App.js

import './App.css';

function App() {

  const handleSubmit = async (event) => {
    event.preventDefault();
    const { target } = event;
    const form = {
      name: target.name.value,
      email: target.email.value,
      message: target.message.value,
    };
    const result = await fetch(
      '/contact_form',
      {
        method: 'POST',
        headers: {
          'Content-Type': 'application/json',
        },
        body: JSON.stringify({ ...form, secret: 'firebaseIsCool' }),
      }
    );
    //if (result.ok).....
    alert("Pronto nos pondremos en contacto contigo!");
    // route to another page...
  }

  return (
    <form onSubmit={handleSubmit}>
      <label>
        Escribe tu nombre
        <input name="name" type="text" />
      </label>
      <label>
        Escribe tu Email
        <input name="email" type="text" />
      </label>
      <label>
        Escribe tu mensaje
        <input name="message" type="text" />
      </label>
      <button>enviar</button>
    </form>
  );
}

export default App;

firebase.json

{
  "hosting": {
    "public": "build",
    "ignore": [
      "firebase.json",
      "**/.*",
      "**/node_modules/**"
    ],
    "rewrites": [
      {
        "source": "/contact_form",
        "function": "contactForm"
      },
      {
        "source": "**",
        "destination": "/index.html"
      }
    ]
  }
}

firebase_function_nodemailer.js

const functions = require("firebase-functions");
const nodemailer = require('nodemailer');

const transport = nodemailer.createTransport({
    service: 'Gmail',
    auth: {
        user: "[email protected]",
        pass: "hwmhrwisevhugrgf",
    },
});

const sendContactForm = (form) => {
    return transport
        .sendMail({
            subject: "👾🤖Nuevo mensaje de tu formulario de contacto😎",
            bcc: ["[email protected]"],
            html: `<h3>¡Tienes un nuevo mensaje!</h3>
			<p> Nombre: ${form.name} </p>
			<p> Correo: ${form.email} </p>
			<p>Mensaje: ${form.message} </p>
			`,
        })
        .then((r) => {
            console.log("Accepted => ", r.accepted)
            console.log("Rejected => ", r.rejected)
        })
        .catch((e) => console.log(e))
}

exports.contactForm = functions.https.onRequest((request, response) => {
    if (req.body.secret !== 'firebaseIsCool') return res.send('Missing secret');
    sendContactForm(req.body);
    res.send("Sending email...");
});