JavaScript

Question 1

What is the purpose of variables?

Answer 1

To store data

Question 2

How do you declare a variable?

Answer 2

Var variablename= variable value

Question 3

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

Answer 3

=

Question 4

What characters are allowed in variable names?

Answer 4

Letter, $, or _, numbers after first character

Question 5

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

Answer 5

Variable name can be different for capital or lowercase same name

Question 6

What is the purpose of a string?

Answer 6

Store text a sequence of characters

Question 7

What is the purpose of a number?

Answer 7

Store a number, calculations

Question 8

What is the purpose of a boolean?

Answer 8

Store true or false values, render a sate of something is or isn’t

Question 9

What does the = operator mean in JavaScript?

Answer 9

Assignment operator, assigning a value to the variable.

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: value can only be assigned, not generated, developer set variable = null purposely. placeholder empty for now
Undefined: empty, nothing here

Question 12

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

Answer 12

To understand which console log is being output

Question 13

Give five examples of JavaScript primitives.

Answer 13

Number, String, Boolean, Undefined, Null and Symbol

Question 14

What data type is returned by an arithmetic operation?

Answer 14

number

Question 15

What is string concatenation?

Answer 15

Joining together 2 or more strings, result will be a string

Question 16

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

Answer 16

cocatenate or add

Question 17

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

Answer 17

boolean

Question 18

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

Answer 18

The addition assignment (+=) operator adds the value of the right operand to a variable and assigns the result to the variable

Question 19

What are objects used for?

Answer 19

Grouping together related variables

Question 20

What are object properties?

Answer 20

Piece of data stored in the object, collection of variables
The key and value in the objects

Question 21

Describe object literal notation.

Answer 21

Var object = {
Key: value,
Key: value
};

Question 22

How do you remove a property from an object?

Answer 22

Delete object.property

Question 23

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

Answer 23

Dot notation object.property
Bracket notation object[property]

Question 24

What are arrays used for?

Answer 24

Lists of data where either order is important or not important

Question 25

Describe array literal notation.

Answer 25

Var arrayname;
Arrayname = [‘value1’, ‘value2’];
bracket separated by commas

Question 26

How are arrays different from “plain” objects?

Answer 26

has a length property and keeps updating
call a method to add data to the array

Question 27

What number represents the first index of an array?

Answer 27

0

Question 28

What is the length property of an array?

Answer 28

Number of items in the array

Question 29

How do you calculate the last index of an array?

Answer 29

Array.length -1

Question 30

What is a function in JavaScript?

Answer 30

Performs a task and returns the output

Question 31

Describe the parts of a function definition.

Answer 31

Function(called keyword) functname (parameters) { Code block;
Return ;
}

Question 32

Describe the parts of a function call.

Answer 32

functionname(argument1, argument2, etc);

Question 33

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

Answer 33

Function call is running the code
Function definition is creating the object, has a codeblock { } and function keyword

Question 34

What is the difference between a parameter and an argument?

Answer 34

Parameter is a placeholder for a variable we don’t know until we call the function and pass an argument
When defining a function, we declare parameters.
When we call a function we pass it an argument

Question 35

Why are function parameters useful?

Answer 35

So we can write the function once and insert any argument we want when we run the function

Question 36

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

Answer 36

Causes the function to produce a value we can use in our program (get the output to wherever the function was called)
Prevents any more code in the function’s code block from being run (stops function from running)

Question 37

Give 6 examples of comparison operators.

Answer 37

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

Question 38

What data type do comparison expressions evaluate to?

Answer 38

Boolean

Question 39

What is the purpose of an if statement?

Answer 39

To guide the computer to make a decision based on criteria

Question 40

Is else required in order to use an if statement?

Answer 40

No If you require code to run only when the statement returns true

Question 41

Describe the syntax (structure) of an if statement.

Answer 41

If (condition) {
code block -Return value;
} else { code;
}

Question 42

What are the three logical operators?

Answer 42

&& || !

Question 43

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

Answer 43

Using a && or ||

Question 44

Why do we log things to the console?

Answer 44

To find errors and warnings as they occur in the JavaScript code.
Debugging, something is broken, understand what’s happening

Question 45

What is a method?

