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doc: clarify require behavior with non .js extensions
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Refs: #41333

PR-URL: #41345
Reviewed-By: Geoffrey Booth <[email protected]>
Reviewed-By: Guy Bedford <[email protected]>
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aduh95 authored and danielleadams committed Jan 31, 2022
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27 changes: 18 additions & 9 deletions doc/api/modules.md
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Expand Up @@ -133,10 +133,13 @@ relative, and based on the real path of the files making the calls to

## The `.mjs` extension

It is not possible to `require()` files that have the `.mjs` extension.
Attempting to do so will throw [an error][]. The `.mjs` extension is
reserved for [ECMAScript Modules][] which cannot be loaded via `require()`.
See [ECMAScript Modules][] for more details.
Due to the synchronous nature of `require()`, it is not possible to use it to
load ECMAScript module files. Attempting to do so will throw a
[`ERR_REQUIRE_ESM`][] error. Use [`import()`][] instead.

The `.mjs` extension is reserved for [ECMAScript Modules][] which cannot be
loaded via `require()`. See [Determining module system][] section for more info
regarding which files are parsed as ECMAScript modules.

## All together...

Expand Down Expand Up @@ -373,11 +376,15 @@ correctly within an application.

If the exact filename is not found, then Node.js will attempt to load the
required filename with the added extensions: `.js`, `.json`, and finally
`.node`.
`.node`. When loading a file that has a different extension (e.g. `.cjs`), its
full name must be passed to `require()`, including its file extension (e.g.
`require('./file.cjs')`).

`.js` files are interpreted as JavaScript text files, and `.json` files are
parsed as JSON text files. `.node` files are interpreted as compiled addon
modules loaded with `process.dlopen()`.
`.json` files are parsed as JSON text files, `.node` files are interpreted as
compiled addon modules loaded with `process.dlopen()`. Files using any other
extension (or no extension at all) are parsed as JavaScript text files. Refer to
the [Determining module system][] section to understand what parse goal will be
used.

A required module prefixed with `'/'` is an absolute path to the file. For
example, `require('/home/marco/foo.js')` will load the file at
Expand Down Expand Up @@ -1032,19 +1039,21 @@ This section was moved to
* <a id="modules_sourcemap_payload" href="module.html#sourcemappayload">`sourceMap.payload`</a>
* <a id="modules_sourcemap_findentry_linenumber_columnnumber" href="module.html#sourcemapfindentrylinenumber-columnnumber">`sourceMap.findEntry(lineNumber, columnNumber)`</a>

[Determining module system]: packages.md#determining-module-system
[ECMAScript Modules]: esm.md
[GLOBAL_FOLDERS]: #loading-from-the-global-folders
[`"main"`]: packages.md#main
[`ERR_REQUIRE_ESM`]: errors.md#err_require_esm
[`Error`]: errors.md#class-error
[`__dirname`]: #__dirname
[`__filename`]: #__filename
[`import()`]: https://wiki.developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Statements/import#Dynamic_Imports
[`module.children`]: #modulechildren
[`module.id`]: #moduleid
[`module` object]: #the-module-object
[`package.json`]: packages.md#nodejs-packagejson-field-definitions
[`path.dirname()`]: path.md#pathdirnamepath
[`require.main`]: #requiremain
[an error]: errors.md#err_require_esm
[exports shortcut]: #exports-shortcut
[module resolution]: #all-together
[native addons]: addons.md

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