tshy
TypeScript HYbridizer - Hybrid (CommonJS/ESM) TypeScript node package builder
Last updated a year ago by isaacs .
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tshy - TypeScript HYbridizer

Hybrid (CommonJS/ESM) TypeScript node package builder. Write modules that Just Work in ESM and CommonJS, in easy mode.

This tool manages the exports in your package.json file, and builds your TypeScript program using tsc 5.2, emitting both ESM and CommonJS variants, providing the full strength of TypeScript’s checking for both output formats.

USAGE

Install tshy:

npm i -D tshy

Put this in your package.json to use it with the default configs:

{
  "files": ["dist"],
  "scripts": {
    "prepare": "tshy"
  }
}

Put your source code in ./src.

The built files will end up in ./dist/esm (ESM) and ./dist/commonjs (CommonJS).

Your exports will be edited to reflect the correct module entry points.

Configuration

Mostly, this just uses opinionated convention, and so there is very little to configure.

Source must be in ./src. Builds are in ./dist/commonjs for CommonJS and ./dist/esm for ESM.

There is very little configuration for this. The only thing to decide is the exported paths. If you have a ./index.ts file, then that will be listed as the main "." export by default.

exports

You can set other entry points by putting this in your package.json file:

{
  "tshy": {
    "exports": {
      "./foo": "./src/foo.ts",
      "./bar": "./src/bar.ts",
      ".": "./src/something-other-than-index.ts",
      "./package.json": "./package.json"
    }
  }
}

Any exports pointing to files in ./src will be updated to their appropriate build target locations, like:

{
  "exports": {
    "./foo": {
      "import": {
        "types": "./dist/esm/foo.d.ts",
        "default": "./dist/esm/foo.js"
      },
      "require": {
        "types": "./dist/commonjs/foo.d.ts",
        "default": "./dist/commonjs/foo.js"
      }
    }
  }
}

Any exports that are not within ./src will not be built, and can be anything supported by package.json exports, as they will just be passed through as-is.

{
  "tshy": {
    "exports": {
      ".": "./src/my-built-module.ts",
      "./package.json": "./package.json"
      "./thing": {
        "import": "./lib/thing.mjs",
        "require": "./lib/thing.cjs",
        "types": "./lib/thing.d.ts"
      },
      "./arraystyle": [
        { "import": "./no-op.js" },
        { "browser": "./browser-thing.js" },
        { "require": [{ "types": "./using-require.d.ts" }, "./using-require.js"]},
        { "types": "./blah.d.ts" },
        "./etc.js"
      ]
    }
  }
}

Making Noise

On failure, all logs will be printed.

To print error logs and a success! message at the end, set TSHY_VERBOSE=1 in the environment.

To print debugging and other extra information, set TSHY_VERBOSE=2 in the environment.

Selecting Dialects

You can tell tshy which dialect you're building for by setting the dialects config to an array of strings:

{
  "tshy": {
    "dialects": ["esm", "commonjs"]
  }
}

The default is ["esm", "commonjs"] (ie, both of them). If you set it to just one, then only that dialect will be built and exported.

Suppressing the self-link

See below about Local Package exports for an explanation of what this is.

Suppress the symlink to the project folder into a node_modules folder in dist and src by doing this:

{
  "tshy": {
    "selfLink": false
  }
}

Old Style Exports

Versions of node prior to 12.10.0 (published in early to mid 2016) did not have support for exports as a means for defining package entry points.

By default, tshy deletes the main field, rather than maintain this affordance for versions of node that met their end of life long ago. However, some tools still rely on main and have not been updated to read the package entry points via exports.

Warning: this will likely cause incorrect types to be loaded in some scenarios.

Use with extreme caution. It's almost always better to not define top-level main and types fields if you are shipping a hybrid module. Users will need to update their module and moduleResolution tsconfigs appropriately. That is a good thing, and will save them future headaches.

You can tell tshy to export a top-level main and types field by setting main to true.

If the commonjs dialect is not built, or if a "." export is not created, or if the "." export does not support the commonjs dialect, then the build will fail.

For example, this config:

{
  "tshy": {
    "exports": {
      ".": "./src/index.ts"
    },
    "main": true
  }
}

will produce:

{
  "main": "./dist/commonjs/index.js",
  "types": "./dist/commonjs/index.d.ts",
  "type": "module",
  "exports": {
    ".": {
      "require": {
        "types": "./dist/commonjs/index.d.ts",
        "default": "./dist/commonjs/index.js"
      },
      "import": {
        "types": "./dist/esm/index.d.ts",
        "default": "./dist/esm/index.js"
      }
    }
  }
}

CommonJS Dialect Polyfills

Sometimes you have to do something in different ways depending on the JS dialect in use. For example, maybe you have to use import.meta.url in ESM, but polyfill with pathToFileURL(__filename) in CommonJS.

To do this, create a polyfill file with the CommonJS code in <name>-cjs.cts. (The cts extension matters.)

// src/source-dir-cjs.cts
//     ^^^^^^^^^^--------- matching name
//               ^^^^----- "-cjs" tag
//                   ^^^^- ".cts" filename suffix
// this one has a -cjs.cts suffix, so it will override the
// module at src/source-dir.ts in the CJS build,
// and be excluded from the esm build.
import { pathToFileURL } from 'node:url'
//@ts-ignore - Have to ignore because TSC thinks this is ESM
export const sourceDir = pathToFileURL(__dirname)

Then put the "real" ESM code in <name>.ts (not .mts!)

