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ReactJs + Webapi How do you do external authentication?

I am building a reactjs website that will communicate with asp.net web api 2 to save and retreive data.
but I am not sure how to do this.
I know to accomplish this on a high level it would be something like
User comes to my site and hits signup/log
Chooses which provider then want to use(google, facebook and etc). I am only want to support external providers(ie I don't want to have to deal with usernames/pwds)
User it sent to authenticated part of site
User clicks "add course" that data send via ajax to webapi with some sort of token to prove they have access to these methods.
I am not sure how to implement this problems I see is
Reactjs I guess is handling the authentication part? then once they been authenticated it would have to be saved in my db via webapi so it knows about this new user?
Reactjs would have to block users from going to secure pages till they are authenticated
Web api would have to generate a token for the user for that session so they can access the web api(I want to stop people from consuming my api).
Is there some simple example out there how to achieve this?

Reactjs I guess is handling the authentication part? then once they been authenticated it would have to be saved in my db via webapi so it knows about this new user
Better use some third party auth library here like PassportJS that does the auth for you using strategies like Passport-Facebook. This will give you an Oauth access token from Facebook upon authentication. You can now save this token in your cookies (or localStorage), take a look at the security considerations.
Should you store it in a DB? Here are some arguments about it.
Reactjs would have to block users from going to secure pages till they are authenticated
This can be done by checking if they have a valid token.
Web api would have to generate a token for the user for that session so they can access the web api(I want to stop people from consuming my api).
This can be easily achieved by using JSON Web Tokens. Note that you will have to store the JWT in your client side locally, along side your FB-Google oauth tokens (or you can relegate that to a single API by storing them in DB?. Its a design choice, I would prefer to store them separately and save a lot of hassle).

Can Firebase handle logins over REST?

The documentation claims:
"The Firebase JavaScript, Java, and Objective-C libraries provide built-in functionality for many login providers, but the REST API works a little differently. ... generate user login tokens with your own server side code."
There appears to be no library for .NET/Xamarin, so I need to use the REST API. But there is no "own server side code" on Firebase.
I'm interpreting this to mean I need a separate service (not Firebase) to host software to provide the registration and authentication endpoints. Do I have that right?

Yes it can! (answering the question in title title)
I just fiddlered (is that a word?) how the javascript SDK talks to firebase servers and it turns out it is quite straightforward.
At the beginning you let your user authenticate with your provider of choice (e.g. facebook). You grab the oauth access token and you POST that (don't forget the "application/json" header) to the following location:
https://www.googleapis.com/identitytoolkit/v3/relyingparty/verifyAssertion?key=<YOUR API KEY>
The post data would like like this:
{
"postBody":"access_token=<OAUTH ACCESS TOKEN>&providerId=<PROVIDER ID>",
"requestUri":"http://localhost",
"returnSecureToken":true
}
The <PROVIDER ID> can be one of the following
facebook.com
google.com
twitter.com
github.com
In return you get a nice json which contains a lot of information, one of which is the "idToken" which acts as the authentication token for making REST requests to firebase. You can set it as the "auth" URL parameter of your request.
I wrote a library to handle this (along with email+password & anonymous logins):
https://github.com/step-up-labs/firebase-authentication-dotnet
And I wrote a blog post about it:
http://blog.bezysoftware.net/firebase-authentication-csharp-library/

Yes and No.
You could create binding projects and use the existing FirebaseUI iOS and Android libraries
Xamarin Binding walk-throughs:
https://developer.xamarin.com/guides/android/advanced_topics/binding-a-java-library/
https://developer.xamarin.com/guides/ios/advanced_topics/binding_objective-c/
Firebase mobile libraries:
https://www.firebase.com/docs/android/
https://www.firebase.com/docs/ios/
Otherwise, yes, you are correct, you would need to provide your own server side authorization system.

The need for a server when using REST is not for the actual authentication (Although you will have to solve this issue too somehow), but for generating the tokens that can be used for the client to authenticate against the Firebase REST API.
From the documentation
To authenticate a user with Custom Login, we must provide each client with a secure JWT that has been generated on a server. There are several helper libraries for generating JWTs given a Firebase app's secret. Our secret can be found under the Secrets tab in our Firebase app's dashboard.
Then you could attache this token to REST requests to be authenticated:
'https://docs-examples.firebaseio.com/rest/saving-data/auth-example.json?auth=SERVER_GENERATED_TOKEN'

Secure webapi 2 without authorisation or user login

I looked everywhere for an answer about securing my webApi but always i'm pointed to OAuth or openID, but i don't want the user to login or authenticate. This is my problem:
I have a webapi which is called via Javascript to get the data in Json. This data is used in the Html of that page. (deployed on Azure)
Now i don't want someone else to get that data via Javascript or with a simple GET request. Only my html page is allowed to do so.
I'm looking for something to secure my Webapi to be only consumed by the applications i want. If i look to the OAuth and Azure stuff, people always have to login, but i want this page to be public, but only the webapi needs to be secure.
I can't check on IP, because the call is done at client side in the browser.

