JavaScript

Question 1

What is the purpose of variables?

Answer 1

To store data that needs to be referenced later

Question 2

How do you declare a variable?

Answer 2

Using the keyword var (const and let)

Question 3

How do you initialize (assign a value to) a variable?

Answer 3

var variableName = value;

Question 4

What characters are allowed in variable names?

Answer 4

Letters, numbers, $, _ (numbers cannot be first)

Question 5

What does it mean to say that variable names are “case sensitive”?

Answer 5

They are sensitive to uppercase and lowercase letters

Question 6

What is the purpose of a string?

Answer 6

Store a series of character, text content

Question 7

What is the purpose of a number?

Answer 7

To store a numeric value, counting, measurement, math

Question 8

What is the purpose of a boolean?

Answer 8

A data type that only has two possible values, to make decisions

Question 9

What does the = operator mean in JavaScript?

Answer 9

Assigning a value to something

Question 10

How do you update the value of a variable?

Answer 10

variableName = newValue

Question 11

What is the difference between null and undefined?

Answer 11

Null: empty value, must be assigned by you, it represents something that does not have a value assigned to it yet,
Undefined: empty value, assigned by JavaScript

Question 12

Where is var scoped

Answer 12

Var is scoped to the function that it is inside of. If you leave out the var keyword when using a variable in a function, JavaScript will look for the next variable that is scoped closest

Question 13

Why is it a good habit to include “labels” when you log values to the browser console?

Answer 13

So you know what is being logged to the console

Question 14

Give five examples of JavaScript primitives.

Answer 14

String, number, boolean, undefined, null

Question 15

What data type is returned by an arithmetic operation?

Answer 15

Number

Question 16

What is string concatenation?

Answer 16

Combining a string with another value and makes the result of that expression into a new string

Question 17

What purpose(s) does the + plus operator serve in JavaScript?

Answer 17

Adding numbers, concatenating strings

Question 18

What data type is returned by comparing two values (, ===, etc)?

Answer 18

boolean

Question 19

What does the += “plus-equals” operator do?

Answer 19

Adds the value of the variable on the right to the variable on left, then assigns the result of that expression to the variable on the left

Question 20

What are objects used for?

Answer 20

Storing data in a group or collection that makes sense

Question 21

What are object properties?

Answer 21

Pieces of data stored within an object

Question 22

Describe object literal notation.

Answer 22

Var variableName = {};

Question 23

How do you remove a property from an object?

Answer 23

The delete operator: delete object.property

Question 24

What are the two ways to get or update the value of a property?

Answer 24

Dot notation: object.propertyName

Bracket notation: object[‘propertyName’]

Question 25

What are arrays used for?

Answer 25

To list data in a specific order

Question 26

Describe array literal notation.

Answer 26

Var variableName = [];

Question 27

How are arrays different from "plain" objects?

Answer 27

They are numerically indexed

Question 28

What number represents the first index of an array?

Answer 28

0

Question 29

What is the length property of an array?

Answer 29

How many elements are in the array

Question 30

How do you calculate the last index of an array?

Answer 30

Array.length - 1

Question 31

What is a function in JavaScript?

Answer 31

A special type of object that can be called, a set of reusable code

Question 32

Describe the parts of a function definition.