You will generally have to //@ts-ignore a bunch of stuff to get the CommonJS build to ignore it, so it's best to keep the polyfill surface as small as possible.

// src/source-dir.ts
// This is the ESM version of the module
//@ts-ignore
export const sourceDir = new URL('.', import.meta.url)

Then in your code, you can just import { sourceDir } from './source-dir.js' and it'll work in both builds.

Excluding from a build using .cts and .mts files

Files named *.mts will be excluded from the CommonJS build.

Files named *.cts will be excluded from the ESM build.

If you need to do something one way for CommonJS and another way for esm, use the "Dialect Switching" trick, with the ESM code living in src/<whatever>.ts and the CommonJS polyfill living in src/<whatever>-cjs.cts.

Atomic Builds

Code is built in ./.tshy-build-tmp and then copied over only if the build succeeds. This makes it work in monorepo cases where you may have packages that depend on one another and are all being built in parallel (as long as they've been built one time, of course).

Exports Management

The exports field in your package.json file will be updated based on the tshy.exports configuration, as described above.

If you don't provide that config, then the default is:

{
  "tshy": {
    "exports": {
      ".": "./src/index.ts",
      "./package.json": "./package.json"
    }
  }
}

TSConfigs

Put whatever configuration you want in tsconfig.json, with the following caveats:

  • include - will be overridden based on build, best omitted
  • exclude - will be overridden based on build, best omitted
  • compilerOptions:
    • outDir - will be overridden based on build, best omitted
    • rootDir - will be set to ./src in the build, can only cause annoying errors otherwise.
    • target - will be set to es2022
    • module - will be set to NodeNext
    • moduleResolution - will be set to NodeNext

If you don't have a tsconfig.json file, then one will be provided for you.

Then the tsconfig.json file will be used as the default project for code hints in VSCode, neovim, tests, etc.

src/package.json

As of TypeScript 5.2, the only way to emit JavaScript to ESM or cjs, and also import packages using node-style "exports"-aware module resolution, is to set the type field in the package.json file closest to the TypeScript source code.

During the build, tshy will create a file at src/package.json for this purpose, and then delete it afterwards. If that file exists and wasn't put there by tshy, then it will be destroyed.

Package #imports

If you use "imports" in your package.json, then tshy will set scripts.preinstall to set up some symbolic links to make it work. This just means you can't use scripts.preinstall for anything else if you use "imports".

<summary>tl;dr explanation</summary>

The "imports" field in package.json allows you to set local package imports, which have the same kind of conditional import logic as "exports". This is especially useful when you have a vendored dependency with require and import variants, modules that have to be bundled in different ways for different environments, or different dependencies for different environments.

These package imports are always resolved against the nearest package.json file, and tshy uses generated package.json files to set the module dialect to "type":"module" in dist/esm and "type":"commonjs" in dist/commonjs, and it swaps the src/package.json file between this during the tsc builds.

Furthermore, local package imports may not be relative files outside the package folder. They may only be local files within the local package, or dependencies resolved in node_modules.

To support this, tshy copies the imports field from the project's package.json into these dialect-setting generated package.json files, and creates symlinks into the appropriate places so that they resolve to the same files on disk.

Because symlinks may not be included in npm packages (and even if they are included, they won't be unpacked at install time), the symlinks it places in ./dist wouldn't do much good. In order to work around this restriction, tshy creates a node program at dist/.tshy-link-imports.mjs, which generates the symlinks at install time via the preinstall script.

Local Package exports

In order to facilitate local package exports, tshy will create a symlink to the current package temporarily in ./src/node_modules and permanently in ./dist/node_modules.

If you rely on this feature, you may need to add a paths section to your tsconfig.json so that you don't get nagged constantly by your editor about missing type references.

You can suppress the self-linking by putting this config in package.json but be advised this means that you won't be able to import from local package exports:

{
  "tshy": {
    "selfLink": false
  }
}
<summary>tl;dr explanation</summary>

Similar to local module imports, Node supports importing the exports of the current package as if it was a dependency of itself. The generated package.json files mess with this similar to imports, but it's much easier to work around.

For example, if you had this in your package.json:

{
  "name": "@my/package",
  "exports": {
    "./foo": {
      "import": "./lib/foo.mjs",
      "require": "./lib/foo.cjs"
    }
  }
}

Then any module in the package could do import('@my/package/foo') or require('@my/package/foo') to pull in the appropriate file.

In order to make this wort, tshy links the current project directory into ./src/node_modules/<pkgname> during the builds, and removes the link afterwards, so that TypeScript knows what those things refer to.

The link is also created in the dist folder, but it's only relevant if your tests load the code from ./dist rather than from ./src. In the install, there's no need to re-create this link, because the package will be in a node_modules folder already.

If you use this feature, you can put something like this in your tsconfig.json file so that your editor knows what those things refer to:

{
  "compilerOptions": {
    "paths": {
      "@my/package/foo": ["./src/foo.js"],
      "@my/package/bar": ["./src/bar.js"]
    }
  }
}

Note the .js extension, rather than .ts. Add this for each submodule path that you use in this way, or use a wildcard if you prefer, though this might result in failing to catch errors if you use a submodule identifier that isn't actually exported:

{
  "compilerOptions": {
    "paths": {
      "@my/package/*": ["./src/*.js"]
    }
  }
}

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