next.js/examples/with-firebase/pages/index.js

import { getFirestore, setDoc, doc } from "firebase/firestore";
import Head from "next/head";
import Link from "next/link";
import { useEffect } from "react";
import { useUser } from "../context/userContext";

export default function Home() {
  // Our custom hook to get context values
  const { loadingUser, user } = useUser();

  const profile = { username: "nextjs_user", message: "Awesome!!" };

  useEffect(() => {
    if (!loadingUser) {
      // You know that the user is loaded: either logged in or out!
      console.log(user);
    }
    // You also have your firebase app initialized
  }, [loadingUser, user]);

  const createUser = async () => {
    const db = getFirestore();
    await setDoc(doc(db, "profile", profile.username), profile);

    alert("User created!!");
  };

  return (
    <div className="container">
      <Head>
        <title>Next.js w/ Firebase Client-Side</title>
        <link rel="icon" href="/favicon.ico" />
      </Head>

      <main>
        <h1 className="title">Next.js w/ Firebase Client-Side</h1>
        <p className="description">Fill in your credentials to get started</p>

        <p className="description">
          Cloud Firestore Security Rules write permissions are required for
          adding users
        </p>
        <button onClick={createUser}>Create 'nextjs_user'</button>

        <p className="description">
          Please press the link below after adding the user
        </p>
        <Link href={`/profile/${profile.username}`}>Go to SSR Page</Link>
      </main>

      <style jsx>{`
        .container {
          min-height: 100vh;
          padding: 0 0.5rem;
          display: flex;
          flex-direction: column;
          justify-content: center;
          align-items: center;
        }

        main {
          padding: 5rem 0;
          flex: 1;
          display: flex;
          flex-direction: column;
          justify-content: center;
          align-items: center;
        }

        footer {
          width: 100%;
          height: 100px;
          border-top: 1px solid #eaeaea;
          display: flex;
          justify-content: center;
          align-items: center;
        }

        footer img {
          margin-left: 0.5rem;
        }

        footer a {
          display: flex;
          justify-content: center;
          align-items: center;
        }

        button {
          font-size: 1.5em;
          margin: 1em 0;
        }

        a {
          color: blue;
          font-size: 1.5em;
        }

        .title a {
          color: #0070f3;
          text-decoration: none;
        }

        .title a:hover,
        .title a:focus,
        .title a:active {
          text-decoration: underline;
        }

        .title {
          margin: 0;
          line-height: 1.15;
          font-size: 4rem;
        }

        .title,
        .description {
          text-align: center;
        }

        .description {
          line-height: 1.5;
          font-size: 1.5rem;
        }

        code {
          background: #fafafa;
          border-radius: 5px;
          padding: 0.75rem;
          font-size: 1.1rem;
          font-family:
            Menlo,
            Monaco,
            Lucida Console,
            Liberation Mono,
            DejaVu Sans Mono,
            Bitstream Vera Sans Mono,
            Courier New,
            monospace;
        }

        .grid {
          display: flex;
          align-items: center;
          justify-content: center;
          flex-wrap: wrap;
          max-width: 800px;
          margin-top: 3rem;
        }

        .card {
          margin: 1rem;
          flex-basis: 45%;
          padding: 1.5rem;
          text-align: left;
          color: inherit;
          text-decoration: none;
          border: 1px solid #eaeaea;
          border-radius: 10px;
          transition:
            color 0.15s ease,
            border-color 0.15s ease;
        }

        .card:hover,
        .card:focus,
        .card:active {
          color: #0070f3;
          border-color: #0070f3;
        }

        .card h3 {
          margin: 0 0 1rem 0;
          font-size: 1.5rem;
        }

        .card p {
          margin: 0;
          font-size: 1.25rem;
          line-height: 1.5;
        }

        @media (max-width: 600px) {
          .grid {
            width: 100%;
            flex-direction: column;
          }
        }
      `}</style>

      <style jsx global>{`
        html,
        body {
          padding: 0;
          margin: 0;
          font-family:
            -apple-system,
            BlinkMacSystemFont,
            Segoe UI,
            Roboto,
            Oxygen,
            Ubuntu,
            Cantarell,
            Fira Sans,
            Droid Sans,
            Helvetica Neue,
            sans-serif;
        }

        * {
          box-sizing: border-box;
        }
      `}</style>
    </div>
  );
}