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/__tests__/validationUtils.test.ts

@@ -0,0 +1,64 @@
+/**
+ * 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 Joi from '../Joi';
+import {JoiFrontMatter, validateFrontMatter} from '../validationUtils';
+
+describe('validateFrontMatter', () => {
+  test('should accept good values', () => {
+    const schema = Joi.object<{test: string}>({
+      test: Joi.string(),
+    });
+    const frontMatter = {
+      test: 'hello',
+    };
+    expect(validateFrontMatter(frontMatter, schema)).toEqual(frontMatter);
+  });
+
+  test('should reject bad values', () => {
+    const consoleError = jest.spyOn(console, 'error').mockImplementation();
+    const schema = Joi.object<{test: string}>({
+      test: Joi.string(),
+    });
+    const frontMatter = {
+      test: true,
+    };
+    expect(() =>
+      validateFrontMatter(frontMatter, schema),
+    ).toThrowErrorMatchingInlineSnapshot(`"\\"test\\" must be a string"`);
+    expect(consoleError).toHaveBeenCalledWith(
+      expect.stringContaining('FrontMatter contains invalid values: '),
+    );
+  });
+
+  // Fix Yaml trying to convert strings to numbers automatically
+  // We only want to deal with a single type in the final frontmatter (not string | number)
+  test('should convert number values to string when string schema', () => {
+    const schema = Joi.object<{test: string}>({
+      test: JoiFrontMatter.string(),
+    });
+    const frontMatter = {
+      test: 42,
+    };
+    expect(validateFrontMatter(frontMatter, schema)).toEqual({test: '42'});
+  });
+
+  // Helps to fix Yaml trying to convert strings to dates automatically
+  // We only want to deal with a single type in the final frontmatter (not string | Date)
+  test('should convert date values when string schema', () => {
+    const schema = Joi.object<{test: string}>({
+      test: JoiFrontMatter.string(),
+    });
+    const date = new Date();
+    const frontMatter = {
+      test: date,
+    };
+    expect(validateFrontMatter(frontMatter, schema)).toEqual({
+      test: date.toString(),
+    });
+  });
+});