refactor(docs): extract loadVersion() without changing the behavior (#11230)

* refactor, extract loadVersion() without changing behavior

* Add minimal test case for loadVersion()

* Add minimal test case for loadVersion()

* more refactor, rename index.ts to version.ts

* fix tests

* empty

packages/docusaurus-plugin-content-docs/src/versions/__tests__/loadVersion.test.ts

@@ -0,0 +1,73 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import path from 'path';
+import {fromPartial} from '@total-typescript/shoehorn';
+import {DEFAULT_PARSE_FRONT_MATTER} from '@docusaurus/utils/src';
+import {readVersionsMetadata} from '../version';
+import {DEFAULT_OPTIONS} from '../../options';
+import {loadVersion} from '../loadVersion';
+import type {I18n, LoadContext} from '@docusaurus/types';
+import type {PluginOptions} from '@docusaurus/plugin-content-docs';
+
+const DefaultI18N: I18n = {
+  path: 'i18n',
+  currentLocale: 'en',
+  locales: ['en'],
+  defaultLocale: 'en',
+  localeConfigs: {},
+};
+
+describe('minimal site', () => {
+  async function loadSite() {
+    const siteDir = path.resolve(
+      path.join(__dirname, './__fixtures__', 'minimal-site'),
+    );
+    const options: PluginOptions = fromPartial<PluginOptions>({
+      ...DEFAULT_OPTIONS,
+    });
+    const context = fromPartial<LoadContext>({
+      siteDir,
+      baseUrl: '/',
+      i18n: DefaultI18N,
+      localizationDir: path.join(siteDir, 'i18n/en'),
+      siteConfig: {
+        markdown: {
+          parseFrontMatter: DEFAULT_PARSE_FRONT_MATTER,
+        },
+      },
+    });
+    return {
+      siteDir,
+      options,
+      context,
+    };
+  }
+
+  it('can load current version', async () => {
+    const {options, context} = await loadSite();
+
+    const versionsMetadata = await readVersionsMetadata({
+      options,
+      context,
+    });
+
+    expect(versionsMetadata).toHaveLength(1);
+    expect(versionsMetadata[0]!.versionName).toBe('current');
+
+    const versionMetadata = versionsMetadata[0]!;
+
+    const loadedVersion = loadVersion({
+      context,
+      options,
+      versionMetadata,
+      env: 'production',
+    });
+
+    await expect(loadedVersion).resolves.toMatchSnapshot();
+  });
+});