript entrypoints in package.json#/zshy
• 🤖 Auto-generated "exports" — Writes "exports" map directly into your package.json and jsr.json
• 🧱 ES modules — Builds standard ESM, which import and (on Node.js 20.19+ and 22.12+) require can both load
• 📂 Unopinionated — Use any file structure or import extension syntax you like
• 📦 Asset handling — Non-JS assets are copied to the output directory
• ⚛️ Supports .tsx — Compiles to .js per your tsconfig.json#/jsx* settings
• 🐚 CLI-friendly — First-class "bin" support
• 🐌 Blazing fast — Just kidding, it's slow. But it's worth it

Quickstart

1. Install zshy as a dev dependency:

npm install --save-dev zshy
yarn add --dev zshy
pnpm add --save-dev zshy

zshy builds with your project's own typescript, which must be 7.1 or later. While 7.1 is in beta, install it as typescript@beta. zshy builds ES modules only, so your package needs "type": "module". To build with TypeScript 5 or 6, or to publish CommonJS, use [email protected].

2. Specify your entrypoint(s) in package.json#zshy:

Single entrypoint:

// package.json
{
  "name": "my-pkg",
  "version": "1.0.0",
  "type": "module",
+ "zshy": "./src/index.ts"
}

Multiple entrypoints:

// package.json
{
  "name": "my-pkg",
  "version": "1.0.0",
  "type": "module",

+ "zshy": {
+   "exports": {
+     ".": "./src/index.ts",
+     "./utils": "./src/utils.ts",
+     "./plugins/*": "./src/plugins/*",               // wildcard
+     "./components/**/*": "./src/components/**/*"    // deep wildcard
+   }
+ }
}

3. Run a build

Run a build with npx zshy:

$ npx zshy # use --dry-run to try it out without writing/updating files

→  Starting zshy build 🐒
→  Detected project root: /Users/colinmcd94/Documents/projects/zshy
→  Reading package.json from ./package.json
→  Reading tsconfig from ./tsconfig.json
→  Cleaning up outDir...
→  Determining entrypoints...
   ╔════════════╤════════════════╗
   ║ Subpath    │ Entrypoint     ║
   ╟────────────┼────────────────╢
   ║ "my-pkg"   │ ./src/index.ts ║
   ╚════════════╧════════════════╝
→  Resolved build paths:
   ╔══════════╤════════════════╗
   ║ Location │ Resolved path  ║
   ╟──────────┼────────────────╢
   ║ rootDir  │ ./src          ║
   ║ outDir   │ ./dist         ║
   ╚══════════╧════════════════╝
→  Building...
→  Updating package.json#/exports...
→  Updating package.json#/bin...
→  Build complete! ✅

Add a "build" script to your package.json

{
  // ...
  "scripts": {
+   "build": "zshy"
  }
}

Then, to run a build:

$ npm run build

How it works

Node.js loads an ES module's relative imports by exact path, so from "./util" has to reach your users as from "./util.js". Vanilla tsc won't write that for you: under "moduleResolution": "bundler" it emits the specifier exactly as you wrote it, and under "nodenext" it makes you write .js throughout your source. This is the limitation that pushes library authors toward bundlers or bundler-powered tools like tsup, tsdown, or unbuild...

...until now! zshy builds with tsc through TypeScript's compiler API, then rewrites every relative import in the emitted output to the file it names:

Each .ts file becomes a .js file and its .d.ts declarations:

$ tree .
├── package.json
├── src
│   └── index.ts
└── dist # generated
  ├── index.js
  └── index.d.ts

Finally, zshy automatically writes "exports" into your package.json:

{
  // ...
  "zshy": {
    "exports": "./src/index.ts"
  },
+ "exports": { // auto-generated by zshy
+   ".": {
+     "types": "./dist/index.d.ts",
+     "default": "./dist/index.js"
+   }
+ }
}

The result is a tool that I consider to be the "holy grail" of TypeScript library build tools:

• emits standard ES modules whose imports Node.js can resolve as written
• type checks your code
• leverages tsc for gold-standard transpilation
• doesn't require a bundler
• doesn't require another config file (just package.json and tsconfig.json)

Usage

Flags

$ npx zshy --help
Usage: zshy [options]

Options:
  -h, --help                        Show this help message
  -p, --project <path>              Path to tsconfig (default: ./tsconfig.json)
      --verbose                     Enable verbose output
      --dry-run                     Don't write any files or update package.json
      --fail-threshold <threshold>  When to exit with non-zero error code
                                      "error" (default)
                                      "warn"
                                      "never"

