tests: Add test coverage for fetching markdown frontmatter

src/lib/blog/markdown/markdown-builder.test.ts

@@ -1,28 +1,77 @@
 import { describe, it, expect } from 'vitest';
 import { MarkdownBuilder } from './markdown-builder.js';
 
-const exampleMarkdown = [
-    `---`,
-    `title: "This is a title"`,
-    `---`,
-    `This is a title. This is a body.`,
-    `This is an incredibly long new set`,
-    `of words to read. I hope you`,
-    `enjoy reading them. I hope you`,
-    `enjoy reading them. I hope you`,
-].join('\n');
+const markdownBuilder = (tags: string[] | undefined = undefined): string => {
+    let exampleMarkdown = [`---`, `title: "This is a title"`, `date: 2025-03-13T18:44:00Z`, `draft: false`];
+
+    if (tags !== undefined) {
+        exampleMarkdown = [...exampleMarkdown, `tags:`, ...tags.map((tag) => `  - ${tag}`)];
+    }
+
+    exampleMarkdown = [
+        ...exampleMarkdown,
+        `---`,
+        `This is a title. This is a body.`,
+        `This is an incredibly long new set`,
+        `of words to read. I hope you`,
+        `enjoy reading them. I hope you`,
+        `enjoy reading them. I hope you`,
+    ];
+
+    return exampleMarkdown.join('\n');
+};
 
 describe(`MarkdownBuilder`, () => {
-    // const markdownBuilder = new MarkdownBuilder();
+    const markdown = markdownBuilder();
 
     it(`should build an excerpt`, async () => {
-        // GIVEN
-        const markdown = exampleMarkdown;
-
         // WHEN
         const excerpt = await MarkdownBuilder.getExcerptFromMarkdown(markdown, 10);
 
         // THEN
         expect(excerpt).toBe(`This is a title. This is a body. This is`);
     });
+
+    describe(`getFrontmatter`, () => {
+        it(`should be able to identify the fields in front-matter`, () => {
+            // WHEN
+            const frontmatter = MarkdownBuilder.getFrontmatter<{ title: string; date: Date }>(markdown, 'test.md');
+
+            // THEN
+            expect(frontmatter).toStrictEqual({
+                title: 'This is a title',
+                date: new Date(`2025-03-13T18:44:00Z`),
+                draft: false,
+            });
+        });
+    });
+
+    describe(`getTagsFromMarkdown`, () => {
+        it(`should build the tags`, () => {
+            // WHEN
+            const tags = MarkdownBuilder.getTagsFromMarkdown<{ tags: string[] }>(
+                markdownBuilder(['one-tag', 'another']),
+                'test.md'
+            );
+
+            // THEN
+            expect(tags).toStrictEqual(['one-tag', 'another']);
+        });
+
+        it(`should build when the tags are empty`, () => {
+            // WHEN
+            const tags = MarkdownBuilder.getTagsFromMarkdown<{ tags: string[] }>(markdownBuilder(), 'test.md');
+
+            // THEN
+            expect(tags).toStrictEqual([]);
+        });
+
+        it(`should build when there is no 'tags' field`, () => {
+            // WHEN
+            const tags = MarkdownBuilder.getTagsFromMarkdown<{ tags: string[] }>(markdownBuilder(), 'test.md');
+
+            // THEN
+            expect(tags).toStrictEqual([]);
+        });
+    });
 });

src/lib/blog/markdown/markdown-builder.ts

@@ -18,7 +18,7 @@ export class MarkdownBuilder {
         return value.toString();
     }
 
-    static getFrontmatter<T extends Record<string, any>>(markdownContent: string, fileName: string): T | null {
+    static getFrontmatter<T extends Record<string, any> = {}>(markdownContent: string, fileName: string): T | null {
         const processor = this.getFrontmatterProcessor();
         const parsedMarkdown = processor.parse(markdownContent);
 
@@ -47,6 +47,17 @@ export class MarkdownBuilder {
             .join(' ');
     }
 
+    static getTagsFromMarkdown<T extends { tags?: string[] }>(markdownContent: string, theFileName: string): string[] {
+        const frontMatter = this.getFrontmatter<T>(markdownContent, theFileName);
+
+        if (frontMatter === null) {
+            return [];
+        } else if (!frontMatter.tags) {
+            return [];
+        }
+        return frontMatter.tags;
+    }
+
     private static getFrontmatterProcessor() {
         return unified() //
             .use(remarkParse)