1 - Good practices and debugging

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Intro to ES6

ECMAScript 2015 (or ES6) is a significant update to JavaScript that introduces new syntax for writing complex applications.

const and let

You have already come across the var keyword as a way to create new variables. The let and const keywords are also used for variable creation, but the variables created using these keywords have different scope. Var has "function scope", whereas let and const have "block scope".

This badly designed function will throw the error message is not defined. What is the problem, and how could we fix it?

function compareNumbers(m, n) {
  if (m < n) {
    let message = m + " is smaller than " + n;
  } else {
    let message = m + " is bigger than or equal to " + n;

  return message;

The const keyword is similar to let, the only difference is that a variable declared using const can't be changed after it is assigned.

What advantages might a block scope variable have over a function scope variable? In what situation might you want to use const instead of a variable that can be re-assigned?

How can we update this code to use let and const instead of var?

function getCircleArea(radius) {
  var pi = Math.PI;
  var rSquared = Math.pow(radius, 2);

  return pi * rSquared;

function getCircleAreas(radiusArr) {
  var areasArr = [];

  for (var i = 0; i < radiusArr.length; i++) {
    var circleArea = getCircleArea(radiusArr[i]);

  return areasArr;

Template literals

We do a lot of string concatenation in JavaScript - ES6 introduces a more elegant way of accomplishing the same.

function greeting(name) {
  return "Hello " + name + ", welcome to JS core 3!";

Rewriting this function in ES6, we have

function greeting(name) {
  return `Hello ${name}, welcome to JS core 3!`;

Arrow functions

ES6 also has a new way of declaring functions. Let's see how it works.

// before 
function sum(a, b, c) {
  return a + b + c;

// ES6
const sum = (a, b, c) => {
  return a + b + c;

If the function only contains one expression, the curly braces and the return are optional and we can write the whole function in one line.

const sum = (a, b, c) => a + b + c;

Default parameters

ES6 allows us to declare defaults for function arguments. The default value is used when the argument/parameter is either missing or undefined.

This function returns the sum of three numbers. Let's assume we want to use the same function with only two arguments:

// without default parameter
const sum = (a, b, c) => {
  c = c || 0;
  return a + b + c;

console.log(sum(1, 3, 4)) // 8
console.log(sum(2, 5)) // NaN

// with default parameter 
const sum = (a, b, c = 0) => {
  return a + b + c;
console.log(sum(2, 5)) // 7


In ES6 we can extract data from objects or arrays using destructuring.

// before 
var chicken = {
  name: 'Maggie', 
  age: 2

var name = chicken.name;
var age = chicken.age;

var numbers = [1, 2];

var firstNumber = numbers[0];
var secondNumber = numbers[1];

// in ES6
const chicken = {
  name: 'Maggie', 
  age: 2

const { name, age } = chicken;

const numbers = [1, 2];

const [firstNumber, secondNumber] = numbers;



From exercises-js3-week1 repo in folder InClass/A-ES6 do exercise1.js


Objects in programming languages provide us with an easy way to model data. Let’s say we have an object called polygon. The polygon object has properties: values that contain data about the polygon, and methods: functions that define actions that the polygon can perform. This focus on “objects” and “actions” is the basis of Object Oriented Programming.

Analyze the following code, what is a constructor?

class Polygon {
  constructor(height, width) {
    this.name = "Polygon";
    this.height = height;
    this.width = width;
  sayName() {
    console.log('Hi, I am a ' + this.name);

let p = new Polygon(300, 400);

The extends keyword is used in class declarations or class expressions to create a class as a child of another class. Why do you think are two lines commented?

class Rectangle extends Polygon {
  constructor(height, width) {
    super(height, width);
    this.name = "Rectangle";
    this.sides = 4;
    //this.area = this.height * this.width;
  get area() {return this.width * this.height;}
  sayName() {
    console.log('Hi I am a polygon and my name is ' + this.name + '.');

let r = new Rectangle(50, 60);
//r.area = 3;
console.log('The area of this polygon is ' + r.area);



From exercises-js3-week1 repo in folder InClass/A-ES6 do exercise2.js

Tracing Code

What is Code? Computer code is a set of rules or instructions. It is made up of words and numbers and when you put them in the right order it will tell your computer what you want it to do.

