react-router and several other demos offer up a "fakeAuth". This is a starterAuth that sends an email and password to an external API and then stores the returned auth_token JWT in localStorage. Main benefit of doing so is to persist login between page refreshes.

starterAuth.js

import fetch from "cross-fetch";
const apiUrl = "//localhost:5000"; // or where ever your api lives!

const checkStatus = response => {
  if (response.status >= 200 && response.status < 300) {
    return response;
  }
  const error = new Error(`${response.statusText}`);
  error.status = response.statusText;
  error.response = response;
  throw error;
};

const parseJSON = payload => {
  if (typeof payload === "string") {
    return JSON.parse(payload);
  }
  return payload.json();
};

export const starterAuth = {
  auth_token: localStorage.getItem("auth_token"),
  isAuthenticated() {
    return !!this.auth_token;
  },
  authenticate(email, password) {
    return fetch(`${apiUrl}/auth/login`, {
      method: "POST",
      headers: { "Content-Type": "application/x-www-form-urlencoded" },
      body: `email=${email}&password=${password}`
    })
      .then(checkStatus)
      .then(parseJSON)
      .then(({ auth_token }) => {
        localStorage.setItem("auth_token", auth_token);
        this.auth_token = auth_token;
        return Promise.resolve();
      });
  },
  signout(cb) {
    localStorage.removeItem("auth_token");
    this.auth_token = null;
    return cb();
  }
};