Subpaths and wildcards

Multi-entrypoint packages can specify subpaths or wildcard exports in package.json#/zshy/exports:

{
  "name": "my-pkg",
  "version": "1.0.0",

  "zshy": {
    "exports": {
      ".": "./src/index.ts", // root entrypoint
      "./utils": "./src/utils.ts", // subpath
      "./plugins/*": "./src/plugins/*", // wildcard
      "./components/*": "./src/components/**/*" // deep wildcard
    }
  }
}

$ npx zshy

→  Starting zshy build... 🐒
→  Detected project root: /path/to/my-pkg
→  Reading package.json from ./package.json
→  Reading tsconfig from ./tsconfig.json
→  Determining entrypoints...
   ╔════════════════════╤═════════════════════════════╗
   ║ Subpath            │ Entrypoint                  ║
   ╟────────────────────┼─────────────────────────────╢
   ║ "my-pkg"           │ ./src/index.ts              ║
   ║ "my-pkg/utils"     │ ./src/utils.ts              ║
   ║ "my-pkg/plugins/*" │ ./src/plugins/* (5 matches) ║
   ╚════════════════════╧═════════════════════════════╝
→  Resolved build paths:
   ╔══════════╤════════════════╗
   ║ Location │ Resolved path  ║
   ╟──────────┼────────────────╢
   ║ rootDir  │ ./src          ║
   ║ outDir   │ ./dist         ║
   ╚══════════╧════════════════╝
→  Building...
→  Updating package.json exports...
→  Build complete! ✅

// package.json
{
  // ...
+ "exports": {
+   ".": {
+     "types": "./dist/index.d.ts",
+     "default": "./dist/index.js"
+   },
+   "./utils": {
+     "types": "./dist/utils.d.ts",
+     "default": "./dist/utils.js"
+   },
+   "./plugins/*": {
+     "types": "./dist/src/plugins/*",
+     "default": "./dist/src/plugins/*"
+   }
+ }
}

Building CLIs ("bin" support)

If your package is a CLI, specify your CLI entrypoint in package.json#/zshy/bin. zshy will include this entrypoint in your builds and automatically set "bin" in your package.json.

{
  // package.json
  "name": "my-cli",
  "version": "1.0.0",
  "type": "module",
  "zshy": {
+   "bin": "./src/cli.ts"
  }
}

The "bin" field is automatically written into your package.json:

{
  // package.json
  "name": "my-cli",
  "version": "1.0.0",
  "zshy": {
    "exports": "./src/index.ts",
    "bin": "./src/cli.ts"
  },
+ "bin": {
+   "my-cli": "./dist/cli.js" // CLI entrypoint
+ }
}

{
  // package.json
  "name": "my-cli",
  "version": "1.0.0",
  "type": "module",
  "zshy": {
    "bin": {
      "my-cli": "./src/cli.ts",
      "other": "./src/other.ts"
    }
  }
}

{
  // package.json
  "name": "my-cli",
  "version": "1.0.0",
  "zshy": {
    "exports": "./src/index.ts",
    "bin": {
      "my-cli": "./src/cli.ts",
      "other": "./src/other.ts"
    }
  },
  "bin": {
    "my-cli": "./dist/cli.js",
    "other": "./dist/other.js"
  }
}

Be sure to include a shebang as the first line of your CLI entrypoint file:

#!/usr/bin/env node

// CLI code here

CommonJS

zshy builds ES modules only. Node.js 20.19+ and 22.12+ can require() an ES module, so CommonJS code can still load your package, unless it uses top-level await.

To publish CommonJS output, use [email protected], which builds ESM and CommonJS side by side (with TypeScript 5 or 6). Coming from it:

• "cjs": false is still accepted, and changes nothing.
• "cjs": true, "sealCjsExports", and conditions set to "cjs" are errors.
• zshy points "main" at the ESM entrypoint, replacing the .cjs path a 0.8 build wrote there.

Custom conditions

To specify custom conditions in your export maps, specify a "conditions" map. Each condition name must correspond to one of "src" | "esm".

