4wswt4ex2: Add like and unlike for post cards

src/lib/like-client.ts

@@ -0,0 +1,52 @@
+// Browser-side like / unlike calls and the pure card update they drive.
+
+import type { CardPost } from "./feed-model";
+
+export interface LikeOutcome {
+  isLiked: boolean;
+  likesCount: number | null;
+}
+
+export class LikeError extends Error {
+  constructor(public readonly signedOut: boolean) {
+    super(signedOut ? "sign_in" : "like_failed");
+  }
+}
+
+export async function sendLike(
+  postId: string,
+  liked: boolean,
+  fetchImpl: typeof fetch = fetch,
+): Promise<LikeOutcome> {
+  let res: Response;
+  try {
+    res = await fetchImpl(`/api/posts/${encodeURIComponent(postId)}/like`, {
+      method: liked ? "POST" : "DELETE",
+      credentials: "same-origin",
+    });
+  } catch {
+    throw new LikeError(false);
+  }
+  if (res.status === 401) throw new LikeError(true);
+  if (!res.ok) throw new LikeError(false);
+  try {
+    const b = (await res.json()) as { isLiked?: unknown; likesCount?: unknown };
+    return {
+      isLiked: typeof b.isLiked === "boolean" ? b.isLiked : liked,
+      likesCount:
+        typeof b.likesCount === "number" && Number.isInteger(b.likesCount) && b.likesCount >= 0
+          ? b.likesCount
+          : null,
+    };
+  } catch {
+    throw new LikeError(false);
+  }
+}
+
+/** The card after a successful call: the server count when present, else flip and adjust by one. */
+export function applyLikeOutcome(post: CardPost, wanted: boolean, out: LikeOutcome): CardPost {
+  if (out.likesCount !== null) return { ...post, isLiked: out.isLiked, likesCount: out.likesCount };
+  const changed = post.isLiked !== wanted;
+  const delta = changed ? (wanted ? 1 : -1) : 0;
+  return { ...post, isLiked: wanted, likesCount: Math.max(0, post.likesCount + delta) };
+}