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PR-URL: nodejs#35370
Reviewed-By: Derek Lewis <DerekNonGeneric@inf.is>
Reviewed-By: Geoffrey Booth <webmaster@geoffreybooth.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>
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guybedford authored and joesepi committed Oct 22, 2020
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Expand Up @@ -4,11 +4,13 @@

## Introduction

A package is a folder described by a `package.json` file.
A package is a folder tree described by a `package.json` file. The package
consists of the folder containing the `package.json` file and all subfolders
until the next folder containing another `package.json` file, or a folder
named `node_modules`.

A folder containing a `package.json` file, and all subfolders below that folder
until the next folder containing another `package.json` file, are a _package
scope_.
This page provides guidance for package authors writing `package.json` files
along with a reference for the [`package.json`][] fields defined by Node.js.

## Determining module system

Expand All @@ -18,7 +20,7 @@ initial input, or when referenced by `import` statements within ES module code:
* Files ending in `.mjs`.

* Files ending in `.js` when the nearest parent `package.json` file contains a
top-level field [`"type"`][] with a value of `"module"`.
top-level [`"type"`][] field with a value of `"module"`.

* Strings passed in as an argument to `--eval`, or piped to `node` via `STDIN`,
with the flag `--input-type=module`.
Expand All @@ -45,29 +47,25 @@ future-proof the package in case the default type of Node.js ever changes, and
it will also make things easier for build tools and loaders to determine how the
files in the package should be interpreted.

### Package scope and file extensions
### `package.json` and file extensions

A folder containing a `package.json` file, and all subfolders below that folder
until the next folder containing another [`package.json`][], are a
_package scope_. Package scopes do not carry through `node_modules` folders.

Within a package scope, the [`package.json`][] [`"type"`][] field defines how
Within a package, the [`package.json`][] [`"type"`][] field defines how
Node.js should interpret `.js` files. If a `package.json` file does not have a
`"type"` field, `.js` files are treated as [CommonJS][].

A `package.json` `"type"` value of `"module"` tells Node.js to interpret `.js`
files within that package scope as using [ES module][] syntax.
files within that package as using [ES module][] syntax.

The package scope applies not only to initial entry points (`node my-app.js`)
The `"type"` field applies not only to initial entry points (`node my-app.js`)
but also to files referenced by `import` statements and `import()` expressions.

```js
// my-app.js, in an ES module package scope because there is a package.json
// my-app.js, treated as an ES module because there is a package.json
// file in the same folder with "type": "module".

import './startup/init.js';
// Loaded as ES module since ./startup contains no package.json file,
// and therefore inherits the ES module package scope from one level up.
// and therefore inherits the "type" value from one level up.

import 'commonjs-package';
// Loaded as CommonJS since ./node_modules/commonjs-package/package.json
Expand All @@ -79,10 +77,10 @@ import './node_modules/commonjs-package/index.js';
```

Files ending with `.mjs` are always loaded as [ES modules][] regardless of
package scope.
the nearest parent `package.json`.

Files ending with `.cjs` are always loaded as [CommonJS][] regardless of package
scope.
Files ending with `.cjs` are always loaded as [CommonJS][] regardless of the
nearest parent `package.json`.

```js
import './legacy-file.cjs';
Expand All @@ -93,17 +91,17 @@ import 'commonjs-package/src/index.mjs';
```

The `.mjs` and `.cjs` extensions may be used to mix types within the same
package scope:
package:

* Within a `"type": "module"` package scope, Node.js can be instructed to
* Within a `"type": "module"` package, Node.js can be instructed to
interpret a particular file as [CommonJS][] by naming it with a `.cjs`
extension (since both `.js` and `.mjs` files are treated as ES modules within
a `"module"` package scope).
a `"module"` package).

* Within a `"type": "commonjs"` package scope, Node.js can be instructed to
* Within a `"type": "commonjs"` package, Node.js can be instructed to
interpret a particular file as an [ES module][] by naming it with an `.mjs`
extension (since both `.js` and `.cjs` files are treated as CommonJS within a
`"commonjs"` package scope).
`"commonjs"` package).

### `--input-type` flag

Expand Down Expand Up @@ -211,10 +209,10 @@ To set the main entry point for a package, it is advisable to define both
}
```

The benefit of doing this is that when using the [`"exports"`][] field all
subpaths of the package will no longer be available to importers under
`require('pkg/subpath.js')`, and instead they will get a new error,
`ERR_PACKAGE_PATH_NOT_EXPORTED`.
When defining the [`"exports"`][] field, all subpaths of the package will be
encapsulated and no longer available to importers. For example,
`require('pkg/subpath.js')` would throw an [`ERR_PACKAGE_PATH_NOT_EXPORTED`][]
error.

This encapsulation of exports provides more reliable guarantees
about package interfaces for tools and when handling semver upgrades for a
Expand Down Expand Up @@ -295,24 +293,6 @@ treating the right hand side target pattern as a `**` glob against the list of
files within the package. Because `node_modules` paths are forbidden in exports
targets, this expansion is dependent on only the files of the package itself.

### Package exports fallbacks

> Stability: 1 - Experimental
For possible new specifier support in future, array fallbacks are
supported for all invalid specifiers:

