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 inside the handler function. Restart your server, refresh the browser, then go to 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

If you are using a local environment or the cloud9 terminal to run your code, quit your server in the terminal with ctrl + c. Then restart it to run your new changes.

$ node server.js

Or if you are using the Run button in cloud9, click the Restart (or Stop and then Run) button in the Run Panel.

Now open your favourite browser (we like Chrome), and for your local environment, navigate to http://localhost:3000. Otherwise for cloud9, click the Preview button on the top menu, you will see a browser is opened inside cloud9.

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.