93 lines
2.6 KiB
Markdown
93 lines
2.6 KiB
Markdown
---
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tags:
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- node-js
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---
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# `http` module
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The HTTP Module allows us to create a web server that listens for HTTP requests
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on a given port. It is therefore perfect for creating backends for client-side
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JavaScript.
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## Creating a server
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An HTTP server is another instance of an
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[event emitter](events.md)). It
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therefore has all the same methods as the `EventEmitter` class: `on`, `emit`,
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`addListener` etc. This demonstrates again how much of Node's core functionality
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is based on event emitters.
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_Creating a server_
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```js
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const http = require("http");
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const server = http.createServer(); // Create server as emitter
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// Register functions to run when listener is triggered
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server.on("connection", (socket) => {
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console.log("new connection...");
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});
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server.listen(3000);
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console.log("Listening on port 3000");
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```
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This server is functionally equivalent to a generic event emitter:
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```js
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// Raise an event
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const emitter = new EventEmitter("messageLogged");
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// Register a listener
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emitter.on("messagedLogged", function () {
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console.log("The listener was called.");
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});
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```
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Whenever a request is made to this server, it raises an event. We can therefore
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target it with the `on` method and make it execute a function when requests are
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made.
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If we were to start the server by running the file and we then used a browser to
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navigate to the port, we would see `new connection` logged every time we refresh
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the page.
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### Sockets and `req, res`
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A socket is a generic protocol for client-server communication. Crucially it
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**allows simultaneous communication both ways**. The client can contact the
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server but the server can also contact the client. Our listener function above
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uses a socket as the callback function but in most cases this is quite
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low-level, not distinguishing responses from requests. It is more likely that
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you would initiate a `request, resource` architecture in place of a socket:
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```js
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const server = http.createServer((req, res) => {
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if (req.url === "/") {
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res.write("hello");
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res.end();
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}
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});
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```
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#### Return JSON
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Below is an example of using this architecture to return JSON to the client:
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```js
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const server = http.createServer((req, res) => {
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if (req.url === "/products") {
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res.write(JSON.stringify(["shoes", "lipstick", "cups"]));
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res.end();
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}
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});
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```
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### Express
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In reality you would rarely use the `http` module directly to create a server.
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This is because it is quite low level and each response must be written in a
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linear fashion as with the two URLs in the previous example. Instead we use
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Express which is a framework for creating servers and routing that is an
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abstraction on top of the core HTTP module.
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