JavaScript

Question 1

JavaScript Primitives and Variables:
What is the purpose of variables?

Answer 1

to store data for the future action/reference

Question 2

JavaScript Primitives and Variables:
How do you declare a variable?

Answer 2

variable keyword
variable name ;

Question 3

JavaScript Primitives and Variables:
How do you initialize (assign a value to )a variable?

Answer 3

(variable keyword
variable name)
assignment operator (=)
variable value ;

Question 4

JavaScript Primitives and Variables:
What characters are allowed in variable names?

Answer 4

letters, dollar sign, underscore (only first three can start the variable name) and numbers (no dashes or periods)

Question 5

JavaScript Primitives and Variables:
What does it mean to say that variable names are “case sensitive”?

Answer 5

it means that there’s a difference between a word starting with lowercase and that same word starting with a capital letter.
(bad practice to have same name using difference cases)

Question 6

JavaScript Primitives and Variables:
What is the purpose of a string?

Answer 6

Strings are used when working with text
(typically used to add new content to a page and can contain HTML markup)

Question 7

JavaScript Primitives and Variables:
What is the purpose of a number?

Answer 7

Numbers are used in tasks that involve counting or calculating sums

Question 8

JavaScript Primitives and Variables:
What is the purpose of a boolean?

Answer 8

Booleans are used to determine which parts of a script should run.
Used to make decisions

Question 9

JavaScript Primitives and Variables:
What does the = operator mean in JavaScript?

Answer 9

The equal sign means that you are going to assign a value to a variable

Question 10

JavaScript Primitives and Variables:
How do you update the value of a variable?

Answer 10

variable name
assignment operator
and then the new value ;

Question 11

JavaScript Primitives and Variables:
What is the difference between null and undefined?

Answer 11

null intentional absence of a value
undefined creates a variable but assigns it no value

Question 12

JavaScript Primitives and Variables:
Why is it a good habit to include “labels” when you log values to the browser console?

Answer 12

If you do not include “labels” it can be confusing

Question 13

JavaScript Primitives and Variables:
Give five examples of JavaScript primitives

Answer 13

number, string, boolean, null, undefined

Question 14

JavaScript Operators and Expressions:
What data type is returned by an arithmetic operation?

Answer 14

numbers

Question 15

JavaScript Operators and Expressions:
What is string concatenation?

Answer 15

the process of joining together two or more strings to create one new string

Question 16

JavaScript Operators and Expressions:
What purpose(s) does the + plus operator serve in JavaScript?

Answer 16

the plus operator adds numbers together or concatenates

Question 17

JavaScript Operators and Expressions:
what data type is returned by comparing two values (<, >, ===, etc)?

Answer 17

boolean

Question 18

JavaScript Operators and Expressions:
What does the += “plus-equals” operator do?

Answer 18

it adds the value to the right of the operand to a variable and assigns the result to the variable

Question 19

JavaScript Objects:
What are objects used for?

Answer 19

group together a set of variables and functions to create a model of something you recognize in the real world

Question 20

JavaScript Objects:
what are object properties?

Answer 20

variables that give information about an object

Question 21

JavaScript Objects:
Describe object literal notation.

Answer 21

{
key: value ,
}

Question 22

JavaScript Objects:
How do you remove a property from an object?

Answer 22

delete operator followed by objectName.propertyName
(period is member operator)

Question 23

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

Answer 23

dot notation or square bracket notation

Question 24

JavaScript Arrays:
What are arrays used for?

Answer 24

for lists of data where either the order of the list is extremely important or unimportant

Question 25

JavaScript Arrays:
Describe array literal notation

Answer 25

[item, item, etc]

Question 26

JavaScript Arrays:
How are arrays different from ‘plain’ objects?

Answer 26

all arrays have length property
objects do not have an order
(automatically updates as you add things. Arrays, you call a method to add items. Objects, you assign values directly to property)

Question 27

JavaScript Arrays:
What number represents the first index of an array?

Answer 27

[0]

Question 28

JavaScript Arrays:
What is the length property of an array?

Answer 28

property.length
get the number of items in the array

Question 29

JavaScript Arrays:
How do you calculate the last index of an array?

Answer 29

subtract 1 from property.length

Question 30

JavaScript Functions:
What is a function in JavaScript?