Answer 32

``` Function functionName(parameter) {code block; return statement} Name and return statement are optional ```

Question 33

Describe the parts of a function call.

Answer 33

functionName(argument) argument is optional

Question 34

When comparing them side-by-side, what are the differences between a function call and a function definition?

Answer 34

A call will have arguments, a definition includes a code block

Question 35

What is the difference between a parameter and an argument?

Answer 35

A parameter is a variable related to a function definition, an argument is a value related to a function call Parameter is a placeholder for a variable that will eventually be passed into the function. It is included in the function definition Argument is the actual variable that is passed into the function when it is called

Question 36

Why are function parameters useful?

Answer 36

To pass additional information into a function for the function to use. It allows us to create more generalized, reusable functions

Question 37

What two effects does a return statement have on the behavior of a function?

Answer 37

Causes the function to produce a value, ends the function

Question 38

Why do we log things to the console?

Answer 38

To test and inspect your code and make sure your functions or methods are returning what you expect, or see what the values of variables are.

Question 39

What is a method?

Answer 39

A property of an object that is a function

Question 40

How is a method different from any other function?

Answer 40

A method is a function that is a property of an object

Question 41

How do you remove the last element from an array?

Answer 41

array.pop()

Question 42

How do you round a number down to the nearest integer?

Answer 42

Math.floor()

Question 43

How do you generate a random number?

Answer 43

Math.random() (generates a number from 0 to 1 not including 1)

Question 44

How do you delete an element from an array?

Answer 44

Array.splice(start, deleteCount)

Question 45

How do you append an element to an array?

Answer 45

Array.push()

Question 46

How do you prepend an element to an array?

Answer 46

Array.unshift()

Question 47

How do you break a string up into an array?

Answer 47

String.split()

Question 48

Do string methods change the original string? How would you check if you weren't sure?

Answer 48

No, strings are immutable. Use console.log() to check

Question 49

Is the return value of a function or method useful in every situation?

Answer 49

no

Question 50

What three-letter acronym should you always include in your Google search about a JavaScript method or CSS property?

Answer 50

MDN, also include the name of the language

Question 51

Give 6 examples of comparison operators.

Answer 51

>, =, <=, !=, !==

Question 52

What data type do comparison expressions evaluate to?

Answer 52

Boolean

Question 53

What is the purpose of an if statement?

Answer 53

To execute a block of code if a certain condition is truthy

Question 54

Is else required in order to use an if statement?

Answer 54

no

Question 55

Describe the syntax (structure) of an if statement.

Answer 55

If (condition) {code block}

Question 56

What are the three logical operators?

Answer 56

&&, || , !

Question 57

How do you compare two different expressions in the same condition?

Answer 57

Put each expression in parentheses

Question 58

What is the purpose of a loop?

Answer 58

To allow us to repeat a certain block of code

Question 59

What is the purpose of a condition expression in a loop?

Answer 59

To tell the loop whether or not to run

Question 60

What does "iteration" mean in the context of loops?

Answer 60

It represents each time the code within the code block runs

Question 61

When does the condition expression of a while loop get evaluated?

Answer 61

Before each iteration

Question 62

When does the initialization expression of a for loop get evaluated?

Answer 62

Only once, before the loop starts | Initialization: before anything

Question 63

When does the condition expression of a for loop get evaluated?

Answer 63

After initialization, before each iteration | Condition: before each

Question 64

When does the final expression of a for loop get evaluated?

Answer 64

After each iteration | Final: after each

Question 65

Besides a return statement, which exits its entire function block, which keyword exits a loop before its condition expression evaluates to false?

Answer 65

break

Question 66

What does the ++ increment operator do?

Answer 66

Increments a variable by 1

Question 67

How do you iterate through the keys of an object?

Answer 67

a for...in loop

Question 68

Which "document" is being referred to in the phrase Document Object Model?

Answer 68

The HTML document

Question 69

What is the word "object" referring to in the phrase Document Object Model?

Answer 69

JavaScript object

Question 70

What is a DOM Tree?

Answer 70

A tree representing each element within the DOM, and its child elements

Question 71

Give two examples of document methods that retrieve a single element from the DOM.

Answer 71

document.getElementById(), document.querySelector()

Question 72

Give one example of a document method that retrieves multiple elements from the DOM at once.

Answer 72

document.querySelectorAll()

Question 73

Why might you want to assign the return value of a DOM query to a variable?

Answer 73

So you can access it again more easily

Question 74

What console method allows you to inspect the properties of a DOM element object?

Answer 74

console.dir()

Question 75

Why would a tag need to be placed at the bottom of the HTML content instead of at the top?

Answer 75

So the browser can parse through the document before JavaScript has to access it

Question 76

What does document.querySelector() take as its argument and what does it return?

Answer 76

A string of a CSS selector, returns the first element that matches the selector

Question 77

What does document.querySelectorAll() take as its argument and what does it return?

Answer 77

A string of a CSS selector, returns a NodeList of all elements that match

Question 78

What is the purpose of events and event handling?

Answer 78

It allows us to interact with the webpage, to make our apps dynamic

Question 79

Are all possible parameters required to use a JavaScript method or function?

Answer 79

no

Question 80

What method of element objects lets you set up a function to be called when a specific type of event occurs?

Answer 80

.addEventListener()

Question 81

What is a callback function?

Answer 81

A function that is passed into another function

Question 82

What object is passed into an event listener callback when the event fires?

Answer 82

The event object, which is a report on the event

Question 83

What is the event.target? If you weren't sure, how would you check? Where could you get more information about it?

Answer 83

The element where the event we are dealing with originated from. console.log() it. MDN

Question 84

What is the difference between these two snippets of code? element. addEventListener('click', handleClick) element. addEventListener('click', handleClick())

Answer 84

We are actually calling the function in the second one, which means that the function will run at that line of code, not when the event actually fires

Question 85

What is the className property of element objects?

Answer 85

Used to access or update the value of the class attribute on an element

Question 86

How do you update the CSS class attribute of an element using JavaScript?

Answer 86

Assign a new value to the className property of that element

Question 87

What is the textContent property of element objects?

Answer 87

Used to access or update text content of any DOM element

Question 88

How do you update the text within an element using JavaScript?

Answer 88

Assign a new value to the textContent property of the element

Question 89

Is the event parameter of an event listener callback always useful?

Answer 89

no

Question 90

Why is storing information about a program in variables better than only storing it in the DOM?

Answer 90

We don’t want to check outside of your JavaScript every time you need to access data

Question 91

What event is fired when a user places their cursor in a form control?

Answer 91

focus

Question 92

What event is fired when a user's cursor leaves a form control?

Answer 92

blur

Question 93

What event is fired as a user changes the value of a form control?

Answer 93

input

Question 94

What event is fired when a user clicks the "submit" button within a "form" element?

Answer 94

submit

Question 95

What does the event.preventDefault() method do?

Answer 95

Prevents the default behavior of an event from happening

Question 96

What does submitting a form without event.preventDefault() do?

Answer 96

Refresh the page

Question 97

What property of a form element object contains all of the form's controls.

Answer 97

.elements

Question 98

What property of form a control object gets and sets its value?

Answer 98

.value