Asynchronous APIs

JavaScript Ajax introduced background browser-server comms over HTTP.
But it is still basically "pull", i.e. the REST "request/response" model.
Some applications need to constantly "push" data to the client asynchronously.
There was a need for a true, two-way, push-and-pull model.

WebSockets

True browser-server two-way comms (push-pull).

A typical use of WebSockets is with Node.js, and JS on client and server, as follows:

Node.js installed on server.
Websocket JS library like Socket.IO installed on server. Also has a client-side JS part.
Node runs server-side JS to set up the Node server:
```
 node server.js 
```
server.js starts a http server listening at some new port, say port 3000.
server.js sets up socket connection handling on that Node server.
Client-side JS makes a Websocket connection to something like:
```
 https://host:3000 
```
Client can then transmit data to server. Server can transmit that to other clients (on this socket). Clients may be sent new push data from server at any time.
Multiple clients can talk to each other through server.
Online chat in browser. Multi-player games in browser.

Live Websockets demos on Ancient Brain.

Click to run World: Websocket buttons at Ancient Brain.

Click to run World: Websocket chat at Ancient Brain.

Click to run World: Websockets boxes at Ancient Brain.