After the user authenticates, they are redirected back to your application with an authorization code. Steps:
  1. Retrieve Authorization Code and State: Extract these from the query parameters.
  2. Verify State: Ensure the state matches the one you generated earlier.
  3. Exchange Authorization Code for Tokens: Send a POST request to the token endpoint to exchange the authorization code for ID, access, and refresh tokens.
Example Callback Handling: 
const params = new URLSearchParams(window.location.search)
const code = params.get('code')
const state = params.get('state')

if (state !== savedState) {
  throw new Error('Invalid state')
}

const codeVerifier = localStorage.getItem('code_verifier')
const body = new URLSearchParams({
  grant_type: 'authorization_code',
  code: code,
  redirect_uri: redirectUri,
  client_id: clientId,
  code_verifier: codeVerifier,
})

const response = await fetch(tokenEndpoint, {
  method: 'POST',
  headers: {
    'Content-Type': 'application/x-www-form-urlencoded',
  },
  body: body.toString(),
})

const tokenResponse = await response.json()
localStorage.setItem('access_token', tokenResponse.access_token)
localStorage.setItem('refresh_token', tokenResponse.refresh_token)
localStorage.setItem('id_token', tokenResponse.id_token)
Token Request Parameters
ParameterDescription
grant_typeThe type of grant being requested. For authorization code flow, use authorization_code.
codeThe authorization code received from the authorization endpoint.
redirect_uriThe URI to which the response will be sent. It must match the redirect URI registered with the client.
client_idThe client ID you obtained during client registration.
code_verifierThe PKCE code verifier.
Example Token Request Body 
grant_type=authorization_code&
code=AUTHORIZATION_CODE&
redirect_uri=http://localhost:3000/callback&
client_id=YOUR_CLIENT_ID&
code_verifier=CODE_VERIFIER
Token Response Fields
FieldDescription
access_tokenThe token that can be used to access protected resources.
id_tokenA JWT that contains user identity information.
refresh_tokenA token that can be used to obtain new access tokens.
token_typeThe type of token issued. Typically Bearer.
expires_inThe duration in seconds for which the access token is valid.
Example Token Response 
{
  "access_token": "ACCESS_TOKEN",
  "id_token": "ID_TOKEN",
  "refresh_token": "REFRESH_TOKEN",
  "token_type": "Bearer",
  "expires_in": 3600
}