Answer 45

a function which is a property of an object. Using an object then a . then method name, performs a function

Question 46

How is a method different from any other function?

Answer 46

method is associated with an object, while a function is not.
method: object.method
function: state it

Question 47

How do you remove the last element from an array?

Answer 47

arrayname.pop();, arrayname.shift() removes first element

Question 48

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

Answer 48

Math.floor(numberhere);

Question 49

How do you generate a random number?

Answer 49

Math.random() -random float value from 0 to 1 (is a percentage)

Question 50

How do you delete an element from an array?

Answer 50

arrayname.splice();
splice method at the index array

Question 51

How do you append an element to an array?

Answer 51

arrayname.push() adds to end/ arrayname.unshift() adds to beginning

Question 52

How do you break a string up into an array?

Answer 52

stringname.split(“ “) use space as an argument (thing you want to break it up on)

Question 53

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

Answer 53

does not change it, console log it

Question 54

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

Answer 54

No

Question 55

What is the purpose of a loop?

Answer 55

To check a condition and repeat the code block until a condition is met

Question 56

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

Answer 56

Instruct the code when to stop looping

Question 57

What does “iteration” mean in the context of loops?

Answer 57

A pass through the loop

Question 58

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

Answer 58

before each pass through the loop (before each iteration)

Question 59

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

Answer 59

evaluated once before the loop begins

Question 60

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

Answer 60

evaluated before each loop iteration

Question 61

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

Answer 61

When the condition evaluates to false

Question 62

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

Answer 62

break

Question 63

What does the ++ increment operator do?

Answer 63

Adds 1

Question 64

How do you iterate through the keys of an object?

Answer 64

use a for in loop

Question 65

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

Answer 65

focus

Question 66

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

Answer 66

Blur

Question 67

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

Answer 67

input

Question 68

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

Answer 68

submit

Question 69

What does the event.preventDefault() method do?

Answer 69

prevent the browser from automatically reloading the page with the form’s values in the URL

Question 70

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

Answer 70

Prevent form from submitting

Question 71

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

Answer 71

HTMLFormElement.elements

Question 72

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

Answer 72

HTMLFormElement.elements.value

Question 73

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

Answer 73

Not knowing where to fix it , rewriting

Question 74

What is an advantage of having your console open when writing a JavaScript program?

Answer 74

See errors as they occur

Question 75

What is the event.target?

Answer 75

Target of the event, element interacted width

Question 76

What is the effect of setting an element to display: none?

Answer 76

Removes it from the display and doesn’t take up space but still part of the page

Question 77

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

Answer 77

A string css selector and returns a boolean if it matches the selector

Question 78

How can you retrieve the value of an element’s attribute?

Answer 78

Element.getAttribute(‘ attribute you want here‘);

Question 79

At what steps of the solution would it be helpful to log things to the console?

Answer 79

Every step, after every expression

Question 80

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 80

conditional for each possible view or separate event handler for each view

Question 81

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 81

individual conditional blocks for each view, repetition of effort same code

Question 82

What is JSON?

Answer 82

a text-based data format following JavaScript object syntax

Question 83

What are serialization and deserialization?

Answer 83

Serialization is the process of turning an object in memory into a stream of bytes so you can do stuff like store it on disk or send it over the network.
Deserialization is the reverse process: turning a stream of bytes into an object in memory.
Serialization converts it to a string
Deserialization does the opposite to objects and arrays

Question 84

Why are serialization and deserialization useful?

Answer 84

Serialization lets you send the data easily
Deserialization lets you convert back to an object that is easy to work with

Question 85

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

Answer 85

var jsonString = JSON.stringify(data structure here);

Question 86

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

Answer 86

var jsonParse = JSON.parse(jsonString);

Question 87

How do you store data in localStorage?

Answer 87

localStorage.setItem(‘key name’, ‘value name’);

Question 88

How do you retrieve data from localStorage?

Answer 88

Var varstorage = localStorage.getItem(‘keyName’);

Question 89

What data type can localStorage save in the browser?

Answer 89

String only

Question 90

When does the ‘beforeunload’ event fire on the window object?

