feat(transfer): implement transfer functionality with user selection and balance validation

src/app/api/transfer/route.ts

@@ -0,0 +1,75 @@
+import { NextResponse } from "next/server";
+import { auth } from "~/server/auth";
+import { db } from "~/server/db";
+
+export async function POST(request: Request) {
+  const session = await auth();
+
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const { toUserId, amount } = (await request.json()) as {
+    toUserId: string;
+    amount: number;
+  };
+
+  if (!toUserId || typeof toUserId !== "string") {
+    return NextResponse.json({ error: "Invalid recipient" }, { status: 400 });
+  }
+
+  if (typeof amount !== "number" || isNaN(amount) || amount <= 0) {
+    return NextResponse.json(
+      { error: "Amount must be a positive number" },
+      { status: 400 },
+    );
+  }
+
+  const fromUserId = session.user.id;
+
+  if (fromUserId === toUserId) {
+    return NextResponse.json(
+      { error: "Cannot transfer to yourself" },
+      { status: 400 },
+    );
+  }
+
+  const [sender, recipient] = await Promise.all([
+    db.user.findUnique({
+      where: { id: fromUserId },
+      select: { id: true, balance: true },
+    }),
+    db.user.findUnique({
+      where: { id: toUserId },
+      select: { id: true, name: true },
+    }),
+  ]);
+
+  if (!sender) {
+    return NextResponse.json({ error: "Sender not found" }, { status: 404 });
+  }
+
+  if (!recipient) {
+    return NextResponse.json({ error: "Recipient not found" }, { status: 404 });
+  }
+
+  if (sender.balance < amount) {
+    return NextResponse.json(
+      { error: "Insufficient balance" },
+      { status: 400 },
+    );
+  }
+
+  await db.$transaction([
+    db.user.update({
+      where: { id: fromUserId },
+      data: { balance: { decrement: amount } },
+    }),
+    db.user.update({
+      where: { id: toUserId },
+      data: { balance: { increment: amount } },
+    }),
+  ]);
+
+  return NextResponse.json({ success: true, amount, to: recipient.name });
+}

src/app/api/users/route.ts

@@ -0,0 +1,19 @@
+import { NextResponse } from "next/server";
+import { auth } from "~/server/auth";
+import { db } from "~/server/db";
+
+export async function GET() {
+  const session = await auth();
+
+  if (!session?.user?.id) {
+    return NextResponse.json({ error: "Unauthorized" }, { status: 401 });
+  }
+
+  const users = await db.user.findMany({
+    where: { id: { not: session.user.id } },
+    select: { id: true, name: true },
+    orderBy: { name: "asc" },
+  });
+
+  return NextResponse.json(users);
+}

src/components/HomeClient.tsx

@@ -8,6 +8,7 @@ import CartButton from "~/components/cart";
 import type { Session } from "next-auth";
 import AccountButton from "./account";
 import SellableItemsButton from "./new_items";
+import TransferButton from "./transfer";
 
 type UserResponse = {
   id: string;
@@ -102,9 +103,10 @@ export default function HomeClient({ session }: Props) {
     <main className="flex min-h-screen flex-col items-center justify-center bg-linear-to-b from-[#2e026d] to-[#7f3b3b] text-white">
       <div className="absolute top-4 right-4 flex items-center gap-4">
         {userData && (
-          <div className="rounded-full bg-white/10 px-4 py-2 font-semibold">
-            ${userData.balance ?? 0}
-          </div>
+          <TransferButton
+            balance={userData.balance ?? 0}
+            reloadUser={loadUser}
+          />
         )}
         {session?.user && (
           <>