{
  "zshy": {
    "exports": {
      ".": "./src/index.ts"
    },
+   "conditions": {
+     "my-src-condition": "src",
+     "my-esm-condition": "esm"
+   }
  }
}

With this addition, zshy will add both conditions to the generated "exports" map:

// package.json
{
  "exports": {
    ".": {
+     "my-src-condition": "./src/index.ts",
+     "my-esm-condition": "./dist/index.js",
      "types": "./dist/index.d.ts",
      "default": "./dist/index.js"
    }
  }
}

JSR

For packages that also have a jsr.json file for publishing to JSR, zshy will copy your configured exports to jsr.json/#exports, making your zshy configuration the single source of truth for exports.

This will copy over the paths of the source code entrypoints, not the paths to the transpiled code, since JSR supports and encourages publishing TypeScript source code rather than pairs of .js + .d.ts files.

FAQ for nerds

How does zshy resolve entrypoints?

It reads your package.json#/zshy config:

// package.json
{
  "name": "my-pkg",
  "version": "1.0.0",
  "zshy": {
    "exports": {
      ".": "./src/index.ts",
      "./utils": "./src/utils.ts",
      "./plugins/*": "./src/plugins/*", // shallow match {.ts,.tsx,.cts,.mts} files
      "./components/*": "./src/components/**/*" // deep match *.{.ts,.tsx,.cts,.mts} files
    }
  }
}

A few important notes about package.json#/zshy/exports:

• All keys should start with "./"
• All values should be relative paths to source files (resolved relative to the package.json file)

A few notes on wildcards exports:

• The key should always end in "/*"
• The value should correspond to a glob-like path value that ends in either "/*" (shallow match) or "/**/*" (deep match)
• Do not include a file extensions! zshy matches source files with the following extensions:
  • .ts, .tsx, .cts, .mts
• A shallow match (./<dir>/*) will match both:
  • ./<dir>/*.{ts,tsx,cts,mts}
  • ./<dir>/*/index.{ts,tsx,cts,mts}.
• A deep match (./<dir>/**/*) will match all files recursively in the specified directory, including subdirectories:
  • ./<dir>/**/*.{ts,tsx,cts,mts}
  • ./<dir>/**/*/index.{ts,tsx,cts,mts}

Note — Since zshy computes an exact set of resolved entrypoints, your "files", "include", and "exclude" settings in tsconfig.json are ignored during the build.

Does zshy respect my tsconfig.json compiler options?

Yes! With some strategic overrides:

• module: Overridden to "esnext"
• moduleResolution: Overridden to "bundler", so extensionless imports type-check (zshy then adds the extension)
• declaration/noEmit/emitDeclarationOnly: Overridden to ensure proper output
• verbatimModuleSyntax: Set to false, so a type-only import written without import type is still erased
• esModuleInterop: Set to true (it's a best practice)
• composite: Set to false to avoid resolution issues. zshy will build all files that are reachable from your specified entrypoints.

All other options are respected as defined, though zshy will also set the following reasonable defaults if they are not explicitly set:

• outDir (defaults to ./dist)
• declarationDir (defaults to outDir — you probably shouldn't set this explicitly)
• target (defaults to es2020)

Which versions of TypeScript does zshy support?

TypeScript 7.1 and later. zshy builds with the typescript your project has installed, through the API of TypeScript's native compiler, and 7.1 is the first release where that API is stable. While 7.1 is in beta, install it as typescript@beta.

To build with TypeScript 5 or 6, use [email protected], the last release built on the classic Compiler API.

Coming from [email protected], the ESM output should be unchanged (CommonJS output is gone: see CommonJS). A few declarations come out of TypeScript 7 in a different but equivalent shape, and source maps differ in detail. TypeScript 7 also keeps the defaults TypeScript 6 changed, and zshy checks your code with them. Two that commonly surface:

• types now defaults to [], so Node.js globals like console and process need "types": ["node"].
• Side-effect imports are checked, so import "./styles.css" reports TS2882 unless you set "noUncheckedSideEffectImports": false. zshy still copies the asset.

TypeScript 7 also removes baseUrl: spell paths targets relative to the tsconfig instead, e.g. "./src/*".

Do I need to use a specific file structure?

No. You can organize your source however you like; zshy will transpile your entrypoints and all the files they import, respecting your tsconfig.json settings.

Comparison to tshy — tshy requires you to put your source in a ./src directory, and always builds to ./dist/esm and ./dist/cjs.

What files does zshy create?

