Chapter 2 - Basic Javascript Instructions

Question 1

A statement is…

Answer 1

An individual instruction or step in a script
e.g. var today = new Date();

Question 2

Is Javascript case-sensitive?

Answer 2

Yes

Question 3

Give a detailed description of a statement…

Answer 3

A statement is an individual instruction that the computer should follow.
Each one starts on a new line and ends with a semicolon
The semicolon lets the JS interpreter know when the statement is over, and that it should move on to the next step.

Question 4

What is a code block?

Answer 4

A statement surrounded by a curly brace.
The closing curly brace is not followed by a semicolon
Code blocks will often be used to group together many statements.
This helps programmers organise their code and make it more readable.

Question 5

Why write comments?

Answer 5

Comments help to explain what your code does
They help make your code easier to read and understand.
This can help you and others who ead your code.

Question 6

How would you write a single-line comment?

Answer 6

A single line comment would be anything on a line that follows two forward slash characters

Question 7

How would you write a multi-line comment?

Answer 7

You start with a /*
You end with a */
Anything between these two is not processed by the JS interpreter

Question 8

When might you use a multi-line comment?

Answer 8

To describe how a script works

To stop a section of your script from running when testing it.

Question 9

What is a variable?

Answer 9

A variable is a piece of information that a script needs to store temporarily so that it can do its job.

Question 10

How would variables factor in the calculation of the area of wall?

Answer 10

You would need to multiply the width and the height of a wall to calculate its area
Variables would be used to temporarily store values for the height and width.

Question 11

Which type of human memory might you compare variables to?

Answer 11

Short-term memory

Question 12

If you have a script which can calculate the area of a wall, which part of this task is best represented by variables?

Answer 12

Variables are best used to represent the values in your script that are likely to change
In this case, it would be the specific heights and widths of each wall
Even though these change, the area of a wall will always be height x width.

Question 13

What kind of data is generated by variables, when used together in a script?

Answer 13

Calculated or computed data.

Question 14

How is a variable declared?

Answer 14

var is a keyword that the interpreter recognises and anticipates the creation of a variable
The variable must be given an identifier e.g. quantity in var quantity.

Question 15

Give an example of how you would declare a variable?

Answer 15

var quantity;

Question 16

How would you write a variable name of more than one word?

Answer 16

You would use camelCase

Question 17

What does it mean to assign a value to a variable?

Answer 17

Telling a variable what information you want it to store for you.

Question 18

What are some good practices when naming variables?

Answer 18

A variable name should describe the kind of data that the variable holds.

Question 19

What is the significance of the equals (=) sign?

Answer 19

This is known as the assignment operator

It can assign or update the value given to a variable

Question 20

If a variable is declared, but has now value assigned, it is said to be…

Answer 20

Undefined.

Question 21

What are the three data types?

Answer 21

Numeric - integers, decimals and negatives
String - enclosed within single or double quotes - can contain HTML code
Boolean - True or false

Question 22

What can numbers be used for?

Answer 22

Calculations
Determining screen size
Moving elements on the screen
Timing fading in of elements

Question 23

What are the other data types?

Answer 23

Arrays
Objects
Undefined
Null

Question 24

Do you need to specify a data type when declaring a variable in JavaScript?

Answer 24

No

Question 25

Give and example of using a variable to store a number…

Answer 25

// Create three variables to store the information needed.
var price;
var quantity;
var total;

// Assign values to the price and quantity variables.
price = 5;
quantity = 14;
// Calculate the total by multiplying the price by quantity.
total = price * quantity;

// Get the element with an id of cost.
var el = document.getElementById('cost');
el.textContent = '$' + total;

Question 26

How does variable assignment mirror algebra?

Answer 26

Once a variable has been assigned a value, you can use that variable’s name to represent that value.

Question 27

Write an example of using a variable to store a string…

Answer 27

var username;

username = ‘Molly’;

Question 28

Can you write strings on more than one line?

Answer 28

No.

Question 29

Name two ways in which you can use quotes within a string?

Answer 29

If you want to use a double quote, surround the string in single quotes or vice versa.
Use a single backslash before the relevant quotation character. This is called escaping.

Question 30

Give an example of using a variable to store a Boolean…

Answer 30

var inStock;

inStock = true;

Question 31

What are two popular instances for the use of Booleans?

Answer 31

When the answer can only be true or false.

When your code can take more than one path.

Question 32

Name four ways in which variables can be declared…

Answer 32

Variables declared and values assigned at the same time.
var price = 5;
Variables declared, then values assinged
var price, quantity, total;
price = 5; etc etc.
var price = 5, quantity = 14;
var total = price * quantity;
Variable is used to hold a reference to an to an HTML element
This allows you to work directly with the element stored in that variable.
var el = document.getElementById(‘cost’);
el.textContent = ‘$’ + total;

Question 33

How might you change the value of a variable?

Answer 33

var inStock = true;
inStock = false;

Question 34

What are the six rules for naming variables?

