hello-world.md

Hello World

HelloWorld! with Express

Probably the simplest web server ever:

const express = require("express");
const app = express();
const PORT = 3000;

app.get("/", (req, res) => {
  res.send("Hello World!");
});

const server = app.listen(PORT, () => {
  console.log(`Server started on port ${PORT}...`);
});

To see the first example, run the following command in project root directory:

npm install -g nodemon
nodemon helloworld.js

Now open the browser with address "http://localhost:3000/", you will see "Hello World!" printed on the browser page.

Your API can handle more than HTTP GET requests

If you look at the codes in helloworld.js, you notice that the application can handle other type of HTTP methods (like POST and PUT).

To see it working, you need to use a tool like postman or insomnia.

Using those tools, compose a POST or PUT request to "http://localhost:3000/". You should see corresponding texts in responses.

Syntax for defining a route handler

app.METHOD(PATH, ROUTE_HANDLER);

Where:

• app is an instance of the express module
• METHOD is an HTTP request method (GET, POST, PUT or DELETE)
• PATH is a path on the server (e.g. '/', '/books', '/books/:id').
• ROUTE_HANDLER is a callback function executed when the route is matched. A route handler function usually has a signature like the one below:

function xxxHandler(request, response) {
  // get useful information from request
  // generate response
}

Sometimes, you may define multiple route handlers for the same route. In that case, the route handlers can take one more argument called next. A route handler can call next() within the body of the function to hand off control to the next callback defined for the same route.

function xxxHandler(request, response, next) {
  // get useful information from request
  // update response with response.write()
  // delegate to the next route handler by calling next()
}

To see the a multi route handler example, run the following command:

nodemon express_basic_example_2.js