eolas/zk/http.md

tags
backend node-js

http module

The HTTP Module allows us to create a web server that listens for HTTP requests on a given port. It is therefore perfect for creating backends for client-side JavaScript.

Creating a server

An HTTP server is another instance of an event emitter). It therefore has all the same methods as the EventEmitter class: on, emit, addListener etc. This demonstrates again how much of Node's core functionality is based on event emitters.

Creating a server

const http = require("http");

const server = http.createServer(); // Create server as emitter

// Register functions to run when listener is triggered
server.on("connection", (socket) => {
  console.log("new connection...");
});

server.listen(3000);
console.log("Listening on port 3000");

This server is functionally equivalent to a generic event emitter:

// Raise an event
const emitter = new EventEmitter("messageLogged");

// Register a listener
emitter.on("messagedLogged", function () {
  console.log("The listener was called.");
});

Whenever a request is made to this server, it raises an event. We can therefore target it with the on method and make it execute a function when requests are made.

If we were to start the server by running the file and we then used a browser to navigate to the port, we would see new connection logged every time we refresh the page.

Sockets and req, res

A socket is a generic protocol for client-server communication. Crucially it allows simultaneous communication both ways. The client can contact the server but the server can also contact the client. Our listener function above uses a socket as the callback function but in most cases this is quite low-level, not distinguishing responses from requests. It is more likely that you would initiate a request, resource architecture in place of a socket:

const server = http.createServer((req, res) => {
  if (req.url === "/") {
    res.write("hello");
    res.end();
  }
});

Return JSON

Below is an example of using this architecture to return JSON to the client:

const server = http.createServer((req, res) => {
  if (req.url === "/products") {
    res.write(JSON.stringify(["shoes", "lipstick", "cups"]));
    res.end();
  }
});

Express

In reality you would rarely use the http module directly to create a server. This is because it is quite low level and each response must be written in a linear fashion as with the two URLs in the previous example. Instead we use Express which is a framework for creating servers and routing that is an abstraction on top of the core HTTP module.