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Adding Login to the User Interface

Adding Login to the User Interface

Turning to the SignIn.tsx component, we will make similar changes to the work we did to SignUp.tsx

The first step is to add types. We'll need a type to hold the form information and we'll also need a type for the data that is returned from the login API. Then state to store the user and an error message:

export type LoginUserType = {
email: string
password: string
export type LoginSuccess = {
token: string
user: {
id: number
fullName: string
email: string
const [errorMessage, setErrorMessage] = useState('')
const [user, setUser] = useState<LoginUserType>({
email: '',
password: '',

We can add a <p> tag to show the error message:

errorMessage ? <p>{errorMessage}</p> : null

then we will add a function to handle the user's input in the fields:

function handleStringFieldChange(event: React.ChangeEvent<HTMLInputElement>) {
const value =
const fieldName =
const updatedUser = { ...user, [fieldName]: value }

then add a function to submit the form:

async function loginUser(user: LoginUserType): Promise<LoginSuccess> {
const response = await fetch('/api/Sessions', {
method: 'POST',
headers: { 'content-type': 'application/json' },
body: JSON.stringify(user),
if (response.ok) {
return response.json()
} else {
return await response.json()

Now update the fields and the form to call the handling functions as needed.


This page is much like the Signup page except for how we'll handle the mutation result.

const loginUserMutation = useMutation(loginUser, {
onSuccess: function (apiResponse) {
// TODO: record the authentication information we receive
// recordAuthentication(apiResponse)
onError: function (error: APIError) {
setErrorMessage(Object.values(error.errors).join(' '))
onSubmit={function (event) {

Notice we use window.location to redirect the user instead of history.push. This is so the browser forces a reload and detects that the user is logged in. The information about the logged in user is captured by the recordAuthentication function we are about to introduce.

The recordAuthentication method and many other useful authentication methods exist in the file auth.js. The purpose of recordAuthentication is to store the api response from the login in local storage so we can access it later.

Local Storage

Local storage is a key/value pair mechanism that can, for any string key, store a string of data. This data is persistent across sessions and browser restarts. The storage is per site, so each domain has its own set of key/value pairs.

Local storage makes for a convenient place to store the authentication data. However, we should note that this local storage is available to any javascript that runs on the page that originates from that domain. This security should protect the data from JavaScript running in an injected ad, but if some malicious software can inject JavaScript into the page itself, it will be able to read these values. While local storage is convenient, it may not be the most secure way to store the authentication information.

An alternative is to send the authentication data as a cookie value. However, cookies have security implications and concerns. This article gives a good overview of the differences. For now, we will proceed with local storage.

Redirecting after login

Since we want the user to redirect to the main page, we also want it to reload any authentication data. For this reason, we will use window.location to force a page reload rather than history.push, which would do a local, non-reload navigation.


The contents of auth.ts give some useful client-side methods to:

  • Determine if the user is logged in
  • Fetch the user's ID
  • Fetch the user details
  • Get the elements needed for an authentication header for fetch
  • Store the authentication info recordAuthentication
  • Logout

The template added an auth.ts at the top level of your front end, right next to the App.tsx.

The contents of the auth.ts are:

// Returns an object that can be included in `fetch`
// headers to include the required bearer token
// for authentication
// Example usage:
// fetch('/api/Thing', {
// method: 'POST',
// headers: { 'content-type': 'application/json', ...authHeader() },
// body: JSON.stringify(thing)
// })
import { LoginSuccess } from './types'
// Returns the Authorization header for the the currently logged in in user.
// If there is no authorization data, we'll return an empty object
export function authHeader() {
const auth = authFromStorage()
return auth.token ? `Bearer ${auth.token}` : null
// Save the authentication received from the API
// This method stores the authentication data as
// a JSON string in local storage. Local storage
// requires everything to be in a string.
// This is typically called from a login component
export function recordAuthentication(auth: LoginSuccess) {
localStorage.setItem('auth', JSON.stringify(auth))
// Returns a boolean if the user is logged in.
// Returns TRUE if there is an active user id, FALSE otherwise
export function isLoggedIn() {
return getUserId() !== undefined
// Returns the user id if the logged in user, null otherwise
export function getUserId() {
const auth = authFromStorage()
return auth.user &&
// Returns the user details retrieved from the authentication data
// Example:
// const user = getUser()
// console.log(user.fullName)
export function getUser() {
const auth = authFromStorage()
return auth.user
// Removes the authentication data, effectively "forgetting" the
// session information and logging the user out.
export function logout() {
// Local method to fetch and decode the auth data from local storage
// If there is no local storage value, returns an empty object
function authFromStorage(): LoginSuccess {
const auth = localStorage.getItem('auth')
return auth ? JSON.parse(auth) : {}

Now in the SignIn.tsx we can uncomment the recordAuthentication(apiResponse) and add the corresponding import.

Add a route to the SignIn component in our App.tsx

<Route exact path="/signin">
<SignIn />

and a link in the navigation

<Link to="/signin">Sign In</Link>
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