feat: express example

.eslintignore

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+node_modules
+examples

.eslintrc

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   "parserOptions": {
     "ecmaVersion": 2018
   },
+  "ignorePatterns": ["examples/**/*"],
   "overrides": [
     {
       "files": ["*.ts"],

examples/express/.env.example

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+SGID_CLIENT_ID="Your client ID"
+SGID_CLIENT_SECRET="Your client secret"
+SGID_PRIVATE_KEY="Your private key"

examples/express/.eslintrc.js

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+module.exports = {
+  root: true,
+  env: {
+    commonjs: true,
+    es2021: true,
+    node: true,
+  },
+  extends: ['standard-with-typescript', 'plugin:prettier/recommended'],
+  overrides: [],
+  parserOptions: {
+    ecmaVersion: 'latest',
+    project: 'examples/express/tsconfig.json',
+  },
+  rules: {
+    '@typescript-eslint/no-misused-promises': 'off',
+  },
+}

examples/express/.gitignore

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+dist/*
+.env
+node_modules/

examples/express/.swcrc

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+{
+  "jsc": {
+    "parser": {
+      "syntax": "typescript",
+      "tsx": false
+    }
+  }
+}

examples/express/README.md

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+# ExpressJS example sgID app
+
+The example application code is in `index.ts`. You can copy this code to bootstrap your sgID client application, and run this app locally to understand how this SDK helps you interact with the sgID server.
+
+## Running this example app locally
+
+### Prerequisites
+
+Register a new client at the [sgID developer portal](https://developer.id.gov.sg). For detailed instructions on how to register, check out our [documentation](https://jie-hao-kwa.gitbook.io/sgid-v2-beta/~/changes/75HA2ZSHqQMuHPNECzfs/introduction/getting-started/registration). Feel free to register a test client; there is no limit to the number of clients you can create.
+
+When registering a client, be sure to register the callback URL to be `http://localhost:5001/api/callback`, since this example runs on port `5001` by default.
+
+### Steps to run locally
+
+1. Clone this repo.
+
+```
+git clone https://github.com/opengovsg/sgid-client.git
+```
+
+2. Go to this folder and copy the contents of `example.env` into a new file called `.env`.
+
+```
+cd sgid-client/examples/express
+cat .env.example > .env
+```
+
+3. Replace the values in `.env` with the credentials of your sgID client (see [Prerequisites](#prerequisites)).
+
+4. Run the Express app:
+
+```
+npm install
+npm start
+```
+
+This should start the server on port `5001` by default
+
+5. Clone sgID's example frontend repo:
+
+```
+git clone https://github.com/opengovsg/sgid-demo-frontend-spa.git
+```
+
+6. Install and run the example frontend:
+
+```
+cd sgid-demo-frontend-spa
+npm install
+npm run dev
+```
+
+This should start the frontend on port `5173` by default. Visit `http://localhost:5173` to test the entire sgID flow!
+
+## For contributors
+
+To start the server in debug mode, run:
+
+```
+npm install
+npm run dev
+```

