step04.md

Step 4 - Communicating with the server

Now that we've built the server, we need to communicate with it. We're going to control the server with handler functions.

What is a handler function?

When a request reaches the server, we need a way of responding to it. In comes the handler function. The handler function is just a function which receives requests and handles them, hence the name.

The handler function always takes a request and response object, and sends the response back to the client along with some information. You can decide what to send back in your response.

What does a handler function look like in Express?

The get() method is used to define a handler function in Express. It takes two parameters: the endpoint at which to trigger an action (we'll explain more about this in the next step), and the handler function that tells it exactly what to do. Here's a simple "Hello World!" example:

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

Here, we are telling our server to respond with "Hello World!" when someone tries to access the webpage.

1. Create your own handler function.

We are now making a handler function with a custom message in our response. You can write any message you want.

Update your server.js file with an empty app.get() function:

var express = require("express");
var app = express();

app.get("/", function (req, res) {

});

app.listen(3000, function () {
  console.log("Server is listening on port 3000. Ready to accept requests!");
});

Try to console.log the req object in your terminal to see what it looks like. You should see a lot of data come through.

2. Tell your handler function what to do

We want our handler function to send back a message to the client. To do that, we're going to use the Express send() method. This will update the response object with the message.

Update your handler function like so:

var express = require("express");
var app = express();

app.get("/", function (req, res) {
  res.send("Yay Node Girls!");
});

app.listen(3000, function () {
  console.log("Server is listening on port 3000. Ready to accept requests!");
});

3. Check it out in your browser

Quit your server in the terminal with ctrl + c. Then restart it to run your new changes.

$ node server.js

Now, open your favourite browser (we like Chrome), and navigate to http://localhost:3000. If you see your message in the browser, congratulations! You just sent your first response from the server.

Go to Step 5 >>>>

Keyword	Explanation
handler function	A function that receives requests and tells the server how to respond to them.
endpoint	The part of the URL which comes after /. For example: /chocolate is the "chocolate" endpoint.
get()	The Express method used to set up a handler function in Express. Takes two parameters: the endpoint, and the handler function.
send()	The Express method used to send information back to the client from the server. Updates the response object.