Create getUniqueTags function

src/utils/getUniqueTags.ts

@@ -0,0 +1,18 @@
+import slugify from "./slugify";
+import type { MarkdownInstance } from "astro";
+import type { Frontmatter } from "./types";
+
+const getUniqueTags = (posts: MarkdownInstance<Frontmatter>[]) => {
+  let tags: string[] = [];
+  posts.forEach((post) => {
+    tags = [...tags, ...post.frontmatter.tags]
+      .map((tag) => slugify(tag))
+      .filter(
+        (value: string, index: number, self: string[]) =>
+          self.indexOf(value) === index
+      );
+  });
+  return tags;
+};
+
+export default getUniqueTags;