Let's trace these code samples together:


Debugging is the process of finding and resolving defects or problems within a computer program that prevent correct operation of computer software or a system.

Syntax bugs

A syntax bug is an error caused by something the programmer has typed – it could be a spelling mistake or a command that the computer doesn’t understand.

Logical bugs

A logical bug is an error which means that even though the computer is able to carry out its instructions, it doesn’t act as the programmer intended or the user expects.

Debugging with Chrome

Follow this tutorial about Debugging with Chrome And try to solve the problem in the link 'Open Demo'

Good Design

Design is important if we want our code to be understandable (both to other humans, but also to us in the future), to be easy to use and easy to expand.

There are three main principles you need to know now: clarity, reusability and extensibility. There are also others, but they are deeply related to these three.

  • Ease of Maintenance / Clarity

    • Naming

    • Commenting

    • Clear logic

    • Concise

    • Formatting

    • Avoiding Redundancy

  • Reusability

    • DRY

    • Single Reponsibility

      • Avoiding global state (scope)

      • Predictability and Ease of testing

  • Extensibility

    • Avoiding being unnecessarily specific (e.g. magic numbers)

Now let's take a look at a bigger example of a badly written function

function myFunction(salary, taxCode, incomeTax1, incomeTax2, ownsCar) {
  var totalIncomeTax = incomeTax1 + incomeTax2;
  var studentLoan = (salary - 17775) * 0.09;
  var originalSalary = salary;
  var nationalInsurance = null;

  if (taxCode === "1150L") {
    nationalInsurance = salary * 0.1;
  } else if (taxCode === "ST") {
    nationalInsurance = salary * 0.05;
  } else {
    nationalInsurance = salary * 0.08;

  var deductions = [nationalInsurance, totalIncomeTax, studentLoan];

  salary = salary - deductions[0];
  salary = salary - deductions[1];
  salary = salary - deductions[2];

  return (
    "Your gross income is £" +
    originalSalary.toString() +
    " and your net income is £" +
    salary.toString() +

console.log(myFunction(28000, "1150L", 1000, 580, false));

Comment together what is wrong with this function?


From exercises-js3-week1 repo in folder InClass/C-Good Design do exercise1.js


There are times when we want to make our code better without changing any functionality, for example because we just learnt about a better way to solve a certain problem (like, finding needles in haystacks). This is called refactoring.

When previously GREEN code - working code! - suddenly does not work anymore, we call this a regression. Our existing tests can make sure that when we refactor, the functionality of our code actually stays the same, and does not regress.


So far, all our programs have been in their own single files. But Node programs can become really large, and having all our code in only one file will not be maintainable.

We can therefore split our code into so-called modules. A module is basically a JavaScript file that makes its functionality available to other modules and programs.

Creating modules, exporting code

It is really simple to take existing JavaScript code and turn it into a module by exporting its functionality:

function printName(name) {
  console.log("My name is " + name);

module.exports = printName;

The key here is the line containing module.exports. As you see, this is an assignment, and whatever is assigned to module.exports will be made available to other modules and program when this file is imported.

Using modules, importing code

But how do we make use of another module in our program? We need to import it, and this is done using a function called require().

There are different module formats for JavaScript. The one we are using here, which is natively supported by Node, is called CommonJS.

var printName = require("./printName.js");

The string passed to the require() function is a path to the file you are importing. ./ signifies the current directory, so the above command will import a file called "printName.js" that is in the same directory as our program.

Assuming our program is in the same folder as printName.js, we can use the above code to import the functionality provided by that module and store it in the printName variable.

We can then continue to use the printName function as if it we defined it in our own program!

var printName = require('./printName.js');


Modules can not only export functions, but all variable types you already learned about. Most commonly, they export a function or an object containing multiple functions.

Together: Edit the file modules/main.js and follow the instructions.


Tracing code resources

Check out these videos. They contain code that is not JavaScript being traced on paper.

Hand Tracing - Intro to Java Programming

Tracing code by hand

Tracing a flowchart

Java Tracing Arrays Worksheet 1

Nested loops

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