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.

Dual Package Hazards

If you are exporting both CommonJS and ESM forms of a package,
then it is possible for both versions to be loaded at run-time.
However, the CommonJS build is a different module from the ESM
build, and thus a different thing from the point of view of the
JavaScript interpreter in Node.js.

Consider this contrived example:

 // import the class from ESM
import { SomeClass } from 'module-built-by-tshy'
import { createRequire } from 'node:module'
const req = createRequire(import.meta.url)

// create an object using the commonjs version
function getObject() {
  const { SomeClass } = require('module-built-by-tshy')
  return new SomeClass()
}

const obj = getObject()
console.log(obj instanceof SomeClass) // false!!

In a real program, this might happen because one part of the code
loads the package using require() and another loads it using
import.

The Node.js documentation
recommends
exporting an ESM wrapper that re-exports the CommonJS code, or
isolating state into a single module used by both CommonJS and
ESM. While these strategies do work, they are not what tshy does.

What Does tshy Do Instead?

It builds your program twice, into two separate folders, and sets
up exports. By default, the ESM and CommonJS forms live in
separate universes, unaware of one another, and treats the "Dual
Module Hazard" as a simple fact of life.

Which it is.

"Dual Module Hazard" is a fact of life anyway

Since the advent of npm, circa 2010, module in node have been
potentially duplicated in the dependency graph. Node's nested
node_modules resolution algorithm, added in Node 0.4, made this
even easier to leverage, and more likely to occur.

So: as a package author, you cannot safely rely on there being
exactly one copy of your library loaded at run-time.

This doesn't mean you shouldn't care about it. It means that you
should take it into consideration always, whether you are using
a hybrid build or not.

If you need to ensure that exactly one copy of something exists
at run-time, whether using a hybrid build or not, you need to
guard this with a check that is not dependent on the dependency
graph, such as a global variable.

 const ThereCanBeOnlyOne = Symbol.for('there can be only one')
const g = globalThis as typeof globalThis & {
  [ThereCanBeOnlyOne]?: Thing
}
import { Thing } from './thing.js'
g[ThereCanBeOnlyOne] ??= new Thing
export const thing = g[ThereCanBeOnlyOne]

If you find yourself doing this, it's a good idea to pause and
consider if you would be better off with a type check function or
something other than relying on instanceof. There are certainly
cases where it's unavoidable, but it can be tricky to work with.

Module Local State

There are some cases where you need something to be the same
value whether loaded with CommonJS or ESM, but not necessarily
unique to the entire program.

For example, say that there is some package-local set of data,
and it needs to be updated and accessible whether the user is
accessing your package via import or require.

In this case, we can use a dialect polyfill that pulls in the
state module from a single dialect.

In Node, it's easy for ESM to load CommonJS, but since ESM cannot
be loaded synchronously by CommonJS, I recommend putting the
state in the polyfill, and having the "normal" module access it
from that location.

For example:

 // src/index.ts
import { state } from './state.js'
export const setValue = (key: string, value: any) => {
  state[key] = value
}
export const getValue = (key: string) => state[key]

 // src/state-cjs.cts
// this is the actual "thing"
export const state: Record<string, any> = {}

 // src/state.ts
// this is what will end up in the esm build
// need a ts-ignore because this is a hack.
//@ts-ignore
import cjsState from '../commonjs/state.js'
export const { state } = cjsState as { state: Record<string, any> }

If you need a provide an ESM dialect that doesn't support
CommonJS (eg, deno, browser, etc), then you can do this:

 // src/state-deno.mts
// can't load the CJS version, so no dual package hazard
export const state: Record<string, any> = {}

See below for more on using dialect specific polyfills.

Handling Default Exports

export default is the bane of hybrid TypeScript modules.

When compiled as CommonJS, this results in creating an export
named default, which is not the same as setting
module.exports.

 // esm, beautiful and clean
import foo from 'foo'
// commonjs, unnecessarily ugly and confusing
// even if you like it for some reason, it's not "the same"
const { default: foo } = require('foo')

You can tell TypeScript to do a true default export for CommonJS
by using export = <whatever>. However:

• This is not compatible with an ESM build.
• You cannot export types along with it.

In general, when publishing TypeScript packages as both CommonJS
and ESM, it is a good idea to avoid default exports for any
public interfaces.

• No need to polyfill anything.
• Can export types alongside the values.

However, if you are publishing something that does need to
provide a default export (for example, porting a project to
hybrid and/or TypeScript, and want to keep the interface
consistent), you can do it with a CommonJS polyfill.

 // index.ts
// the thing that gets exported for ESM
import { thing } from './main.ts'
import type { SomeType } from './main.ts'

export default thing
export type { SomeType }

 // index-cjs.cts
// the polyfill for CommonJS
import * as items from './main.ts'
declare global {
  namespace mything {
    export type SomeType = items.SomeType
  }
}
export = items.thing

Then, CommonJS users will get the appropriate thing when they
import 'mything', and can access the type via the global
namespace like mything.SomeType.

But in almost all cases, it's much simpler to just use named
exports exclusively.

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, but a lot of things
can be configured.

exports

By default, if there is a src/index.ts file, then that will be
set as the "." export, and the package.json file will be
exported as "./package.json", because that's just convenient to
expose.

You can set other entry points by putting something like 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"
      ]
    }
  }
}

Glob Exports

You can also specify one or more glob
patterns to define your exported modules. This is handy if you
want to export several things as subpath exports to avoid "bucket
modules".

