Update README.md after commits bff756b and 7709f45 (#259)

* Update README.md after commits bff756b and 7709f45

* Lint code

Author: giovanniruzzi

README.md

@@ -17,6 +17,8 @@ npm i @fastify/oauth2
 
 ## Usage
 
+Two separate endpoints need to be created when using the fastify-oauth2 module, one for the callback from the OAuth2 service provider (such as Facebook or Discord) and another for initializing the OAuth2 login flow.
+
 ```js
 const fastify = require('fastify')({ logger: { level: 'trace' } })
 const oauthPlugin = require('@fastify/oauth2')
@@ -30,26 +32,43 @@ fastify.register(oauthPlugin, {
     },
     auth: oauthPlugin.FACEBOOK_CONFIGURATION
   },
-  // register a fastify url to start the redirect flow
+  // register a fastify url to start the redirect flow to the service provider's OAuth2 login
   startRedirectPath: '/login/facebook',
-  // facebook redirect here after the user login
+  // service provider redirects here after user login
   callbackUri: 'http://localhost:3000/login/facebook/callback'
   // You can also define callbackUri as a function that takes a FastifyRequest and returns a string
   // callbackUri: req => `${req.protocol}://${req.hostname}/login/facebook/callback`,
 })
 
+// This is the new endpoint that initializes the OAuth2 login flow
+fastify.get('/login/facebook', {}, (req, reply) => {
+  fastify.facebookOAuth2.generateAuthorizationUri(
+    req,
+    reply,
+    (err, authorizationEndpoint) => {
+     if (err) console.error(err)
+     reply.redirect(authorizationEndpoint)
+    }
+  );
+});
+
+// The service provider redirect the user here after successful login
 fastify.get('/login/facebook/callback', async function (request, reply) {
   const { token } = await this.facebookOAuth2.getAccessTokenFromAuthorizationCodeFlow(request)
-
+  
   console.log(token.access_token)
 
-  // if later you need to refresh the token you can use
+  // if later need to refresh the token this can be used
   // const { token: newToken } = await this.getNewAccessTokenUsingRefreshToken(token)
 
   reply.send({ access_token: token.access_token })
 })
 ```
 
+In short, it is necessary to initially navigate to the `/login/facebook` endpoint manually in a web browser. This will redirect to the OAuth2 service provider's login screen. From there, the service provider will automatically redirect back to the `/login/facebook/callback` endpoint where the access token can be retrieved and used. The `CLIENT_ID` and `CLIENT_SECRET` need to be replaced with the ones provided by the service provider.
+
+A complete example is provided at [fastify-discord-oauth2-example](https://github.com/fastify/fastify-oauth2/blob/master/examples/discord.js)
+
 ### Usage with `@fastify/cookie`
 
 Since v7.2.0, `@fastify/oauth2` requires the use of cookies to securely implement the OAuth2 exchange. Therefore, if you need `@fastify/cookie` yourself,

examples/discord.js

@@ -0,0 +1,40 @@
+'use strict'
+
+const fastify = require('fastify')({ logger: { level: 'trace' } })
+const oauthPlugin = require('..')
+
+fastify.register(oauthPlugin, {
+  name: 'discordOAuth2',
+  credentials: {
+    client: {
+      id: '<CLIENT_ID>',
+      secret: '<CLIENT_SECRET>'
+    },
+    auth: oauthPlugin.DISCORD_CONFIGURATION
+  },
+  startRedirectPath: '/login/facebook',
+  callbackUri: 'http://localhost:3000/login/discord/callback'
+})
+
+fastify.get('/login/discord/callback', async function (request, reply) {
+  try {
+    const token =
+      await this.discordOAuth2.getAccessTokenFromAuthorizationCodeFlow(request)
+    return reply.send(token)
+  } catch (error) {
+    return reply.send(error)
+  }
+})
+
+fastify.get('/login/discord', {}, (req, reply) => {
+  fastify.discordOAuth2.generateAuthorizationUri(
+    req,
+    reply,
+    (err, authorizationEndpoint) => {
+      if (err) console.error(err)
+      reply.redirect(authorizationEndpoint)
+    }
+  )
+})
+
+fastify.listen({ port: 3000 })