A .js file and its .d.ts declarations for each .ts or .tsx source, plus source maps if your tsconfig.json asks for them:

$ tree .
├── package.json
├── src
│   └── index.ts
└── dist
    ├── index.js
    └── index.d.ts

A .mts source becomes .mjs/.d.mts, and a .cts source becomes .cjs/.d.cts: TypeScript's own output for those extensions, which zshy leaves as it is.

Comparison to tshy — tshy generates plain .js/.d.ts files into separate dist/esm and dist/cjs directories, each with a stub package.json to enable proper module resolution in Node.js. This is more convoluted than the flat file structure generated by zshy. It also causes issues with Module Federation.

How does import rewriting work?

zshy rewrites relative import/export specifiers in the JavaScript and declarations that tsc emits, so each one names a real file:

The TypeScript 7 API has no transformer hook, so zshy works on what tsc emits: each relative specifier is found by its position in the parsed output and rewritten there. A string that merely looks like a path is never touched.

Comparison to tshy — tshy was designed to enable dual-package builds powered by the tsc compiler. To make this work, it relies on a specific file structure and the creation of temporary package.json files to accommodate the various idiosyncrasies of Node.js module resolution. It also requires the use of separate dist/esm and dist/cjs build subdirectories.

Can I use extension-less imports?

Yes! zshy supports whatever import style you prefer:

• from "./utils": classic extensionless imports
• from "./utils.js": ESM-friendly extensioned imports
• from "./util.ts": recently supported natively viarewriteRelativeImportExtensions

Use whatever you like; zshy will rewrite all imports/exports properly during the build process.

Comparison to tshy — tshy forces you to use .js imports throughout your codebase. While this is generally a good practice, it's not always feasible, and there are hundreds of thousands of existing TypeScript codebases reliant on extensionless imports.

What about package.json#/exports?

Your exports map is automatically written into your package.json when you run zshy. The generated exports map looks like this:

{
  "zshy": {
    "exports": {
      ".": "./src/index.ts",
      "./utils": "./src/utils.ts",
      "./plugins/*": "./src/plugins/*"
    }
  },
+ "exports": { // auto-generated by zshy
+   ".": {
+     "types": "./dist/index.d.ts",
+     "default": "./dist/index.js"
+   },
+   "./utils": {
+     "types": "./dist/utils.d.ts",
+     "default": "./dist/utils.js"
+   },
+   "./plugins/*": {
+     "types": "./dist/src/plugins/*",
+     "default": "./dist/src/plugins/*"
+   }
+ }
}

Can it support React Native or non-Node.js environments?

Yes! This is one of the key reasons zshy was originally developed. Some environments ignore package.json#/exports — the React Native Metro bundler does by default — which causes issues for packages that want to use subpath imports to structure their package. Fortunately zshy unlocks a workaround I call a flat build:

1. Set outDir: "." in your tsconfig.json

2. Keep your TypeScript sources out of the published package. If package.json has no "files", zshy sets it to the built extensions for you:

   {
     // ...
     "files": ["**/*.js", "**/*.mjs", "**/*.cjs", "**/*.d.ts", "**/*.d.mts", "**/*.d.cts"]
   }
   

With this setup, your build outputs (index.js, etc) will be written to the package root. Older environments will resolve imports like "your-library/utils" to "your-library/utils/index.js", effectively simulating subpath imports in environments that don't support them.

Can I prevent zshy from modifying my package.json/jsr.json?

Yes. If you prefer to manage your export fields manually, you can prevent zshy from making any changes by setting the noEdit option to true in your package.json#/zshy config.

{
  "zshy": {
    "exports": "./src/index.ts",
    "noEdit": true
  }
}

When noEdit is enabled, zshy will build your files but will not write to package.json or jsr.json. You will be responsible for populating the "exports", "bin", "main", "module", and "types" fields yourself.

Is it fast?

Not really. It uses tsc to typecheck your codebase, which is a lot slower than using a bundler that strips types. That said:

1. You should be type checking your code during builds
2. zshy builds on TypeScript 7, the 10x faster native port

Acknowledgements

The DX of zshy was heavily inspired by tshy by @isaacs, particularly its declarative entrypoint map and auto-updating of package.json#/exports. It proved that there's a modern way to transpile libraries using pure tsc (and various package.json hacks). Unfortunately its approach necessarily involved certain constraints that made it unworkable for Zod (described in the FAQ in more detail). zshy borrows elements of tshy's DX while using the Compiler API to relax these constraints and provide a more "batteries included" experience.