How to share state between users in ReactJS

WebSockets won't work for you as you need one fixed party in WebSocket connection.

You could share the state peer-to-peer style with a WebRTC connection and using the DataChannel in that connection, but that's not an easy task. (see for example: https://www.html5rocks.com/en/tutorials/webrtc/basics/#toc-rtcdatachannel)

The much more common option is to use a central data storage and either sync your state with that or just store it there in the first place. You could use Firebase (see answer by David) or any other kind of database. Firebase is particularly easy to set up, though.

If you have/need/want to use an API-backend anyways, you can use pretty much any kind of database layer in that and just talk REST or GraphQL with the API to share your state between players.

instead of sharing state between players you should explore the possibility of players sharing an instance of game object on the server.

for example :

game={
status:'running',
players:2,
p1:{state:'waiting'},
p2:{state:'active'}
}

• time complexity for common state on server will be [n]

• expected time complexity for state exchange or polling is [n^2] or more.

If you want to try serverless approach, you can look at AWS Lambda and Dynamo DB.

I recommend you use firebase which is a database to send and receive data online because the states of react js are not interlaced between devices