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.
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.
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 require = 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.
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.
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.
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.
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.
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.
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"
]
}
}
}
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
.
Set "liveDev": true
in the tshy config in package.json
to
build in link mode. In this mode, the files are hard-linked into
place in the dist
folder, so that edits are immediately visible.
This is particularly beneficial in monorepo projects, where workspaces may be edited in parallel, and so it's handy to have changes reflected in real time without a rebuild.
Of course, tools that can't handle TypeScript will have a problem
with this, so any generic node
program will not be able to run
your code. For this reason:
liveDev
is always disabled when thenpm_command
environment variable is'publish'
or'pack'
. In these situations, your code is being built for public consumption, and must be compiled.- Code in dist will not be able to be loaded in the node repl
unless you run it with a loader, such as
node --import=tsx
. - Because it links files into place, a rebuild is required when a file is added or removed.
Note: If a package uses Node.js Worker
scripts or bins that
are written in TS and need to be compiled, then liveDev
will
not work, and should not be used. Additionally, any dependencies
of those modules also cannot use liveDev
, unless a loader such
as tsx
is used when run.
See also: "Loading from Source", below.
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.
tl;dr how this works and why it can't be conditional
In the built dist/{dialect}/package.json
files, the ./src
will be stripped from the path and their file extension changed
from ts
to js
(cts
to cjs
and mts
to mjs
).
It shouldn't be conditional, because the condition is already implicit in the build. In the CommonJS build, they should be required, and in the ESM builds, they should be imported, and there's only one thing that it can resolve to from any given build.
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"
}
}
}
tl;dr explanation
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.
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.
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.
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.
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.
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.
Similarly, some tools rely on a top-level module
field, which
is the ESM equivalent to "main"
. If there is an esm
export of
the "."
subpath, and the tshy.module
field in package.json is
not set to false
, then tshy will use that to set the module
field, 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.
If the esm
dialect is not built, or if a "."
export is not
created, or if the "."
export does not support the esm
dialect, and module
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",
"module": "./dist/esm/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"
}
}
}
}
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.
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
.
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"]
}
}
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
If dialect overrides are used, then the "source"
export
condition will refer to the original source for the override. For
example:
{
"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"
}
}
}
}
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.
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.
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"
}
}
}
Put whatever configuration you want in tsconfig.json
, with the
following caveats:
include
- will be overridden based on build, best omittedexclude
- will be overridden based on build, best omitted- compilerOptions:
outDir
- will be overridden based on build, best omittedrootDir
- will be set to./src
in the build, can only cause annoying errors otherwise.target
- will be set toes2022
if not specifiedmodule
- will be set toNodeNext
moduleResolution
- will be set toNodeNext
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.
To facilitate jumping directly to source definitions in some
tools, you can define custom "sourceDialects"
which will be
resolved to the original TypeScript source. These custom dialects
can then be configured to allow build tools (such as tsc) and
editors (such as VS Code and neovim/CoC) to jump directly to
source definitions.
For example, in editors such as VS Code and neovim/CoC that use
the TypeScript language services, you can give them a tsconfig
that contains this:
{
"compilerOptions": {
"customConditions": ["@my-project/source"]
}
}
And then add this to your package.json
file:
{
"tshy": {
"sourceDialects": ["@my-project/source"]
}
}
If you are loading your program with a custom Node.js importer
like tsx
that can load TypeScript
directly, you can specify it like this:
node --import=tsx --conditions=@my-project/source ./script.ts
Other TypeScript-aware tools may have other mechanisms for specifying export conditions. Refer to their documentation for more information.
Note that sourceDialects
are only added to exports whose type
matches the top-level package.json
type
field. For example,
if the package.json
includes "type": "module"
, then
sourceDialects
export conditions will only be added for the
import
condition, not the require
condition.
See also: "Live Dev", above.
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-awaretypescript-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"
}
}
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.
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
}
}
tl;dr explanation
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"]
}
}
}