next.js/examples/convex/pages/index.tsx

import { FormEvent, useEffect, useState } from "react";
import { useMutation, useQuery } from "convex/react";
import { api } from "../convex/_generated/api";

export default function App() {
  const messages = useQuery(api.messages.list);
  const sendMessage = useMutation(api.messages.send);

  const [newMessageText, setNewMessageText] = useState("");
  const [name, setName] = useState("user");

  useEffect(() => {
    setName("User " + Math.floor(Math.random() * 10000));
  }, []);

  async function handleSendMessage(event: FormEvent) {
    event.preventDefault();
    setNewMessageText("");
    await sendMessage({ body: newMessageText, author: name });
  }
  return (
    <main>
      <h1>Convex Chat</h1>
      <p className="badge">
        <span>{name}</span>
      </p>
      <ul>
        {messages?.map((message) => (
          <li key={message._id.toString()}>
            <span>{message.author}:</span>
            <span>{message.body}</span>
            <span>{new Date(message._creationTime).toLocaleTimeString()}</span>
          </li>
        ))}
      </ul>
      <form onSubmit={handleSendMessage}>
        <input
          value={newMessageText}
          onChange={(event) => setNewMessageText(event.target.value)}
          placeholder="Write a message…"
        />
        <input type="submit" value="Send" disabled={!newMessageText} />
      </form>
    </main>
  );
}