refactor(v2): add typing for docs plugin (#1811)

* refactor(v2): add typing for docs plugin

* nits

packages/docusaurus-plugin-content-docs-legacy/src/metadata.ts

@@ -0,0 +1,88 @@
+/**
+ * Copyright (c) 2017-present, Facebook, Inc.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ */
+
+import fs from 'fs-extra';
+import path from 'path';
+import {parse, normalizeUrl} from '@docusaurus/utils';
+import {Order, MetadataRaw} from './types';
+import {DocusaurusConfig} from '@docusaurus/types';
+
+export default async function processMetadata(
+  source: string,
+  docsDir: string,
+  order: Order,
+  siteConfig: Partial<DocusaurusConfig>,
+  docsBasePath: string,
+  siteDir: string,
+): Promise<MetadataRaw> {
+  const filepath = path.join(docsDir, source);
+
+  const fileString = await fs.readFile(filepath, 'utf-8');
+  const {frontMatter: metadata = {}, excerpt} = parse(fileString);
+
+  // Default id is the file name.
+  if (!metadata.id) {
+    metadata.id = path.basename(source, path.extname(source));
+  }
+
+  if (metadata.id.includes('/')) {
+    throw new Error('Document id cannot include "/".');
+  }
+
+  // Default title is the id.
+  if (!metadata.title) {
+    metadata.title = metadata.id;
+  }
+
+  if (!metadata.description) {
+    metadata.description = excerpt;
+  }
+
+  const dirName = path.dirname(source);
+  if (dirName !== '.') {
+    const prefix = dirName;
+    if (prefix) {
+      metadata.id = `${prefix}/${metadata.id}`;
+    }
+  }
+
+  // Cannot use path.join() as it resolves '../' and removes the '@site'. Let webpack loader resolve it.
+  const aliasedPath = `@site/${path.relative(siteDir, filepath)}`;
+  metadata.source = aliasedPath;
+
+  // Build the permalink.
+  const {baseUrl} = siteConfig;
+
+  // If user has own custom permalink defined in frontmatter
+  // e.g: :baseUrl:docsUrl/:langPart/:versionPart/endiliey/:id
+  if (metadata.permalink) {
+    metadata.permalink = path.resolve(
+      metadata.permalink
+        .replace(/:baseUrl/, baseUrl)
+        .replace(/:docsUrl/, docsBasePath)
+        .replace(/:id/, metadata.id),
+    );
+  } else {
+    metadata.permalink = normalizeUrl([baseUrl, docsBasePath, metadata.id]);
+  }
+
+  // Determine order.
+  const {id} = metadata;
+  if (order[id]) {
+    metadata.sidebar = order[id].sidebar;
+    metadata.category = order[id].category;
+    metadata.subCategory = order[id].subCategory;
+    if (order[id].next) {
+      metadata.next = order[id].next;
+    }
+    if (order[id].previous) {
+      metadata.previous = order[id].previous;
+    }
+  }
+
+  return metadata as MetadataRaw;
+}