Answer 30

chunk of code that performs a task that returns something (most the time)

Question 31

JavaScript Functions:
Describe the parts of a function definition

Answer 31

functionKeyword optionalFunctionName (parameters) {function code block optionalReturnStatement} end of function code block

Question 32

JavaScript Functions:
Describe the parts of a function call

Answer 32

functionName(arg1, arg2, etc);

Question 33

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

Answer 33

a function definition has the functionKeyword and a code block, function call does not

Question 34

JavaScript Functions:
What is the difference between a parameter and an argument?

Answer 34

parameters are part of a function declaration and act as placeholders
arguments are part of function call and take the place of parameters

Question 35

JavaScript Functions:
Why are function parameters useful?

Answer 35

they act as placeholders for arguments

Question 36

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

Answer 36

1. causes the function to produce a value we can use in our program
2. prevents any more code in the function’s code block from being run

Question 37

JavaScript If:
Give 6 examples of comparison operators

Answer 37

strictly equal to (===), strictly not equal to (!==), greater than, less than, greater than or equal to, less than or equal to, equal to (==)

Question 38

JavaScript If
What data type do comparison expressions evaluated to?

Answer 38

boolean

Question 39

JavaScript If
What is the purpose of an if statement?

Answer 39

to make decisions

Question 40

JavaScript If:
Is else required in order to use an if statement?

Answer 40

no

Question 41

JavaScript If:
Describe the syntax(structure) of an if statement?

Answer 41

if keyword, condition,

Question 42

JavaScript If:
What are the three logical operators?

Answer 42

logical and (&&), logical or (||), logical not (!)

Question 43

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

Answer 43

logical and
logical or

Question 44

JavaScript Methods:
Why do we log things to the console?

Answer 44

logging things into the console helps with debugging, so we can see where the browser prints errors and warnings in our code

Question 45

JavaScript Methods:
What is a method?

Answer 45

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

Question 46

JavaScript Methods:
How is a method different from any other function?

Answer 46

methods are associated with objects

Question 47

JavaScript Methods:
How do you remove the last element from an array?

Answer 47

pop()

Question 48

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

Answer 48

Math.floor()

Question 49

JavaScript Methods:
How do you generate a random number?

Answer 49

Math.random()

Question 50

JavaScript Methods:
How do you delete an element from an array?

Answer 50

splice()

Question 51

JavaScript Methods:
How do you append an element to an array?

Answer 51

push()

Question 52

JavaScript Methods:
How do you break a string up into an array?

Answer 52

str.split()

Question 53

JavaScript Methods:
Do string methods change the original string? How would you check if you weren’t sure?

Answer 53

doesn’t change the string
console.log to check

Question 54

JavaScript Loops:
What is the purpose of a loop?

Answer 54

Way to repeat a code block until the condition is no longer true.

Question 55

JavaScript Loops:
What is the purpose of a condition expression in a loop?

Answer 55

it creates a stopping point

Question 56

JavaScript Loops:
What does ‘iteration’ mean in the context of loops?

Answer 56

One completion of the loop code block

Question 57

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

Answer 57

beginning of the loop

Question 58

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

Answer 58

once before the loop

Question 59

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

Answer 59

after the initialization and before beginning each loop

Question 60

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

Answer 60

at the end of each the loop

Question 61

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

Answer 61

break;

Question 62

JavaScript Loops:
What does the ++ increment operator do?

Answer 62

it increases the value of the variable by one

Question 63

JavaScript Loops:
How do you iterate through the keys of an object;

Answer 63

for..in loop

Question 64

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

Answer 64

focus

Question 65

JavaScript Forms:
What event is fired when a user’s cursor leaves a form control?

Answer 65

blur

Question 66

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

Answer 66

input

Question 67

JavaScript Forms:
What event is fired when a user clicks the “submit” button within a <form>?

Answer 67

submit
don’t have submit handler on submit button, needs to be on the form, don’t have click on submit button either

Question 68

JavaScript Forms:
What does the event.preventDefault() method do?

Answer 68

it prevents default behavior
always use preventDefault() on forms

Question 69

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

Answer 69

resets the page

Question 70

JavaScript Forms:
What property of a form element object contains all of the form’s controls.

Answer 70

form.elements

Question 71

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

Answer 71

value property

Question 72

JavaScript Forms:
What is one risk of writing a lot of code without checking to see if it works so far?

Answer 72

you can’t catch errors when they show up, so you have to go back and look for them

Question 73

JavaScript View Swapping:
What is the event.target?

