Learn OAuth

Facebook (Setup steps)

First, go to https://developers.facebook.com/apps and create a new app. Choose consumer as the app type. Different app type offer different products such as facebook login (oauth), webhook, and much more, which can be added to your app later on.
After the app has been created, go to the app dashboard and add facebook login product to your app.
Now, go to the setting of the facebook login product of your app, then add your api URL for facebook callback at Valid OAuth Redirect URIs field. Example: https://localhost:3000/auth/facebook/callback
Remember to prepare 2 endpoints in your app. One for the client to be redirected to facebook app authentication page. Another one is for the oauth response (the callback at the previous step).
You may request for additional user information from facebook such as user likes, profile picture, last checked in and much more. Most of the information require you to request an app review before you can access it.

First, go to https://console.cloud.google.com, and create a new project.
Next, go to OAuth consent screen, and select external type.
Enter all the necessary information required
After setup the OAuth consent screen, click Credentials and create OAuth Client ID
Select Web Application as the application type.
Then, fill up https://localhost:3000 for the Authorised Javascript Origin field, and https://localhost:3000/auth/google/callback for the Authorised redirect URIs field.

First, when user navigate to the url https://localhost:3000/auth/facebook, the server will response with 302 status code, which will redirect the browser to facebook oauth dialog page. Eg: https://www.facebook.com/v3.2/dialog/oauth?response_type=code&redirect_uri=https%3A%2F%2Flocalhost%3A3000%2Fauth%2Ffacebook%2Fcallback&client_id=166453568983174. The url will display a page with oauth of the app, which ask the user to either authorize or reject the app.
After the user authorize or rejected the app, the facebook will response with 302 status code, together with code query string to the callback endpoint you registered in the facebook login app. Eg: https://localhost:3000/auth/facebook/callback?code=AQDr9UQYeiNs2ZRWrDPiBJdSc6yN3Dl-22jkbcG5ZHd4fC6iyfR9-IOq7xzY-zvuPGWH2xx6-Y0ysSb7slsbAH9K01Zl82orEjMNv8Pqo0rMKrG-yAGrsgjeMNIlMf82PDc-QvVJKEiW9lSCastC7-XAh36ItaALZIl0dq2F6-jqCJtISN2iEZlnagF6Q4bkEb34EFdHf8r5hk0UN4MwzdoRA0Tw4fXkrFbAdR73h3G4JPPVON5RRIrKBVFsaLUbwk_2nBeT0Y0K7ucZuakk8JFozoEB9Y6pe9EPewPiqklfPKKbB2dFxWaNBqLwEFIJJB19Mcl9Os6hEirqj3blT29pUKrL7bwDGdkyLXpZd9gR6A4_TUFDxoVx95dTMc54Ga5XejECsEVpaSlEsVyIsoXWJ2qS8oxMbIOQgF3b0KP7xg#_=_
Next, the browser will redirect the whole code query string to your local callback api.
After your server received the information from the callback, the server should decide whether to create a new user, authorize the user, or reject it.

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