Answer 34

The name must begin with a letter, a dollar sign or an underscore.
Names can contain letters, numbers , dollar sign and underscore.
Names must not use keywords or reserved words
Variables are case-sensitive. Don’t create two with the same name in different cases.
Use a name that describes the type of information the variable stores.
If your variable name is more than one word, use camelCase.

Question 35

What is an array?

Answer 35

A special type of variable used when with a list or a set of values related to eachother.
When using an array you do not need to know or specify how many values it will hold.

Question 36

How would you create an array?

Answer 36

ARRAY LITERAL
var colors;
colors = [‘white, ‘black’, ‘custom’];

ARRAY CONSTRUCTOR
var colors = new Array(‘white’, ‘black’, ‘custom’);

Question 37

Which method might you use to retrieve data from an array?

Answer 37

item() - index number of the item goes in the parenthese.

Question 38

What is the standard first index value of the first item in an array?

Answer 38

zero (0)

Question 39

How would you access an item in an array?

Answer 39

var itemThree;

itemThree = colors[2];

Question 40

How would you find the length of an array?

Answer 40

var numColors;

numColors = colors.length;

Question 41

How would you change a value in an array?

Answer 41

colors[2] = ‘beige’;`

Question 42

What is an expression?

Answer 42

Something that evaluates into a single value

Question 43

What are the two main types of expression?

Answer 43

Expressions that assign a value to a variable.
var color = ‘beige’;

Expressions that use two or more values to return a single value
var area = 3 * 2;

Question 44

What types or operator are commonly used in JavaScript?

Answer 44

Assignment Operators - color = ‘beige’;
Arithmetic Operators - area = 3*2;
String Operators - greeting = ‘Hi’ + ‘Molly’;
Comparison Operators - buy = 3 > 5;
Logical Operators - buy = (5 > 3) && (2

Question 45

What are the types of arithmetic operators?

Answer 45

Addition +
Subtraction -
Division /
Multiplication *
Increment ++
Decrement --
Modulus %

Question 46

In what order are arithmetical operations executed?

Answer 46

Multiplication and Division before Subtraction and Addition

Brackets can be used to overcome this.

Question 47

Give an example of using Arithmetic Operators…

Answer 47

// Create a variable for the subtotal and make a calculation
var subtotal = (13 + 1) * 5; // Subtotal is 70

// Create a variable for the shipping and make a calculation
var shipping = 0.5 * (13 + 1); // Shipping is 7

// Create the total by combining the subtotal and shipping values
var total = subtotal + shipping; // Total is 77

// Write the results to the screen
var elSub = document.getElementById('subtotal');
elSub.textContent = subtotal;

var elShip = document.getElementById('shipping');
elShip.textContent = shipping;

var elTotal = document.getElementById('total');
elTotal.textContent = total;

// Note: textContent does not work in IE8 or earlier - see explanation on website

Question 48

Joining two or more strings together is called…

Answer 48

Concatenation

Question 49

What do you need to remember when mixing numbers and strings?

Answer 49

When you place quotes around a number it becomes a string. It will be concatenated, not added.
When you add numeric data to a string, it becomes part of the string.
If you try to use any arithmetic operators on string apart from +, the result is NaN.

Question 50

Give an example of using String Operators

Answer 50

// Store the greeting in a variable
var greeting = 'Howdy ';

// Store the users name in a variable
var name = 'Molly';

/* Create the welcome message by concatenating the strings in the two variables */
var welcomeMessage = greeting + name + '!';

// Get the element that has an id of greeting
var el = document.getElementById('greeting');

// Replace the content of this element with the personal message
el.textContent = welcomeMessage;

// Note: textContent does not work in IE8 or earlier - see explanation on website

Question 51

Give an example of Basic JavaScript Instructions.

Answer 51

// Create variables for the welcome message
var greeting = 'Howdy ';
var name = 'Molly';
var message = ', please check your order:';
// Concatenate the three variables above to create the welcome message
var welcome = greeting + name + message;

// Create variables to hold details about the sign
var sign = 'Montague House';
var tiles = sign.length;
var subTotal = tiles * 5;
var shipping = 7;
var grandTotal = subTotal + shipping;

// Get the element that has an id of greeting
var el = document.getElementById('greeting');
// Replace the content of that element with the personalized welcome message
el.textContent = welcome;

// Get the element that has an id of userSign then update its contents
var elSign = document.getElementById('userSign');
elSign.textContent = sign;

// Get the element that has an id of tiles then update its contents
var elTiles = document.getElementById('tiles');
elTiles.textContent = tiles;

// Get the element that has an id of subTotal then update its contents
var elSubTotal = document.getElementById('subTotal');
elSubTotal.textContent = '$' + subTotal;

// Get the element that has an id of shipping then update its contents
var elShipping = document.getElementById('shipping');
elShipping.textContent = '$' + shipping;

// Get the element that has an id of grandTotal then update its contents
var elGrandTotal = document.getElementById('grandTotal');
elGrandTotal.textContent = '$' + grandTotal;

// Note: textContent does not work in IE8 or earlier - see explanation on website