How to add auth system to Express.js

Sometimes you have great app idea, but you need to make authentication. Without further ado, I'll show you how to do it.

Part 1

Let's assume you have installed express and now you need to:

npm i body-parser cookie-parser

Now you need to make file called accounts.json
Replace content of accounts.json file with this:

[
  {
    "name": "user",
    "password": "user",
    "data": "I'm user"
  },
  {
    "name": "user2",
    "password": "user2",
    "data": "I'm second user"
  },
  {
    "name": "user3",
    "password": "user3",
    "data": "I'm third *special* user"
  }
]

⚠ This is only example. Storing unhashed passwords is dangerous. If you know how to use databases you can use database.

Part 2

Now you need to have accounts list in an object and have sessions array.
Add this lines below const app in your index.js

const accounts = require("accounts.json")
const sessions = {};
app.use(bodyParser.urlencoded());
app.use(cookieParser());

function randomSID() {
    var sID = Math.random().toString(36).substring(2, 15) + Math.random().toString(36).substring(2, 15)
    if(!sessions.hasOwnProperty(sID)){
        return sID;
    }
    return randomSID();
  }

Part 3

Create an /login API endpoints.

app.get('/login', (req,res)=>{
  res.send(`
<form type="post">
<input name="name">
<input name="password" type="password">
<button type="submint">Log In</button>
</form>
`)
})
app.post('/login', (req,res)=>{
  if(accounts.find(x=>x.name == req.body.name && x.password == req.body.password){
    const newSessionID = randomSID()
    res.cookie('sID', newSessionID)
    sessions[newSessionID] = accounts.findIndex(x => x.name === 
    body.name && x.password === body.password)
    res.redirect('/')
  }
  else {
  res.send("Bad password")  
  }
})

Create logout API endpoint:

app.get('/logout', (req, res) => {
  delete loggedInSessions[req.cookies.sID]
  res.clearCookie('sID')
  res.redirect('/')
})

Part 4

Create an account middleware

function accountMiddleware(req,res,next){
  if(accounts[sessions[req.cookies.sID]]){
    req.account = accounts[sessions[req.cookies.sID]]
  }
  else {
    req.account = null;
  }
  next()
}

Also add this line next to app.use's.

app.use(accountMiddleware)

Part 5

Now you can easily get account data using req.account.
Example endpoint:

app.get('/', (req, res) => {
  if(req.account){
    res.send(`You are logged in as ${req.account.name} and here is your data: ${req.account.data}`)
  }
  else {
    res.send("You are logged out")
  }
});