Answer 73

it is the element the event interacted with

Question 74

JavaScript View Swapping:
What is the affect of setting an element to display: none?

Answer 74

it isn’t visible and doesn’t take up space (no longer part of the flow)

Question 75

JavaScript View Swapping:
What does the element.matches() method take as an argument and what does it return?

Answer 75

css selectors
they return a boolean
true if the element matches the element

Question 76

JavaScript View Swapping:
How can you retrieve the value of an element’s attribute?

Answer 76

element.getAttribute( attributeName )

Question 77

JavaScript View Swapping:
At what steps of the solution would it be helpful to log things to the console?

Answer 77

each step, every line

Question 78

JavaScript View Swapping:
If you were to add another tab and view to your HTML, but you didn’t use event delegation, how would your JavaScript code be written instead?

Answer 78

you would have to write a new conditional for each tabs (and for future tabs)

Question 79

JavaScript View Swapping:
If you didn’t use a loop to conditionally show or hide the views in the page, how would your JavaScript code be written instead?

Answer 79

you would need to have individual conditional blocks

Question 80

JavaScript and JSON:
what is JSON?

Answer 80

JSON (JavaScript Object Notation) is a (text-based) data interchange format and it sends and stores information in computer systems.

Question 81

JavaScript and JSON:
What are serialization and deserialization?

Answer 81

serialization is when you turn an object in memory into a stream of bytes so you can do things like store it or transport it over a network (stringify)

deserialization is the reverse: turns a stream of bytes into an object in memory
- fetching a stream of bytes from network or storage and converting it back into the object (parse)

Question 82

JavaScript and JSON:
Why are serialization and deserialization useful?

Answer 82

they are easier to work with

Question 83

JavaScript and JSON:
How do you serialize a data structure into a JSON string using JavaScript?

Answer 83

JSON.stringify()

Question 84

JavaScript and JSON:
How do you deserialize a JSON string into a data structure using JavaScript?

Answer 84

JSON.parse()

Question 85

JavaScript Local Storage:
how do you store data in localStorage?

Answer 85

localStorage.setItem(‘keyName’, ‘keyValue’)

Question 86

JavaScript Local Storage:
How do you retrieve data from localStorage?

Answer 86

localStorage.getItem(‘keyName’)

Question 87

JavaScript Local Storage:
What data type can localStorage save in the browser?

Answer 87

string

Question 88

JavaScript Local Storage:
When does the ‘beforeunload’ event fire on the window object?

Answer 88

when the window, document, and its resources are about to be unloaded

Question 89

JavaScript Custom Methods:
What is a method?

Answer 89

function stored in a property, stored in an object

Question 90

JavaScript Custom Methods:
how can you tell the difference between a method definition and a method call?

Answer 90

it has a code block with a return

Question 91

JavaScript Custom Methods:
Describe method definition syntax (structure)

Answer 91

var nameOfObj = {
nameOfMethod: function () {
code block with return;
}
}

Question 92

JavaScript Custom Methods:
Describe method call syntax (structure)

Answer 92

nameOfObj.nameOfMethod()

Question 93

JavaScript Custom Methods:
How is a method different from any other function?

Answer 93

anObj.method()
tied to an obj

Question 94

JavaScript Custom Methods:
What is the defining characteristic of Object-Oriented-Programming?

Answer 94

objects can contain both data (as properties) and behavior (as methods)

Question 95

JavaScript Custom Methods:
What are the four ‘principles’ of Object Oriented Programming?

Answer 95

Abstraction
Encapsulation
Inheritance
Polymorphism

Question 96

JavaScript Custom Methods:
What is ‘abstraction’?

Answer 96

when you remove (generalize) physical, spatial, and temporal details or attributes in the study of objects to focus on details that are more important
[being able to work with (possibly) complex things in simple ways]

Question 97

JavaScript Custom Methods:
What does API stand for?

Answer 97

application programming interface

Question 98

JavaScript Custom Methods:
What is the purpose of an API?

Answer 98

to give programmers a way to interact with a system in a simplified, consistent way

Question 99

JavaScript This:
What is ‘this’ in JavaScript?

Answer 99

keyword that is an implicit parameter, the object that a method is called on

Question 100

JavaScript This:
What does it mean to say that ‘this’ is an “implicit parameter”?

