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doc: clarify module system selection · nodejs/node@8429216 · GitHub

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doc: clarify module system selection
 Refs: #41345 (comment) PR-URL: #41383 Reviewed-By: Guy Bedford <guybedford@gmail.com> Reviewed-By: Geoffrey Booth <webadmin@geoffreybooth.com>
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‎doc/api/cli.md‎

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# Command-line options
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# Command-line API
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<!--introduced_in=v5.9.1-->
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## Synopsis
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`node [options] [V8 options] [script.js | -e "script" | -] [--] [arguments]`
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`node [options] [V8 options] [<program-entry-point> | -e "script" | -] [--] [arguments]`
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`node inspect [script.js | -e "script" | <host>:<port>] …`
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`node inspect [<program-entry-point> | -e "script" | <host>:<port>] …`
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`node --v8-options`
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Execute without arguments to start the [REPL][].
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For more info about `node inspect`, see the [debugger][] documentation.
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## Program entry point
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The program entry point is a specifier-like string. If the string is not an
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absolute path, it's resolved as a relative path from the current working
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directory. That path is then resolved by [CommonJS][] module loader. If no
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corresponding file is found, an error is thrown.
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If a file is found, its path will be passed to the [ECMAScript module loader][]
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under any of the following conditions:
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* The program was started with a command-line flag that forces the entry
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point to be loaded with ECMAScript module loader.
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* The file has an `.mjs` extension.
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* The file does not have a `.cjs` extension, and the nearest parent
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`package.json` file contains a top-level [`"type"`][] field with a value of
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`"module"`.
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Otherwise, the file is loaded using the CommonJS module loader. See
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[Modules loaders][] for more details.
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### ECMAScript modules loader entry point caveat
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When loading [ECMAScript module loader][] loads the program entry point, the `node`
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command will only accept as input only files with `.js`, `.mjs`, or `.cjs`
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extensions; and with `.wasm` extensions when
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[`--experimental-wasm-modules`][] is enabled.
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## Options
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<!-- YAML
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added: v9.0.0
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-->
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Specify the `module` of a custom experimental [ECMAScript Module loader][].
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`module` may be either a path to a file, or an ECMAScript Module name.
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Specify the `module` of a custom experimental [ECMAScript module loader][].
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`module` may be any string accepted as an [`import` specifier][].
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### `--experimental-policy`
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```
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[Chrome DevTools Protocol]: https://chromedevtools.github.io/devtools-protocol/
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[ECMAScript Module loader]: esm.md#loaders
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[CommonJS]: modules.md
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[ECMAScript module loader]: esm.md#loaders
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[Modules loaders]: packages.md#modules-loaders
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[OSSL_PROVIDER-legacy]: https://www.openssl.org/docs/man3.0/man7/OSSL_PROVIDER-legacy.html
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[REPL]: repl.md
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[ScriptCoverage]: https://chromedevtools.github.io/devtools-protocol/tot/Profiler#type-ScriptCoverage
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[Source Map]: https://sourcemaps.info/spec.html
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[Subresource Integrity]: https://developer.mozilla.org/en-US/docs/Web/Security/Subresource_Integrity
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[V8 JavaScript code coverage]: https://v8project.blogspot.com/2017/12/javascript-code-coverage.html
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[`"type"`]: packages.md#type
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[`--cpu-prof-dir`]: #--cpu-prof-dir
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[`--diagnostic-dir`]: #--diagnostic-dirdirectory
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[`--experimental-wasm-modules`]: #--experimental-wasm-modules
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[`--heap-prof-dir`]: #--heap-prof-dir
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[`--openssl-config`]: #--openssl-configfile
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[`--redirect-warnings`]: #--redirect-warningsfile
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[`dns.lookup()`]: dns.md#dnslookuphostname-options-callback
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[`dns.setDefaultResultOrder()`]: dns.md#dnssetdefaultresultorderorder
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[`dnsPromises.lookup()`]: dns.md#dnspromiseslookuphostname-options
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[`import` specifier]: esm.md#import-specifiers
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[`process.setUncaughtExceptionCaptureCallback()`]: process.md#processsetuncaughtexceptioncapturecallbackfn
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[`tls.DEFAULT_MAX_VERSION`]: tls.md#tlsdefault_max_version
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[`tls.DEFAULT_MIN_VERSION`]: tls.md#tlsdefault_min_version