examples/express/index.ts

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+import express, { Router } from 'express'
+import cors from 'cors'
+import SgidClient, { generatePkcePair } from '@opengovsg/sgid-client'
+import * as dotenv from 'dotenv'
+import crypto from 'crypto'
+import cookieParser from 'cookie-parser'
+import open from 'open'
+
+dotenv.config()
+
+const PORT = 5001
+const redirectUri = String(
+  process.env.SGID_REDIRECT_URI ?? `http://localhost:${PORT}/api/callback`,
+)
+const frontendHost = String(
+  process.env.SGID_FRONTEND_HOST ?? 'http://localhost:5173',
+)
+
+const sgid = new SgidClient({
+  clientId: String(process.env.SGID_CLIENT_ID),
+  clientSecret: String(process.env.SGID_CLIENT_SECRET),
+  privateKey: String(process.env.SGID_PRIVATE_KEY),
+  redirectUri,
+})
+
+const app = express()
+
+const apiRouter = Router()
+
+const SESSION_COOKIE_NAME = 'exampleAppSession'
+const SESSION_COOKIE_OPTIONS = {
+  httpOnly: true,
+}
+
+type SessionData = Record<
+  string,
+  | {
+      nonce?: string
+      // Store state as search params to easily stringify key-value pairs
+      state?: URLSearchParams
+      accessToken?: string
+      codeVerifier?: string
+      sub?: string
+    }
+  | undefined
+>
+
+/**
+ * In-memory store for session data.
+ * In a real application, this would be a database.
+ */
+const sessionData: SessionData = {}
+
+app.use(
+  cors({
+    credentials: true,
+    origin: frontendHost,
+  }),
+)
+
+apiRouter.get('/auth-url', (req, res) => {
+  const iceCreamSelection = String(req.query.icecream)
+  const sessionId = crypto.randomUUID()
+  // Use search params to store state so other key-value pairs
+  // can be added easily
+  const state = new URLSearchParams({
+    icecream: iceCreamSelection,
+  })
+
+  // Generate a PKCE pair
+  const { codeChallenge, codeVerifier } = generatePkcePair()
+
+  const { url, nonce } = sgid.authorizationUrl({
+    // We pass the user's ice cream preference as the state,
+    // so after they log in, we can display it together with the
+    // other user info.
+    state: state.toString(),
+    codeChallenge,
+    // Scopes that all sgID relying parties can access by default
+    scope: ['openid', 'myinfo.name'],
+  })
+  sessionData[sessionId] = {
+    state,
+    nonce,
+    codeVerifier,
+  }
+  return res
+    .cookie(SESSION_COOKIE_NAME, sessionId, SESSION_COOKIE_OPTIONS)
+    .json({ url })
+})
+
+apiRouter.get('/callback', async (req, res): Promise<void> => {
+  const authCode = String(req.query.code)
+  const state = String(req.query.state)
+  const sessionId = String(req.cookies[SESSION_COOKIE_NAME])
+
+  const session = { ...sessionData[sessionId] }
+  // Validate that the state matches what we passed to sgID for this session
+  if (session?.state?.toString() !== state) {
+    res.redirect(`${frontendHost}/error`)
+    return
+  }
+
+  // Validate that the code verifier exists for this session
+  if (!session?.codeVerifier) {
+    res.redirect(`${frontendHost}/error`)
+    return
+  }
+
+  // Exchange the authorization code and code verifier for the access token
+  const { accessToken, sub } = await sgid.callback({
+    code: authCode,
+    nonce: session.nonce,
+    codeVerifier: session.codeVerifier
+  })
+
+  session.accessToken = accessToken
+  session.sub = sub
+  sessionData[sessionId] = session
+
+  // Successful login, redirect to logged in state
+  res.redirect(`${frontendHost}/logged-in`)
+})
+
+apiRouter.get('/userinfo', async (req, res) => {
+  const sessionId = String(req.cookies[SESSION_COOKIE_NAME])
+
+  // Retrieve the access token and sub
+  const session = sessionData[sessionId]
+  const accessToken = session?.accessToken
+  const sub = session?.sub
+
+  // User is not authenticated
+  if (session === undefined || accessToken === undefined || sub == undefined) {
+    return res.sendStatus(401)
+  }
+  const userinfo = await sgid.userinfo({
+    accessToken,
+    sub,
+  })
+
+  // Add ice cream flavour to userinfo
+  userinfo.data.iceCream = session.state?.get('icecream') ?? 'None'
+  return res.json(userinfo)
+})
+
+apiRouter.get('/logout', async (_req, res) => {
+  return res
+    .clearCookie(SESSION_COOKIE_NAME, SESSION_COOKIE_OPTIONS)
+    .sendStatus(200)
+})
+
+const initServer = async (): Promise<void> => {
+  try {
+    app.use(cookieParser())
+    app.use('/api', apiRouter)
+
+    app.listen(PORT, () => {
+      console.log(`Server listening on port ${PORT}`)
+      void open(`http://localhost:${PORT}`)
+    })
+  } catch (error) {
+    console.error(
+      'Something went wrong while fetching the static files. Please restart the server.',
+    )
+    console.error(error)
+  }
+}
+
+void initServer()