It is not possible to authenticate and thus secure the API to be used by a specific client (run entirely in the browser - like SPAs) app. You cannot protect the data the app sends to the API. (see for more details Protecting REST API behind SPA against data thiefs)
You could build some modules server side and then use some token based server to server communication if you do not want to introduce users and end user authentication.
Of course it is also a question of how you interpret security.
If it is just that you do not want other web apps to use the data -
CORS policies will do the job. It is unlikely that there will be more
than some isolated cases of users using a browser other than the
popular once which respect CORS.
If it is you do not want data to be mass downloaded you could
implement some client filtering based on IP. This could even be done
on the networking layer so the API do not need to be modified in any
way.

As a suggestion, you can have it secured making sure the request has some headers defined by you. For example, you can set an Authorization header with a token that only you know. You can for example, create a token based on a key that you and the webapi share, encrypt it with the time you are calling the api. The web api receives the request and:
1 - Check if the request has the token it requires;
2 - if it does, it creates a token the same way your page did;
3 - matches its token with the requests token;
If you are calling the webapi via javascript, the method may be exposed. However, it's one idea

Take a look to CORS (Cross Origin Resource Sharing), it may be your solution.
In synthesis you can allow requests to the Api only from some specific websites. Most, nearly all browsers, support it.
This is how you enable and configure it in Web Api 2.

Handling security in an ASP MVC application that uses JS to a REST API

I have an ASP MVC4 web site. Originally, most of the content was served via controllers as one would expect. I have moved the data storage from SQL Server to MongoDB. I have also added a lot of ajax to update data client side, without a full refresh. This is working fine, but my controllers now have lots of methods that take json and return json. I was able to build a Node.js server that hits the database and exposes exactly the same functionality, without lots of going to and from C#.
My javascript client-side is now calling a Node.js REST API, this works great. My 'secure' code (like adding a new user) hits the same REST API from the server side.
My question is this: How can I handle security properly with this? I have three scenarios:
GET api/messages: No need for security, I want to expose my site's messages to anyone who is interested via a Json REST API.
GET api/my/messages: I need to allow access to this only if the user is logged in (it gets the user's messages).
POST api/users: This is a function that should only be called from the server, and nothing else should be able to use it.
As the user is already logging in to my ASP website, how can I use their logged in credentials to authenticate them with my REST service? While the user is logged in, the pages client side will hit it regularly for updates.
Is there any sensible/standard way to do this? The core idea is that the client side code uses a REST API that is at least partially open to the public, and that in fact that API offers all of my business logic - only parts of it (like creating a user) are locked down to super-admins only.
Thanks in advance!

Create two authentication middleware handlers. One you add to all your "my" routes and another which you add to your POST routes.
The "my" authenticator takes the asp.net auth cookie that is present in the request and makes a http call to your asp.net mvc site with it.
You'll need an action which either returns a 401 if the cookie is invalid otherwise it returns some info about that user's permissions perhaps.
If the request into node doesn't have a cookie, return a 401 again.
In addition, to prevent excessive calls to your mvc site to check the cookie, you could use the cookiesession middleware to set a cookie on the client with a flag of authenticated. That will result in 2 cookies for your client, but that shouldn't be an issue. Just make the node one expire before the aspx one.
The POST authenticator middleware can use any shared secret you like between your node and mvc server. e.g. a special header in the request.

If the user is required to login you can use [Authorize] on your controller actions. Autorization will be handled like any other webrequest.
Furthermore you might consider to add a key to your api requests which you can provide in the initial page load. A autorized user will have a GUID which he will sent with the api call. You can check if this key was issued by your app to a valid user.

As you said all the secure calls already go through your MVC server code which in turn calls the Node.js code, am I right? Basically you need a way to block calls to this Node.js from other clients that are not your MVC code.
Thinking out loud, these are the ideas that pop into my mind:
Use SSL only between MVC and Node. You can set up client and server certificates so that the Node code will only respond after authentication (I don't know how Node handles SSL so you will need some documentation here
If you want, the Node server could also check the call origin and so you can filter based on IP and only allow IPs where your MVC code is sitting
Use an encrypted authentication token on the secure methods on the Node code. Again I'm not really a Node expert but I can imagine it has ways of decrypting a token, or you can simply base it on a random number with a common seed... If noone has access to your server code ideally noone should be able to guess this token. Again, SSL will help against traffic sniffing
I am quite sure that people will come up with other ideas. For me, the most basic thing is anyway ensure that the secure methods are only accessible through an SSL connection and on this connection you can exchange all the info (token, passwords, etc.) you desire.

Use multiple databases in a webservice

My question is: I have a web application built using ASP.net and a web service. The web service has to use a different database, depending on which client is logged on.
Is there a easy and elegant solution to do this ?

You can check it by passing client credentials in each request to service.
Ask client to call init(id,pass,...) method with credentials before calling any web method. You can also ask for optional settings in Init, based on inputs you can give client the access token and let client pass this access token to each request, you can check this access token and parse the settings to take action i.e. to call a particular DB connection. Facebook api works in similar way.