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TEACHING-MATERIALS/01 Introduction to ECMAScript 6/README.md

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Introduction to NodeJS 1 Configuring Node 2 Variables and Scope 2.1 Variables and Constants 2.2 Test Your Knowledge 3 Exception Handling 3.1 Test Your Knowledge Extension Activity Installing Node
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Introduction to NodeJS

In this worksheet you will be introduced to NodeJS, a runtime environment that allows you to run JavaScript code on a web server. NodeJS is based on the Chrome V8 JavaScript engine that runs inside the Chrome browser but adapted for server use.

It uses an event-driven, non-bocking I/O model which makes it very efficient when handling large numbers of connections.

The runtime uses a single thread running an event loop to to handle all incoming requests. When a connection is received we typically fire a callback to handle the request, returning control to the main thread once the work is done. This is termed non-blocking because the callback is used to handle the work leaving the main event loop free to deal with other requests.

Before you start this or any other worksheet you should make sure you have the latest commits from the upstream repository on GitHub.

git pull upstream master

There is a slide deck https://goo.gl/EGvS5Z to accompay this worksheet. Make sure you are familiar with its contents before proceeding.

1 Configuring Node

During this module you will be using NodeJS, a server-side version of ECMAScript 6 (JavaScript) to develop an API. Before wer start, this will need to be installed. To check this we can try to view the version.

node -v
  -bash: node: command not found

The tool used to install and upgrade NodeJS is called the Node Version Management tool (nvm).

Installation is carried out through the Terminal. Start by downloadeding the installation script using the curl command and executing it.

curl -o- https://raw.githubusercontent.com/creationix/nvm/v0.33.1/install.sh | bash

The installation script has added a new path to your shell configuration and this needs to be loaded again using source ~/.bashrc, then we can check for the versions of NodeJS available. The latest version (7.2.0 at the time of writing) will be at the end of the list.

nvm list-remote
         ...
         v7.0.0
         v7.1.0
         v7.2.0
         ...
         v7.9.0
nvm install 7.9.0
  Downloading https://nodejs.org/dist/v7.9.0/node-v7.2.0-linux-x64.tar.xz...                        
  ######################################################################## 100.0%
  Now using node v7.9.0 (npm v3.10.9)  
  default -> 7.9.0 (-> v7.9.0) 
node -v
  v7.9.0

We install the latest version and then set this as default. Finally, to check everything worked we check to see what version is installed.

2 Variables and Scope

In the following examples we will be using a node module to capture user input.

Load up the todo.js script and read through it to understand how it works. There are lots of code comments to assist you.

Before you can run the script you need to install the module dependencies. These are listed in the package.json file. This is formatted as a json document, you will learn more about this in a later topic. Open this up and look for the dependences array, it specifies that we need to install the readline-sync module. To install this you need to use the terminal to navigate to the directory then run npm install. Notice that you now have a new directory called node_modules/.

Use the terminal to navigate to the 01 Introduction to ECMAScript 6 directory and run the script by entering node todo.js. Once running you can use the add command to add new items to the list and the list command to print out the list items. The final command will terminate the application.

node todo.js
add cheese
add bread
add butter
list
exit

2.1 Variables and Constants

There are three ways to declare a variable in the latest version of JavaScript (ECMA6).

  1. Standard variables are function scoped which means they are available anywhere within the function in which they are declared. These are declared with the var keyword.
  2. Immutable variables (constants) are also function scoped. These are declared using the const keyword.
  3. Finally, any variable declared with the let keyword is block scoped. This means the variable is only visible within its block (defined by curly braces).

2.2 Test Your Knowledge

  1. locate the input variable declaration (just inside the do loop)
  • define it as a block-scoped variable by replacing the var with let, what effect does this have?
  • modify the script so that it still works (keep the let variable declaration).
  • substitute a constant by substituting const for let, what effect does this have?
  1. the array at the top of the script is defined using var. What happens if you make this immutable (use const)?
  2. Items are added to the array using its push() method.
  • substute the unshift method. How does this change the script?
  1. modify the code to prevent duplicate items being added. You will need to use the Array.indexOf() method.
  2. create a remove option so an item such as cheese can be removed using the syntax remove cheese. You may need to use the Array.splice() method.
  3. The current version is case sensitive. Modify the code so that items are converted to lowercase before being added or searched for. You will need to use the String.toLowerCase() method.

3 Exception Handling

When JavaScript executes code errors and exceptions may occur. These may be due to incorrect user input or a broken network connection for example. JavaScript includes a rich set of tools for handling these, based on the Error object.

  1. Errors are serious problems that normally mean the application will terminate
  2. Exceptions on the other hand are problems that can be handled by the program logic and thus prevent the application from terminating. In this task we will be focussing on exception handling.

Open the contact.js script and study it carefully, as before, the code includes detailed comments to explain how it works.

  1. All code that could throw an exception must be in a try{} block.
  2. If an exception is thrown the execution moves to the catch{} block.
  • the error object thrown will be passed as the parameter.
  • the error object contains three properties: the name of the error, the message passed and the stack trace.
  • the stack trace is a list of the method calls that the application was in the middle of when an Exception was thrown and can help identify some of the more insidious errors. You should learn to read and understand what information it contains.

3.1 Test Your Knowledge

Implement the validateEmail() function and thoroughly test it, you should avoid using regular expressions at this stage:

  1. Check that the string is at least 5 character long
  2. Check that there is a @ character and that it is not at the start of the string (HINT: use the indexOf String prototype method.
  3. Check that there is a period (.) character after the @ character but before the end of the string.

Extension Activity

By now you have learned:

  1. How to retrieve the lab materials via GitHub
  2. How to install NodeJS on the CodeAnywhere platform
  3. How to edit and run scripts on the CodeAnywhere platform

For you extension activity you are required to set up a development environment on your laptop or desktop computer. If you are running MacOS or Linux you will be able to follow a similar set of steps to those used in CodeAnywhere. If you are running a Windows OS you have three choices:

  1. Work out how to configure your windows machine.
  2. Install Virtualbox and create a virtualised instance of Ubuntu 16 Desktop.
  3. Dual boot with Ubuntu.

You should install and evaluate a range of editors to decide on the one you prefer. There are a wide range of choices but a starting point might be:

  • Visual Studio Code
  • Atom
  • Brackets
  • Sublime

Installing Node

As part of your configuration you will need to install the latest version of NodeJS. On Ubuntu Desktop 16. When you use it you will need to run it as nodejs.

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv 68576280
sudo apt-add-repository "deb https://deb.nodesource.com/node_6.x $(lsb_release -sc) main"
sudo apt-get update
sudo apt-get install nodejs
nodejs -v

If you are running MacOS or Windows you will need to download the NodeJS installer from https://nodejs.org/en/download/current/, make sure you choose the Current version and not the LTS version.

Once you have installed NodeJS you can install the Typescript Definition Language tool and use it to install the node and jasmine definitions. This will add both node and jasmine to the autocomplete data. Most modern editors include support for this.

npm install tsd -g
tsd query -r -o -a install node jasmine