module: add module.customConditions

Add module.customConditions which exposes the
custom resolution conditions specified by the
user.

Fixes: #55824

Author: sapphi-red

doc/api/module.md

@@ -66,6 +66,42 @@ const require = createRequire(import.meta.url);
 const siblingModule = require('./sibling-module');
 ```
 
+### `module.customConditions`
+
+<!-- YAML
+added: REPLACEME
+-->
+
+* Type: {Set<string>}
+
+The custom resolution conditions specified by the user. The default Node.js conditions are not included.
+
+For example, assuming the following script for `module-customconditions.js`:
+
+```mjs
+import module from 'node:module';
+
+console.log('conditions:', module.customConditions);
+```
+
+```cjs
+const module = require('node:module');
+
+console.log('conditions:', module.customConditions);
+```
+
+Launching the Node.js process as:
+
+```console
+$ node -C additional module-customconditions.js
+```
+
+Would generate the output:
+
+```text
+conditions: Set(1) { 'additional' }
+```
+
 ### `module.findPackageJSON(specifier[, base])`
 
 <!-- YAML

lib/module.js

@@ -1,5 +1,11 @@
 'use strict';
 
+const {
+  ObjectDefineProperty,
+  SafeArrayIterator,
+  Set,
+} = primordials;
+const { getOptionValue } = require('internal/options');
 const {
   findSourceMap,
   getSourceMapsSupport,
@@ -23,6 +29,16 @@ const { stripTypeScriptTypes } = require('internal/modules/typescript');
 
 Module.register = register;
 Module.constants = constants;
+ObjectDefineProperty(Module, 'customConditions', {
+  __proto__: null,
+  get() {
+    const value = new Set(new SafeArrayIterator(getOptionValue('--conditions')));
+    ObjectDefineProperty(this, 'customConditions', { __proto__: null, value, configurable: false });
+    return value;
+  },
+  enumerable: true,
+  configurable: true,
+});
 Module.enableCompileCache = enableCompileCache;
 Module.findPackageJSON = findPackageJSON;
 Module.flushCompileCache = flushCompileCache;

test/parallel/test-module-customConditions.js

@@ -0,0 +1,9 @@
+'use strict';
+
+// Flags: -C additional
+
+require('../common');
+const assert = require('node:assert');
+const { customConditions } = require('module');
+
+assert.deepStrictEqual(customConditions, new Set(['additional']));