src/components/transfer.tsx

@@ -0,0 +1,149 @@
+"use client";
+
+import { useState } from "react";
+
+type User = {
+  id: string;
+  name: string | null;
+};
+
+type Props = {
+  balance: number;
+  reloadUser: () => Promise<void>;
+};
+
+export default function TransferButton({ balance, reloadUser }: Props) {
+  const [isOpen, setIsOpen] = useState(false);
+  const [users, setUsers] = useState<User[]>([]);
+  const [toUserId, setToUserId] = useState("");
+  const [amount, setAmount] = useState("");
+  const [error, setError] = useState<string | null>(null);
+  const [sending, setSending] = useState(false);
+
+  const openModal = async () => {
+    setError(null);
+    setAmount("");
+    setToUserId("");
+    try {
+      const res = await fetch("/api/users");
+      if (!res.ok) throw new Error("Failed to load users");
+      const data = (await res.json()) as User[];
+      setUsers(data);
+      if (data[0]) setToUserId(data[0].id);
+      setIsOpen(true);
+    } catch {
+      setError("Could not load users");
+    }
+  };
+
+  const send = async () => {
+    setError(null);
+    const parsed = parseFloat(amount);
+    if (!toUserId) return setError("Select a recipient");
+    if (isNaN(parsed) || parsed <= 0) return setError("Enter a valid amount");
+    if (parsed > balance) return setError("Insufficient balance");
+
+    setSending(true);
+    try {
+      const res = await fetch("/api/transfer", {
+        method: "POST",
+        headers: { "Content-Type": "application/json" },
+        body: JSON.stringify({ toUserId, amount: parsed }),
+      });
+      const data = (await res.json()) as { error?: string; to?: string };
+      if (!res.ok) {
+        setError(data.error ?? "Transfer failed");
+        return;
+      }
+      setIsOpen(false);
+      setAmount("");
+      await reloadUser();
+    } catch {
+      setError("Transfer failed");
+    } finally {
+      setSending(false);
+    }
+  };
+
+  return (
+    <>
+      <button
+        onClick={openModal}
+        className="rounded-full bg-white/10 px-4 py-2 font-semibold transition hover:bg-white/20"
+      >
+        ${balance}
+      </button>
+
+      {isOpen && (
+        <div className="fixed inset-0 z-50 flex items-center justify-center">
+          {/* backdrop */}
+          <div
+            className="absolute inset-0 bg-black/60"
+            onClick={() => setIsOpen(false)}
+          />
+
+          <div className="relative z-10 w-full max-w-sm rounded-xl bg-neutral-900 p-6 text-white shadow-xl">
+            <h2 className="mb-5 text-xl font-bold">Transfer Balance</h2>
+
+            <div className="mb-4">
+              <label className="mb-1 block text-sm font-semibold text-white/70">
+                Recipient
+              </label>
+              <select
+                value={toUserId}
+                onChange={(e) => setToUserId(e.target.value)}
+                className="w-full rounded bg-white/10 px-3 py-2 text-sm text-white focus:outline-none"
+              >
+                {users.map((u) => (
+                  <option key={u.id} value={u.id} className="bg-neutral-800">
+                    {u.name ?? u.id}
+                  </option>
+                ))}
+              </select>
+            </div>
+
+            <div className="mb-5">
+              <label className="mb-1 block text-sm font-semibold text-white/70">
+                Amount{" "}
+                <span className="text-white/40">
+                  (your balance: ${balance})
+                </span>
+              </label>
+              <input
+                type="number"
+                min={1}
+                step={1}
+                value={amount}
+                onChange={(e) => setAmount(e.target.value)}
+                placeholder="0"
+                className="w-full rounded bg-white/10 px-3 py-2 text-sm focus:outline-none"
+              />
+            </div>
+
+            {error && (
+              <p className="mb-3 rounded bg-red-500/20 px-3 py-2 text-sm text-red-300">
+                {error}
+              </p>
+            )}
+
+            <div className="flex gap-2">
+              <button
+                onClick={send}
+                disabled={sending}
+                className="flex-1 rounded bg-purple-600 py-2 font-bold transition hover:bg-purple-700 disabled:opacity-50"
+              >
+                {sending ? "Sending…" : "Send"}
+              </button>
+              <button
+                onClick={() => setIsOpen(false)}
+                className="rounded bg-white/10 px-4 py-2 font-semibold transition hover:bg-white/20"
+              >
+                Cancel
+              </button>
+            </div>
+          </div>
+        </div>
+      )}
+    </>
+  );
+}