Answer 100

it means that ‘this’ is available in the function’s code block even though it wasn’t included in the function’s parameter list or declared with var/let/const

Question 101

JavaScript This:
When is the value of ‘this’ determined in a function; call time or definition time?

Answer 101

when the function is called

Question 102

JavaScript This:
What does ‘this’ refer to in the following code snippet?

var character = {
firstName: ‘Mario’,
greet: function () {
var message = ‘It's-a-me, ‘ + this.firstName + ‘!’;
console.log(message);
}
};

Answer 102

nothing because it’s not being called yet

Question 103

JavaScript This:
Given the above character object, what is the result of the following code snippet? Why?

character.greet();

Answer 103

“It’s-a-me, Mario!” because we’re calling greet off of character and character obj has a firstName property with a value

Question 104

JavaScript This:
Given the above character object, what is the result of the following code snippet? Why?

var hello = character.greet;
hello();

Answer 104

“It’s-a-me, undefined!”
it says undefined because an object is not given so it default to windows and windows doesn’t have a firstName property

Question 105

JavaScript This:
How can you tell what the value of ‘this’ will be for a particular function or method definition?

Answer 105

you can’t know the value yet because this can only have a value when the function is being called, not in a function definition

Question 106

JavaScript This:
How can you tell what the value of ‘this’ is for a particular function or method call?

Answer 106

it would be the object to the left of the dot where the function is being called

Question 107

JavaScript Prototypes:
What kind of inheritance does the JavaScript programming language use?

Answer 107

prototype-based (prototypal)

Question 108

JavaScript Prototypes:
What is a prototype in JavaScript?

Answer 108

an object that contains properties and methods that can be reused by other objects

Question 109

JavaScript Prototypes:
How is it possible to call methods on strings, arrays, and numbers even though those methods don’t actually exist on strings, arrays, and numbers?

Answer 109

they borrow the methods
defined on a prototype object

Question 110

JavaScript Prototypes:
If an object does not have it’s own property or method by a given key, where does JavaScript look for it?

Answer 110

a prototype object

Question 111

JavaScript Constructors:
What does the ‘new’ operator do?

Answer 111

1. create a blank, plain JavaScript object from scratch
2. points the blank JS obj’s prototype to the constructor function’s prototype property (if the prototype is an obj) Otherwise it stays as a plain obj with Object.prototype as its prototype
3. Executes the constructor function with the given arguments, binding the blank JS obj as the ‘this’ context
4. if the constructor function returns a non-primitive, the return value becomes the result of the whole new expression. Otherwise, it it doesn’t return anything or returns a primitive, the blank JS obj is returned instead

Question 112

JavaScript Constructors:
What property of JavaScript functions can store shared behavior for instances created with ‘new’?

Answer 112

.prototype

Question 113

JavaScript Constructors:
What does the instanceof operator do?

Answer 113

It tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object

Question 114

JavaScript Timers:
What is a “callback” function?

Answer 114

a function that is passed into another function as an argument, which is then invoked inside the outer function to complete a routine or action

Question 115

JavaScript Timers:
Besides adding an event listener callback function to an element or the ‘document’, what is one way to delay the execution of a JavaScript function until some point in the future?

Answer 115

setTimeout()

Question 116

JavaScript Timers:
How can you set up a function to be called repeatedly without using a loop?

Answer 116

setInterval()

Question 117

JavaScript Timers:
What is the default time delay if you omit the ‘delay’ parameter from setTimeout() or setInterval()?

Answer 117

immediate

Question 118

JavaScript Timers:
What do setTimeout() and setInterval() return?

Answer 118

an interval id

Question 119

JavaScript Ajax:
What is AJAX?

Answer 119

it is a technique for loading data into part of a page without having to refresh the entire page

Question 120

JavaScript Ajax:
What does the AJAX acronym stand for?

Answer 120

Asynchronous JavaScript and XML

Question 121

JavaScript Ajax:
which object is built into the browser for making HTTP requests in JavaScript?

Answer 121

XMLHttpRequest

Question 122

JavaScript Ajax:
What event is fired by XMLHttpRequest objects when they are finished loading the data from the server?

Answer 122

‘load’

Question 123

JavaScript Ajax:
Bonus Question, An XMLHttpRequest object has an addEventListener() method just like DOM elements. How is it possible that they both share this functionality?

Answer 123

they’re both objects so they can use the prototype methods (addEventListener)