```json
{
"exports": {
"./submodule": ["not:valid", "./submodule.js"]
}
}
```

Since `"not:valid"` is not a valid specifier, `"./submodule.js"` is used
instead as the fallback, as if it were the only target.

### Exports sugar

> Stability: 1 - Experimental
Expand Down Expand Up @@ -560,9 +540,10 @@ could be intended only for other environments such as browsers. Such a package
would be usable by any version of Node.js, since `import` can refer to CommonJS
files; but it would not provide any of the advantages of using ES module syntax.

A package could also switch from CommonJS to ES module syntax in a breaking
change version bump. This has the disadvantage that the newest version
of the package would only be usable in ES module-supporting versions of Node.js.
A package could also switch from CommonJS to ES module syntax in a [breaking
change](https://semver.org/) version bump. This has the disadvantage that the
newest version of the package would only be usable in ES module-supporting
versions of Node.js.

Every pattern has tradeoffs, but there are two broad approaches that satisfy the
following conditions:
Expand Down Expand Up @@ -773,6 +754,19 @@ This section describes the fields used by the Node.js runtime. Other tools (such
as [npm](https://docs.npmjs.com/creating-a-package-json-file)) may use
additional fields which are ignored by Node.js and not documented here.

The following fields in `package.json` files are used in Node.js:

* [`"name"`][] - Relevant when using named imports within a package. Also used
by package managers as the name of the package.
* [`"type"`][] - The package type determining whether to load `.js` files as
CommonJS or ES modules.
* [`"exports"`][] - Package exports and conditional exports. When present,
limits which submodules may be loaded from within the package.
* [`"main"`][] - The default module when loading the package, if exports is not
specified, and in versions of Node.js prior to the introduction of exports.
* [`"imports"`][] - Package imports, for use by modules within the package
itself.

### `"name"`
<!-- YAML
added:
Expand Down Expand Up @@ -815,7 +809,7 @@ changes:
* Type: {string}

The `"type"` field defines the module format that Node.js will use for all
`.js` files within a particular `package.json` file’s [package scope][].
`.js` files that have that `package.json` file as their nearest parent.

Files ending with `.js` will be loaded as ES modules when the nearest parent
`package.json` file contains a top-level field `"type"` with a value of
Expand All @@ -839,8 +833,8 @@ node my-app.js # Runs as ES module

If the nearest parent `package.json` lacks a `"type"` field, or contains
`"type": "commonjs"`, `.js` files are treated as [CommonJS][]. If the volume
root is reached and no `package.json` is found, Node.js defers to the default
treatment as [CommonJS][].
root is reached and no `package.json` is found, `.js` files are treated as
[CommonJS][].

`import` statements of `.js` files are treated as ES modules if the nearest
parent `package.json` contains `"type": "module"`.
Expand Down Expand Up @@ -957,26 +951,27 @@ where `import '#dep'` would now get the resolution of the external package
`dep-node-native` (including its exports in turn), and instead get the local
file `./dep-polyfill.js` relative to the package in other environments.

Unlike the exports field, import maps permit mapping to external packages
because this provides an important use case for conditional loading and also can
be done without the risk of cycles, unlike for exports.
Unlike the `"exports"` field, import maps permit mapping to external packages,
providing an important use case for conditional loading scenarios.

Apart from the above, the resolution rules for the imports field are otherwise
analogous to the exports field.

[Babel]: https://babeljs.io/
[Conditional exports]: #packages_conditional_exports
[CommonJS]: modules.html
[entry points]: #packages_package_entry_points
[`esm`]: https://github.com/standard-things/esm#readme
[`ERR_PACKAGE_PATH_NOT_EXPORTED`]: errors.html#errors_err_package_path_not_exported
[ES modules]: esm.html
[ES module]: esm.html
[`esm`]: https://github.com/standard-things/esm#readme
[`"exports"`]: #packages_exports
[`"main"`]: #packages_main
[`"name"`]: #packages_name
[`"imports"`]: #packages_imports
[`"type"`]: #packages_type
[entry points]: #packages_package_entry_points
[`package.json`]: #packages_node_js_package_json_field_definitions
[package scope]: #packages_package_scope_and_file_extensions
[self-reference]: #packages_self_referencing_a_package_using_its_name
[subpath exports]: #packages_subpath_exports
[the full specifier path]: modules_esm.html#modules_esm_mandatory_file_extensions
[the dual CommonJS/ES module packages section]: #packages_dual_commonjs_es_module_packages
[`"type"`]: #packages_type

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