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JavaScript Core I - 1

Mentor Notes

What we will learn today?

Please make sure you've forked the js-exercises repo at the start of the class. This is the repo we will use during the class, and for homework.

Install Node

Windows and Mac users

Download the installer (.msi or .pkg) from https://nodejs.org/en/download

Linux (Ubuntu) users

Run the following commands in your terminal:

  curl -sL https://deb.nodesource.com/setup_8.x | sudo -E bash -
  sudo apt-get install -y nodejs

Source: https://nodejs.org/en/download/package-manager/#debian-and-ubuntu-based-linux-distributions

A note on security: This will require you to enter a password. Don't execute a script on your terminal if you don't trust its source, especially if they use the sudo command to get admin access to your machine.

2. Test installation

Go to your terminal and run the command: node -v.

You should get the node version printed on your terminal. For example, v8.9.3.

Setup IDE

There are some tools that will help you to write code. One of these, Prettier, formats your code, making it easier for you and others to read.

1. Install prettier

2. Enable formatting on save

Hello World

It is programming tradition that the first thing you do in any language is make it output 'Hello world!'.

We'll do this in JavaScript, using a command called console.log().

Inside of exercise.js there's a line of code that will print "Hello world!".

1. Run the program

  • Open a terminal window
  • Change directory to this folder (cd week-1/C-hello-world)
  • Run the program using node (node exercise.js)

2. Experiment

  • Try to console.log() something different. For example, 'Hello World. I just started learning JavaScript!'.
  • Try to console.log() several things at once.
  • What happens when you get rid of the quote marks?
  • What happens when you console.log() just a number without quotes?

Variables

When you write code, you'll want to create shortcuts to data values so you can don't have to write out the same value every time.

We can use variable to create a reference to a value.

var greeting = "Hello world";

console.log(greeting);

The program above will print "Hello world" to the console. Notice how it uses the value assigned to the variable greeting.

Exercise

  • Add a variable greeting to exercise.js (make sure it comes before the console.log)
  • Print your greeting to the console 3 times

Remember: to run this exercise you must change directory to the D-variables. If you already have a terminal window open for the previous exercise you can do this by running the command cd ../D-variables.

Expected result

Hello world
Hello world
Hello world

Strings

In programming there are different types of data. You've used one data type already: string.

Computers recognise strings as a sequence of characters but to humans, strings are simply lines of text.

var message = "This is a string";

Notice that strings are always wrapped inside of quote marks. We do this so that the computer knows when the string starts and ends.

You can check that the data is a string by using the typeof operator:

var message = "This is a string";
var messageType = typeof message;

console.log(messageType); // logs 'string'

Exercise

  • Write a program that logs a message and its type

Expected result

This is a string
string

String concatenation

You can add two strings together using the plus operator (+):

var greetingStart = "Hello, my name is ";
var name = "Daniel";

var greeting = greetingStart + name;

console.log(greeting); // Logs "Hello, my name is Daniel"

Exercise

  • Write a program that logs a message with a greeting and your name

Expected result

Hello, my name is Daniel

Numbers

The next data type we will learn is number.

Unlike strings, numbers do not need to be wrapped in quotes.

var age = 30;

You can use mathematical operators to caclulate numbers:

var sum = 10 + 2; // 12
var product = 10 * 2; // 20
var quotient = 10 / 2; // 5
var difference = 10 - 2; // 8

Exercise

  • Create two variables numberOfStudents and numberOfMentors
  • Log a message that displays the total number of students and mentors

Expected result

Number of students: 15
Number of mentors: 8
Total numnber of students and mentors: 23

Floats

Numbers can be integers (whole numbers) or floats (numbers with a decimal).

var preciseAge = 30.612437;

Floats can be rounded to the nearest whole number using the Math.round function:

var preciseAge = 30.612437;
var roughAge = Math.round(preciseAge); // 30

Exercise

  • Using the variables provided in the exercise calculate the percentage of mentors and students in the group

Expected result

Percentage students: 65%
Percentage mentors: 35%

Functions

Functions are blocks of code that can do a task as many times as you ask it to. They take an input and return an output.

Here's a function that doubles a number:

function double(number) {
  return number * 2;
}

To use the function we need to: a) call it with an input and b) assign the returned value to a variable

var result = double(2);

console.log(result); // 4

Exercise 1

  • Complete the function in exercise.js so that it halves the input
  • Try calling the function more than once with some different numbers

Remember to use the return keyword to get a value out of the function

Expected result

6

Exercise 2

  • Complete the function in exercise2.js so that it triples the input

Expected result

24

Function Parameters

The input given to a function is called a parameter.

A function can take more than one parameter:

function add(a, b) {
  return a + b;
}

When you write a function (sometimes called declaring a function) you assign names to the parameters inside of the parentheses (()). Parameters can be called anything.

This function is exactly the same as the on above:

function add(num1, num2) {
  return num1 + num2;
}

Exercise 1

  • Write a function that multiplies two numbers together

Expected result

12

Exercise 2

  • From scratch, write a function that divides two numbers

Expected result

0.75

Exercise 3

  • Write a function that takes a name (a string) and returns a greeting

Expected result

Hello, my name is Daniel

Exercise 4

  • Write a function that adds two numbers together
  • Call the function, passing 13 and 124 as parameters, and assigning the returned value to a variable sum

Expected result

137

Exercise 5

  • Write a function that takes a name (a string) and an age (a number) and returns a greeting (a string)

Expected result

Hello, my name is Daniel and I'm 30 years old

Nested Functions

Functions are very powerful.

  • You can write more than one line of code inside of functions.
  • You can use variables inside of functions.
  • You can call other functions inside of functions!
function getAgeInDays(age) {
  return age * 365;
}

function createCreeting(name, age) {
  var ageInDays = getAgeInDays(age);
  var message =
    "My Name is " + name + " and I was born over " + ageInDays + " days ago!";
  return message;
}

Exercise 1

  • In exercise.js write a program that displays the percentage of students and mentors in the group
  • The percentage should be rounded to the nearest whole number (use a search engine to find out how to this with JavaScript)
  • You should have one function that calculates the percentage, and one function that creates a message

Consider: should your percentage function do the rounding, or should it be done when the greeting is created?

Expected result

Percentage students: 65%
Percentage mentors: 35%

Exercise 2

  • In exercise2.js you have been provided with the names of some mentors. Write a program that logs a shouty greeting to each one.
  • Your program should include a function that spells their name in uppercase, and a function that creates a shouty greeting.
  • Log each greeting to the console.

Expected result

HELLO DANIEL
HELLO IRINA
HELLO MIMI
HELLO ROB
HELLO YOHANNES

Homework

  1. The repo that you have forked during this class contains few challenges - solve all the pending exercises in week-1 and week-1-practise in JavaScript!

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