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](events.md)). 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_

```js
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:

```js
// 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:

```js
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:

```js
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.