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

$3

Yes! With some strategic overrides:

- module: Overridden ("commonjs" for CJS build, "esnext" for ESM build)
- moduleResolution: Overridden ("node10" for CJS, "bundler" for ESM)
- declaration/noEmit/emitDeclarationOnly: Overridden to ensure proper output
- verbatimModuleSyntax: Set to false to allow multiple build formats
- 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)

$3

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.

$3

It depends on your package.json#/type field. If your package is ESM (that is, "type": "module" in package.json):

- .js + .d.ts (ESM)
- .cjs + .d.cts (CJS)

`bash
$ tree dist

.
├── package.json # if type == "module"
├── src
│   └── index.ts
└── dist
   ├── index.js
   ├── index.d.ts
   ├── index.cjs
   └── index.d.cts
`

Otherwise, the package is considered _default-CJS_ and the ESM build files will be rewritten as .mjs/.d.mts.

- .mjs + .d.mts (ESM)
- .js + .d.ts (CJS)

`bash
$ tree dist
.
├── package.json # if type != "module"
├── src
│   └── index.ts
└── dist
   ├── index.js
   ├── index.d.ts
   ├── index.mjs
   └── index.d.mts
`

> 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.

$3

zshy uses the TypeScript Compiler API to rewrite file extensions during the tsc emit step.

- If "type": "module"
- .ts becomes .js/.d.ts (ESM) and .cjs/.d.cts (CJS)
- Otherwise:
- .ts becomes .mjs/.d.mts (ESM) and .js/.d.ts (CJS)

Similarly, all relative import/export statements are rewritten to account for the new file extensions.

| Original path | Result (ESM) | Result (CJS) |
| ------------------ | ------------------ | ------------------- |
| from "./util" | from "./util.js" | from "./util.cjs" |
| from "./util.ts" | from "./util.js" | from "./util.cjs" |
| from "./util.js" | from "./util.js" | from "./util.cjs" |

TypeScript's Compiler API provides dedicated hooks for performing such transforms (though they are criminally under-utilized).

- ts.TransformerFactory: Provides AST transformations to rewrite import/export extensions before module conversion
- ts.CompilerHost#writeFile: Handles output file extension changes (.js → .cjs/.mjs)

> 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.

$3

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.

$3

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

`diff
{
"zshy": {
"exports": {
".": "./src/index.ts",
"./utils": "./src/utils.ts",
"./plugins/": "./src/plugins/"
}
},
+ "exports": { // auto-generated by zshy
+ ".": {
+ "types": "./dist/index.d.cts",
+ "import": "./dist/index.js",
+ "require": "./dist/index.cjs"
+ },
+ "./utils": {
+ "types": "./dist/utils.d.cts",
+ "import": "./dist/utils.js",
+ "require": "./dist/utils.cjs"
+ },
+ "./plugins/*": {
+ "import": "./dist/src/plugins/*",
+ "require": "./dist/src/plugins/*"
+ }
+ }
}
`

$3

The "types" field always points to the CJS declaration file (.d.cts). This is an intentional design choice. It solves the "Masquerading as ESM" issue. You've likely seen this dreaded error before:

`ts
import mod from "pkg"; ^^^^^
// ^ The current file is a CommonJS module whose imports will produce 'require' calls; however, the referenced file is an ECMAScript module and cannot be imported with 'require'. Consider writing a dynamic 'import("pkg")' call instead.
`

Simply put, an ESM file can import CommonJS, but CommonJS files can't require ESM. By having "types" point to the .d.cts declarations, we can always avoid the error above. Technically we're tricking TypeScript into thinking our code is CommonJS; in practice, this has no real consequences and maximizes compatibility.

To learn more, read the "Masquerading as ESM" writeup from ATTW.

> Comparison to tshy — tshy generates independent (but identical) .d.ts files in dist/esm and dist/cjs. This can cause Excessively Deep errors if users of the library use declaration merging (declare module {}) for plugins/extensions. Zod, day.js, and others rely on this pattern for plugins.

$3

This is expected behavior when running the "Are The Types Wrong" tool. This warning does not cause any resolution issues (unlike "Masquerading as ESM"). Technically, we're tricking TypeScript into thinking our code is CommonJS; when in fact it may be ESM. The ATTW tool is very rigorous and flags this; in practice, this has no real consequences and maximizes compatibility (Zod has relied on the CJS masquerading trick since it's earliest days.)

To learn more, read the "Masquerading as CJS" writeup from ATTW.

$3

CJS interop transform — When a file contains a single export default ... and _no named exports_...

`ts
function hello() {
console.log("hello");
}

export default hello;
`

...the built .cjs code will assign the exported value directly to module.exports:

`ts
function hello() {
console.log("hello");
}
exports.default = hello;
module.exports = exports.default;
`

...and the associated .d.cts files will use export = syntax:

`ts
declare function hello(): void;
export = hello;
`

The ESM build is not impacted by this transform.

ESM interop transform — Similarly, if a source .ts file contains the following syntax:

`ts
export = ...
`

...the generated _ESM_ build will transpile to the following syntax:

`ts
export default ...
`

$3

Yes! This is one of the key reasons zshy was originally developed. Many environments don't support package.json#/exports yet:

- Node.js v12.7 or earlier
- React Native - The Metro bundler does not support "exports" by default
- TypeScript projects with legacy configs — e.g. "module": "commonjs"

This 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. Remove "type": "module" from your package.json (if present)
2. Set outDir: "." in your tsconfig.json
3. Configure "exclude" in package.json to exclude all source files:

`jsonc
{
// ...
"exclude": ["/.ts", "/.tsx", "/.cts", "/.mts", "node_modules"]
}
`

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.

$3

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.

`jsonc
{
"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.

$3

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. TypeScript is about to get 10x faster

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.