Add anonymous upvote functionality

static/upvote.js

@@ -0,0 +1,37 @@
+(async function () {
+  // Define the slug
+  const slug = location.pathname.replace(/^\/|\/$/g, "").replaceAll("/", "-");
+  const API = "http://localhost:3000";
+
+  const btn = document.getElementById("upvote-btn");
+  const count = document.getElementById("upvote-count");
+
+  //  On load fetch curent vote count and whether the button should be clicked on or off
+  const res = await fetch(`${API}/posts/${slug}/votes`, {
+    credentials: "include",
+  });
+  const data = await res.json();
+
+  count.textContent = data.vote_count;
+  if (data.voted) btn.setAttribute("voted", true);
+
+  btn.addEventListener("click", async () => {
+    const alreadyVoted = btn.hasAttribute("voted");
+
+    const method = alreadyVoted ? "DELETE" : "POST";
+    const r = await fetch(`${API}/posts/${slug}/vote`, {
+      method,
+      credentials: "include",
+    });
+
+    if (r.ok) {
+      const updated = await fetch(`${API}/posts/${slug}/votes`, {
+        credentials: "include",
+      });
+      const d = await updated.json();
+      count.textContent = d.vote_count;
+      if (d.voted) btn.setAttribute("voted", false);
+      else btn.removeAttribute("voted");
+    }
+  });
+})();