fix(v2): fix too strict markdown frontmatter validation (#4654)

* start work

* use orta.vscode-jest

* node 14

* add some better  infra to validate markdown frontmatter

* better docs frontmatter validation

* fix Yaml / Joi validation issues

* fix Yaml / Joi validation issues

Co-authored-by: slorber <lorber.sebastien@gmail.com>

packages/docusaurus-plugin-content-blog/src/__tests__/blogFrontMatter.test.ts

@@ -0,0 +1,67 @@
+/**
+ * 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 {
+  BlogPostFrontMatter,
+  validateBlogPostFrontMatter,
+} from '../blogFrontMatter';
+
+describe('validateBlogPostFrontMatter', () => {
+  test('accept empty object', () => {
+    const frontMatter = {};
+    expect(validateBlogPostFrontMatter(frontMatter)).toEqual(frontMatter);
+  });
+
+  test('accept valid values', () => {
+    const frontMatter: BlogPostFrontMatter = {
+      id: 'blog',
+      title: 'title',
+      description: 'description',
+      date: 'date',
+      slug: 'slug',
+      draft: true,
+      tags: ['hello', {label: 'tagLabel', permalink: '/tagPermalink'}],
+    };
+    expect(validateBlogPostFrontMatter(frontMatter)).toEqual(frontMatter);
+  });
+
+  // See https://github.com/facebook/docusaurus/issues/4591#issuecomment-822372398
+  test('accept empty title', () => {
+    const frontMatter: BlogPostFrontMatter = {title: ''};
+    expect(validateBlogPostFrontMatter(frontMatter)).toEqual(frontMatter);
+  });
+
+  // See https://github.com/facebook/docusaurus/issues/4591#issuecomment-822372398
+  test('accept empty description', () => {
+    const frontMatter: BlogPostFrontMatter = {description: ''};
+    expect(validateBlogPostFrontMatter(frontMatter)).toEqual(frontMatter);
+  });
+
+  // See https://github.com/facebook/docusaurus/issues/4642
+  test('convert tags as numbers', () => {
+    const frontMatter: BlogPostFrontMatter = {
+      tags: [
+        // @ts-expect-error: number for test
+        42,
+        {
+          // @ts-expect-error: number for test
+          label: 84,
+          permalink: '/tagPermalink',
+        },
+      ],
+    };
+    expect(validateBlogPostFrontMatter(frontMatter)).toEqual({
+      tags: [
+        '42',
+        {
+          label: '84',
+          permalink: '/tagPermalink',
+        },
+      ],
+    });
+  });
+});