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Express Study Notes: Five Steps To Develop My First Express App


In the last post, I shared my study note about one of the most important reason that Node is very popular is because Node has the largest package ecosystem of open source libraries in the world.

Node's NPM, the package manager for Javascript, contains almost 200,000 packages which are centralized on the NPM website. And more importantly, NPM has a command line tool which help us to install packages really easily.

The course instructor told us that we are going to learn Express.js to develop Apps in this course.

screenshot for the Express.js.com homepage

What is Express.js?

Express.js is a web development framework.

According to the course, framework is a bunch of code that someone else wrote just like a library is except when we use a framework, we have less control.

The goal of using this framework is to make our development of APPs faster.

Why do we use Express.js?

1. It is the most popular Node web development framework so far.
2. It is a very lightweight framework, so it doesn't hide things from us, and it doesn't do things that we don't expect it to do.

Develop First Express APP

Step One: Install Express

We need to open Goormide terminal first. Then, create a directory "FirstExpressApp" by using "mkdir" command. Then, we create a new Javascript file "App.js" in this folder.

In order to install express, we need to type "npm install express", and the syntax will find the Express.js package and install for us. As installation is done, we can see the "express" folder on the left menu bar.


Step Two: Create The First Route

Open the "App.js" file, we need to write the code to require "express" first. Then, we can start to develop our App.

The App is very simple. Basically, when it get slash request, we receive "Hello" message; when it get slash bye, we receive " Goodbye!" message; and when it get slash cat, we receive "MEOW!".

In order to let the App run, we need to write route. Route is how we listen for a particular requests and then run some code depending on the request we get.

In this project, we have three different routes. All we need to start with: 

app.get("/", function(req, res){
res.send("message");})

According to the course, app.get takes two different parameters:

(1) The URL or the path, which is "/"
(2) A callback function to run this code. 

The callback function takes two arguments: request (req) and response (res).

"res.send()" is a way of responding with some text.

Step Three: Run The Code in Node.js

I finish the rest two routes by using the same function. The only changes are the message. 

Even though we have all the app code done, we still can't run because we need to add an app listener.

   app.listen(3000, process.env.PORT, process.env.IP)

This code will tell Express.js to listen on a particular port that the server wants to and a particular IP that this server expects to.

Then, we save the file and type "node app.js" in the terminal to run the code. By the way, every time you add new route, you have to restart the node app.js.


Step Four: Find The Specific URL

In the Node, we can't actually see the app result. We need to find a specific url from Goormide Project "Running URL and Port".  



Step Five: Run The URL on Browser To Test App

We paste this URL on the browser, and hit enter. You can see the first message "Hello!".


When we add "/bye" on the link, it shows "Goodbye!".


When we change the slash to "/cat", it shows "MEOW!".




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