‎doc/api/esm.md‎

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<!-- type=misc -->
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Node.js treats JavaScript code as CommonJS modules by default.
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Authors can tell Node.js to treat JavaScript code as ECMAScript modules
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Node.js has two module systems: [CommonJS][] modules and ECMAScript modules.
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Authors can tell Node.js to use the ECMAScript modules loader
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via the `.mjs` file extension, the `package.json` [`"type"`][] field, or the
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`--input-type` flag. See
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[Modules: Packages](packages.md#determining-module-system) for more
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details.
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[`--input-type`][] flag. Outside of those cases, Node.js will use the CommonJS
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module loader. See [Determining module system][] for more details.
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<!-- Anchors to make sure old links find a target -->
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[CommonJS]: modules.md
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[Conditional exports]: packages.md#conditional-exports
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[Core modules]: modules.md#core-modules
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[Determining module system]: packages.md#determining-module-system
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[Dynamic `import()`]: https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Dynamic_Imports
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[ES Module Integration Proposal for WebAssembly]: https://github.com/webassembly/esm-integration
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[Import Assertions]: #import-assertions
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[WHATWG JSON modules specification]: https://html.spec.whatwg.org/#creating-a-json-module-script
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[`"exports"`]: packages.md#exports
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[`"type"`]: packages.md#type
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[`--input-type`]: cli.md#--input-typetype
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[`ArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/ArrayBuffer
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[`SharedArrayBuffer`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/SharedArrayBuffer
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[`TypedArray`]: https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/TypedArray

‎doc/api/modules.md‎

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};
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```
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The module system is implemented in the `require('module')` module.
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The CommonJS module system is implemented in the [`module` core module][].
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## Enabling
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<!-- type=misc -->
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Node.js has two module systems: CommonJS modules and [ECMAScript modules][].
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By default, Node.js will treat the following as CommonJS modules:
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* Files with a `.cjs` extension;
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* Files with a `.js` extension when the nearest parent `package.json` file
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contains a top-level field [`"type"`][] with a value of `"commonjs"`.
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* Files with a `.js` extension when the nearest parent `package.json` file
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doesn't contain a top-level field [`"type"`][]. Package authors should include
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the [`"type"`][] field, even in packages where all sources are CommonJS. Being
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explicit about the `type` of the package will make things easier for build
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tools and loaders to determine how the files in the package should be
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interpreted.
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* Files with an extension that is not `.mjs`, `.cjs`, `.json`, `.node`, or `.js`
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(when the nearest parent `package.json` file contains a top-level field
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[`"type"`][] with a value of `"module"`, those files will be recognized as
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CommonJS modules only if they are being `require`d, not when used as the
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command-line entry point of the program).
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See [Determining module system][] for more details.
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Calling `require()` always use the CommonJS module loader. Calling `import()`
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always use the ECMAScript module loader.
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## Accessing the main module
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[ECMAScript Modules]: esm.md
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[GLOBAL_FOLDERS]: #loading-from-the-global-folders
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[`"main"`]: packages.md#main
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[`"type"`]: packages.md#type
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[`ERR_REQUIRE_ESM`]: errors.md#err_require_esm
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[`Error`]: errors.md#class-error
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[`__dirname`]: #__dirname
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[`__filename`]: #__filename
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[`import()`]: https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Dynamic_Imports
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[`module.children`]: #modulechildren
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[`module.id`]: #moduleid
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[`module` core module]: module.md
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[`module` object]: #the-module-object
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[`package.json`]: packages.md#nodejs-packagejson-field-definitions
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[`path.dirname()`]: path.md#pathdirnamepath

