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-utils-validation/src/validationUtils.ts

@@ -83,3 +83,44 @@ export function normalizeThemeConfig<T>(
   }
   return value;
 }
+
+// Enhance the default Joi.string() type so that it can convert number to strings
+// If user use frontmatter "tag: 2021", we shouldn't need to ask the user to write "tag: '2021'"
+// Also yaml tries to convert patterns like "2019-01-01" to dates automatically
+// see https://github.com/facebook/docusaurus/issues/4642
+// see https://github.com/sideway/joi/issues/1442#issuecomment-823997884
+const JoiFrontMatterString: Joi.Extension = {
+  type: 'string',
+  base: Joi.string(),
+  // Fix Yaml that tries to auto-convert many things to string out of the box
+  prepare: (value) => {
+    if (typeof value === 'number' || value instanceof Date) {
+      return {value: value.toString()};
+    }
+    return {value};
+  },
+};
+export const JoiFrontMatter: typeof Joi = Joi.extend(JoiFrontMatterString);
+
+export function validateFrontMatter<T>(
+  frontMatter: Record<string, unknown>,
+  schema: Joi.ObjectSchema<T>,
+): T {
+  try {
+    return JoiFrontMatter.attempt(frontMatter, schema, {
+      convert: true,
+      allowUnknown: true,
+    });
+  } catch (e) {
+    console.error(
+      chalk.red(
+        `FrontMatter contains invalid values: ${JSON.stringify(
+          frontMatter,
+          null,
+          2,
+        )}`,
+      ),
+    );
+    throw e;
+  }
+}