module: implement the "module-sync" exports condition · nodejs/node@df8a045

formats include CommonJS, JSON, native addons, and ES modules

if `--experimental-require-module` is enabled. _Always mutually

exclusive with `"import"`._

* `"module-sync"` - matches no matter the package is loaded via `import`,

`import()` or `require()`. The format is expected to be ES modules that does

not contain top-level await in its module graph - if it does,

`ERR_REQUIRE_ASYNC_MODULE` will be thrown when the module is `require()`-ed.

* `"default"` - the generic fallback that always matches. Can be a CommonJS

or ES module file. _This condition should always come last._

### Community Conditions Definitions

Condition strings other than the `"import"`, `"require"`, `"node"`,

Condition strings other than the `"import"`, `"require"`, `"node"`, `"module-sync"`,

`"node-addons"` and `"default"` conditions

[implemented in Node.js core](#conditional-exports) are ignored by default.

## Dual CommonJS/ES module packages

<!-- This section should not be in the API documentation:

1. It teaches opinionated practices that some consider dangerous, see

https://github.com/nodejs/node/issues/52174

2. It will soon be obsolete when we unflag --experimental-require-module.

3. It's difficult to understand a multi-file structure via long texts and snippets in

a markdown document.

TODO(?): Move this section to its own repository with example folders.

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Prior to the introduction of support for ES modules in Node.js, it was a common

pattern for package authors to include both CommonJS and ES module JavaScript

sources in their package, with `package.json` [`"main"`][] specifying the

Node.js can now run ES module entry points, and a package can contain both

CommonJS and ES module entry points (either via separate specifiers such as

`'pkg'` and `'pkg/es-module'`, or both at the same specifier via [Conditional

exports][]). Unlike in the scenario where `"module"` is only used by bundlers,

exports][]). Unlike in the scenario where top-level `"module"` field is only used by bundlers,

or ES module files are transpiled into CommonJS on the fly before evaluation by

Node.js, the files referenced by the ES module entry point are evaluated as ES

modules.