Anything named src/index.* that is matched in this way will be
used as the main "." export. Anything else will have the
./src stripped from the front, and the file extension removed
from the end. ./package.json will always be exported, and any
pattern matches outside of the ./src folder will be ignored.

 {
  "tshy": {
    "exports": "./src/**"
  }
}

If you use this config, and you have files at ./src/index.ts
and ./src/component/foo.tsx, then the resulting exports will
be:

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

You may also specify an array of glob exports, like so:

 {
  "tshy": {
    "exports": [
      "./src/*.ts",
      "./src/utils/*.ts"
    ]
  }
}

This would export a file at ./src/foo.ts as ./foo, and a file
at ./src/utils/bar.ts as ./utils/bar, but would ignore a file
at ./internal/private.ts.

Package #imports

You can use "imports" in your package.json, and it will be
handled in the following ways.

Any "imports" that resolve to a file built as part of your
program must be a non-conditional string value pointing to the
file in ./src/. For example:

 {
  "imports": {
    "#name": "./src/path/to/name.ts",
    "#utils/*": "./src/path/to/utils/*.ts"
  }
}

In the ESM build, import * from '#name' will resolve to
./dist/esm/path/to/name.js, and will be built for ESM. In the
CommonJS build, require('#name') will resolve to
./dist/commonjs/path/to/name.js and will be built for CommonJS.

If there are any "imports" that resolve to something not
built by tshy, then tshy will set scripts.preinstall to set up
symbolic links at install time to make it work. This just means
that you can't use scripts.preinstall for anything else if you
have "imports" that aren't managed by tshy. For example:

 {
  "imports": {
    "#dep": "@scope/dep/submodule",
    "#conditional": {
      "types": "./vendor/blah.d.ts",
      "require": "./vendor/blah.cjs",
      "import": "./vendor/blah.mjs"
    }
  }
}

If a tshy.imports is present (a previous iteration of this
behavior), it will be merged into the top-level "imports" and
deleted from the tshy section.

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
  }
}

If the selfLink config is not explicitly set, and creating the
symlink fails (common on Windows systems where fs.symlink() may
require elevated permissions), then the error will be ignored.

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. Unfortunately, even 7 years later at the
time of this writing, some projects are still using outdated
tools that are not capable of understanding this interface.

If there is a commonjs export of the "." subpath, and the
tshy.main field in package.json is not set to false, then
tshy will use that to set the main and types fields, for
compatibility with these tools.

Warning: relying on top-level main/types 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.

If the commonjs dialect is not built, or if a "." export is
not created, or if the "." export does not support the
commonjs dialect, and main is explicitly set to true, then
the build will fail.

For example, this config:

 {
  "tshy": {
    "exports": {
      ".": "./src/index.ts"
    }
  }
}

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.

Excluding Files Entirely From All Builds

If you want to keep some files from being processed by tshy's
builds entirely, you can add an exclude string[] field to the
tshy object in package.json. For example:

 {
  "tshy": {
    "exclude": ["src/**/*.test.ts"]
  }
}

Other Targets: browser, deno, etc.

If you have any other dialects that you'd like to support, you
can list them as either commonjsDialects or esmDialects,
depending on whether you want them to be built as CommonJS or
ESM.

Note that each added dialect you create will result in another
build in the ./dist folder, so you may wish to use sparingly if
shipping a large project.

For example:

 {
  "tshy": {
    "exports": {
      ".": "./src/index.ts"
    },
    "esmDialects": ["deno", "browser"],
    "commonjsDialects": ["webpack"]
  }
}

Will result in:

 {
  "exports": {
    ".": {
      "deno": {
        "types": "./dist/deno/index.d.ts",
        "default": "./dist/deno/index.js"
      },
      "browser": {
        "types": "./dist/browser/index.d.ts",
        "default": "./dist/browser/index.js"
      },
      "webpack": {
        "types": "./dist/webpack/index.d.ts",
        "default": "./dist/webpack/index.js"
      },
      "require": {
        "types": "./dist/commonjs/index.d.ts",
        "default": "./dist/commonjs/index.js"
      },
      "import": {
        "types": "./dist/esm/index.d.ts",
        "default": "./dist/esm/index.js"
      }
    }
  }
}

In each of these, you can use the same kind of dialect override
that works for CommonJS polyfills described above. For
commonjsDialects types, create a file named
<filename>-<dialect>.cts, and for esmDialects types, create a
file named <filename>-<dialect>.mts.

For example, to provide deno, browser, and webpack overrides in
the setup above, the following files would be relevant:

src/index.ts           # normal esm/cjs version
src/index-cjs.cts      # cjs variant for default commonjs
src/index-browser.mts  # esm variant for the browser
src/index-deno.mts     # esm variant for deno
src/index-webpack.cts  # cjs variant for webpack

Note that the commonjs override uses the abbreviated cjs
name (historical reasons, it was originally the only override
supported), and that the file extension must be cts or mts
depending on the dialect type that it is.

Atomic Builds

Code is built in ./.tshy-build 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).

If you use "incremental": true in your tsconfig, then this
folder will persist, so that TSC can benefit from the
.tsbuildinfo files it creates in there.

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 if not specified
  • 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.

Custom project

Configure tshy.project if you want tshy to extend from a custom
tsconfig file. This is often useful when you have multiple
tsconfig files for different tools:

• A default tsconfig.json for typechecking and type-aware
  typescript-eslint, specifying "noEmit": true and
  "include": ["**/*.ts"]
• A tsconfig.build.json for compilation, with "noEmit": false. Note that the caveats above still apply.

 {
  "tshy": {
    "project": "./tsconfig.build.json"
  }
}

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.

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
  }
}

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

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

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