Dev.to Implement Passwordless sign-in to your apps

App.svelte

<script lang="ts">
  import Auth from "./lib/components/Auth.svelte";
  import Code from "./lib/components/Code.svelte";

  let sentLink = false;
  let token = localStorage.getItem("token");

  async function getUser() {
    if (!token) return;
    try {
      const res = await fetch("http://localhost:5000/api/auth/user", {
        headers: { Authorization: "Bearer " + token }
      });
      const data = await res.json();
      console.log(data);
      if (res.ok && data.ok && data.user) {
        return data.user;
      }
      localStorage.removeItem("token");
      return null;
    } catch (e) {
      console.error(e);
      localStorage.removeItem("token");
      alert("An unknown error occured");
      return null;
    }
  }
</script>

<h1 class="w3-center">Welcome</h1>
{#if !token}
  <div class="w3-container">
    {#if !sentLink}
      <Auth on:prompt-code="{() => (sentLink = true)}" />
    {:else}
      <Code
        on:authenticated="{({ detail: token }) => {
          localStorage.setItem('token', token);
          window.location.reload();
        }}"
      />
    {/if}
  </div>
{:else}
  {#await getUser()}
    <p class="w3-center">Fetching user information</p>
  {:then user}
    {#if user}
      <p class="w3-center">Hello, {user.username}</p>
      <p class="w3-center">
        <button
          class="w3-button w3-black w3-hover-black"
          on:click="{() => {
            localStorage.removeItem('token');
            window.location.reload();
          }}">Logout</button
        >
      </p>
    {:else}
      <p class="w3-center">An error occured while fetching user data</p>
    {/if}
  {/await}
{/if}

Auth.svelte

<script lang="ts">
  import { createEventDispatcher } from "svelte";

  const d = createEventDispatcher();

  async function requestCode() {
    const email = (document.getElementById("email") as HTMLInputElement)?.value;
    if (!email?.trim()) return;

    try {
      const res = await fetch("http://localhost:5000/api/auth/send_magic_link", {
        headers: {
          "Content-Type": "application/json"
        },
        body: JSON.stringify({ email }),
        method: "POST"
      });

      const data = await res.json();
      if (res.ok && data.ok) d("prompt-code");
      else {
        console.error(data);
        alert(data.error || res.statusText);
      }
    } catch (e) {
      console.error(e);
      alert("An unknown error occured");
    }
  }
</script>

<div class="w3-border w3-border-gray w3-padding w3-rounded">
  <h2 class="w3-center">Authenticate</h2>

  <form class="w3-margin" on:submit|preventDefault="{requestCode}">
    <p>
      <label for="email">Email</label>
      <input type="email" id="email" class="w3-input w3-border w3-border-gray" />
    </p>
    <p>
      <button class="w3-button w3-black w3-hover-black" style="width: 100%"
        >Get magic link</button
      >
    </p>
  </form>
</div>

auth.ts

import { Router } from "express";
import User from "../models/User";
import Code from "../models/Code";
import nodemailer from "nodemailer";
import { sign, verify } from "jsonwebtoken";

const router = Router();

router.post("/send_magic_link", async (req, res) => {
  const { email } = req.body;

  if (typeof email !== "string" || !email.trim())
    return res.status(400).json({
      error: "Invalid email",
      error_description: "Please provide a valid email",
    });

  const userId = (await User.findOne({ email }))?.id;

  const code = Math.floor(Math.random() * 899999 + 100000);

  // Expire after 15 minutes
  const c = new Code({
    code,
    userId,
    email,
    expiresAt: Date.now() + 15 * 60 * 1000,
  });
  await c.save();

  const transport = nodemailer.createTransport({
    host: "smtp.mailtrap.io",
    port: 2525,
    auth: {
      user: "d9c780094b04e2",
      pass: "875f89b26f98c2",
    },
  });

  transport.verify((e) => {
    if (e) console.error(e);
  });

  const message = {
    from: "test@example.com",
    to: email,
    text: `Enter this code: ${code}`,
    html: `<p>Enter this code: <b>${code}</b></p>`,
  };

  transport.sendMail(message, (err) => {
    if (err) console.error("An error occured while sending email", err);
    else console.log("Mail sent");
  });

  return res.status(200).json({ ok: true });
});

router.get("/token", async (req, res) => {
  const { code: codeFromQs } = req.query;

  if (typeof codeFromQs !== "string" || isNaN(parseInt(codeFromQs)))
    return res.status(400).json({
      error: "Invalid code",
      error_description: "Please send a valid code in the querystring",
    });

  const code = parseInt(codeFromQs);
  const c = await Code.findOne({ code }).exec();
  if (!c)
    return res.status(400).json({
      error: "Invalid code",
      error_description: "Please send a valid code in the querystring",
    });

  const { email, userId } = c as any;
  let user = null;
  if (userId) {
    user = await User.findById(userId).exec();
    if (!user)
      return res.status(400).json({
        error: "Invalid code",
        error_description: "Please send a valid code in the querystring",
      });
  } else {
    user = new User({ email, username: email.split("@")[0] });
    await user.save();
  }

  // Exp in 1 week
  const token = sign(
    { id: user._id.toString() },
    process.env.SECRET || "secret",
    {
      expiresIn: 604800,
    }
  );

  return res.status(200).json({ ok: true, token, user });
});

router.get("/user", async (req, res) => {
  const authHeader = req.headers.authorization;
  if (
    !authHeader ||
    typeof authHeader !== "string" ||
    authHeader.split(" ")?.length !== 2 ||
    authHeader.split(" ")[0].toLowerCase() !== "bearer"
  )
    return res.status(401).json({ error: "Invalid auth header" });

  const identity = verify(
    authHeader.split(" ")[1],
    process.env.SECRET || "secret"
  ) as any;

  if (typeof identity === "string")
    return res.status(401).json({ error: "Invalid token" });

  if (typeof identity.id !== "string")
    return res.status(401).json({ error: "Invalid token" });

  const user = await User.findById(identity.id);
  if (!user) return res.status(401).json({ error: "Invalid token" });

  return res.status(200).json({ ok: true, user });
});

export default router;

Code.svelte

<script lang="ts">
  import { createEventDispatcher } from "svelte";

  const d = createEventDispatcher();

  async function getToken() {
    const code = (document.getElementById("code") as HTMLInputElement)?.value;
    if (!code) return;

    try {
      const res = await fetch(
        "http://localhost:5000/api/auth/token?code=" + code
      );

      const data = await res.json();
      if (res.ok && data.ok) d("authenticated", data.token);
      else {
        console.error(data);
        alert(data.error || res.statusText);
      }
    } catch (e) {
      console.error(e);
      alert("An unknown error occured");
    }
  }
</script>

<div class="w3-border w3-border-gray w3-padding w3-rounded">
  <h2 class="w3-center">Enter code</h2>

  <form class="w3-margin" on:submit|preventDefault="{getToken}">
    <p>
      <label for="code">Enter the code sent to you by email</label>
      <input type="number" id="code" class="w3-input w3-border w3-border-gray" />
    </p>
    <p>
      <button class="w3-button w3-black w3-hover-black" style="width: 100%"
        >Authenticate</button
      >
    </p>
  </form>
</div>