‎doc/api/packages.md‎

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## Determining module system
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Node.js will treat the following as [ES modules][] when passed to `node` as the
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initial input, or when referenced by `import` statements within ES module code:
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initial input, or when referenced by `import` statements or `import()`
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expressions:
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* Files ending in `.mjs`.
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* Files with an `.mjs` extension.
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* Files ending in `.js` when the nearest parent `package.json` file contains a
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top-level [`"type"`][] field with a value of `"module"`.
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* Files with a `.js` extension when the nearest parent `package.json` file
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contains a top-level [`"type"`][] field with a value of `"module"`.
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* Strings passed in as an argument to `--eval`, or piped to `node` via `STDIN`,
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with the flag `--input-type=module`.
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preserve backward compatibility. However, now that Node.js supports both
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CommonJS and ES modules, it is best to be explicit whenever possible. Node.js
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will treat the following as CommonJS when passed to `node` as the initial input,
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or when referenced by `import` statements within ES module code:
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or when referenced by `import` statements, `import()` expressions, or
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`require()` expressions:
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* Files ending in `.cjs`.
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* Files with a `.cjs` extension.
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* Files ending in `.js` when the nearest parent `package.json` file contains a
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top-level field [`"type"`][] with a value of `"commonjs"`.
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* Files with a `.js` extension when the nearest parent `package.json` file
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contains a top-level field [`"type"`][] with a value of `"commonjs"`.
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* Strings passed in as an argument to `--eval` or `--print`, or piped to `node`
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via `STDIN`, with the flag `--input-type=commonjs`.
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it will also make things easier for build tools and loaders to determine how the
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### Modules loaders
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Node.js has two systems for resolving a specifier and loading modules.
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There is the CommonJS module loader:
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* It is fully synchronous.
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* It is responsible for handling `require()` calls.
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* It is monkey patchable.
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* It supports [folders as modules][].
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* When resolving a specifier, if no exact match is found, it will try to add
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extensions (`.js`, `.json`, and finally `.node`) and then attempt to resolve
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[folders as modules][].
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* It treats `.json` as JSON text files.
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* `.node` files are interpreted as compiled addon modules loaded with
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`process.dlopen()`.
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* It treats all files that lack `.json` or `.node` extensions as JavaScript
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text files.
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* It cannot be used to load ECMAScript modules (although it is possible to
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[load ECMASCript modules from CommonJS modules][]). When used to load a
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JavaScript text file that is not an ECMAScript module, it loads it as a
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CommonJS module.
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There is the ECMAScript module loader:
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* It is asynchronous.
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* It is responsible for handling `import` statements and `import()` expressions.
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* It is not monkey patchable, can be customized using [loader hooks][].
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* It does not support folders as modules, directory indexes (e.g.
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`'./startup/index.js'`) must be fully specified.
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* It does no extension searching. A file extension must be provided
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when the specifier is a relative or absolute file URL.
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* It can load JSON modules, but an import assertion is required (behind
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`--experimental-json-modules` flag).
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* It accepts only `.js`, `.mjs`, and `.cjs` extensions for JavaScript text
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files.
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* It can be used to load JavaScript CommonJS modules. Such modules
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are passed through the `es-module-lexer` to try to identify named exports,
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which are available if they can be determined through static analysis.
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Imported CommonJS modules have their URLs converted to absolute
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paths and are then loaded via the CommonJS module loader.
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### `package.json` and file extensions
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Within a package, the [`package.json`][] [`"type"`][] field defines how
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[`esm`]: https://github.com/standard-things/esm#readme
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[`package.json`]: #nodejs-packagejson-field-definitions
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[entry points]: #package-entry-points
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[folders as modules]: modules.md#folders-as-modules
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[load ECMASCript modules from CommonJS modules]: modules.md#the-mjs-extension
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[loader hooks]: esm.md#loaders
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[self-reference]: #self-referencing-a-package-using-its-name
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[subpath exports]: #subpath-exports
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[subpath imports]: #subpath-imports

‎doc/api/synopsis.md‎

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If the browser displays the string `Hello, World!`, that indicates
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[Command-line options]: cli.md#command-line-options
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[Command-line options]: cli.md#options
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[Installing Node.js via package manager]: https://nodejs.org/en/download/package-manager/
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[web server]: http.md

‎tools/doc/links-mapper.json‎

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{
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"doc/api/synopsis.md": {
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"command line options": "cli.html#command-line-options",
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"command line options": "cli.html#options",
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"web server": "http.html"
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}
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}
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}

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