Answer 90

fired when the window, the document and its resources are about to be unloaded. before page closes

Question 91

What is a method?

Answer 91

a function which is a property of an object.

Question 92

How can you tell the difference between a method definition and a method call?

Answer 92

A method definition has the code block
Method call is running the code block with a argument

Question 93

Describe method definition syntax (structure).

Answer 93

var obj = {
propertyName : function def(parameter) {
}
};

Question 94

Describe method call syntax (structure).

Answer 94

objectname.method(arguments);

Question 95

How is a method different from any other function?

Answer 95

Methods are associated with an object, exampleobject.methodname
Functions run on their own

Question 96

What is the defining characteristic of Object-Oriented Programming?

Answer 96

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

Question 97

What are the four “principles” of Object-Oriented Programming?

Answer 97

Abstraction
Encapsulation
Inheritance
Polymorphism

Question 98

What is “abstraction”?

Answer 98

being able to work with (possibly) complex systems in simple ways. lightswitches

Question 99

What does API stand for?

Answer 99

application programming interface

Question 100

What is the purpose of an API?

Answer 100

Application that is gonna interface with another program to do things in a simple way

Question 101

What is this in JavaScript?

Answer 101

an implicit parameter of all JavaScript functions.

Question 102

What does it mean to say that this is an “implicit parameter”?

Answer 102

it is available in a function’s code block even though it was never included in the function’s parameter list or declared with var.

Question 103

When is the value of this determined in a function; call time or definition time?

Answer 103

call time

Question 104

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 104

nothing, doesn’t exist yet bc its not called

Question 105

Given the above character object, what is the result of the following code snippet? Why?
character.greet();

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

Answer 105

The variable message : “it’s a me mario’. Runs the greet property of the character variable which is a function.
greet method being invoked of he character object. the object being called off of is chjaracer and gets firstname Property

Question 106

Given the above character object, what is the result of the following code snippet? Why?
var hello = character.greet;
hello();

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

Answer 106

no dot in hello() so becomes the window, has nofirstname, returns “it’s a me undefined”

Question 107

How can you tell what the value of this will be for a particular function or method definition?

Answer 107

You can’t, undefined. only when being called

Question 108

How can you tell what the value of this is for a particular function or method call?

Answer 108

the value of this can be recognized as “the object to the left of the dot” when the function is called (as a method).

“This” is “the object to the left of the dot”. if no dot, it is window

Question 109

What kind of inheritance does the JavaScript programming language use?

Answer 109

prototype-based (or prototypal) inheritance

Question 110

What is a prototype in JavaScript?

Answer 110

object that can allow JavaScript objects to inherit features from one another

Question 111

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 111

JavaScript objects give certain behaviors (methods) or data (properties) to other objects.

within prototype object
each of those methods a re stored on the prototype object of that kind of data type

Question 112

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

Answer 112

Prototype

Question 113

What does the new operator do?

Answer 113

1 Creates a new blank object, attached to the prototype of the new object
2 Takes value of the prototype constructor
3 Makes the new object the value of “this” and runs code block
4 If No return statement, the object created gets returned automatically

Question 114

What property of JavaScript functions can store shared behavior for instances created with new?

Answer 114

prototype property
.prototype

Question 115

What does the instanceof operator do?

Answer 115

tests to see if the prototype property of a constructor appears anywhere in the prototype chain of an object. The return value is a boolean value.

Question 116

What is a “callback” function?

Answer 116

passing a function as an argument to another function

Question 117

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 117

setTimeout()

Question 118

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

Answer 118

setInterval()

Question 119

What is the default time delay if you omit the delay parameter from setTimeout() or setInterval()?

Answer 119

0 seconds, immediate

Question 120

What do setTimeout() and setInterval() return?

Answer 120

Return a intervalID a numeric, non-zero value which identifies the timer created by the call to
setInterval(); this value can be passed to clearInterval() to cancel the interval.

starts at 1, pos integer

Question 121

What is AJAX?

Answer 121

Technique for loading data into part of a page without having to reload the entire page

Question 122

What does the AJAX acronym stand for?

Answer 122

Asynchronous JavaScript And XML

Question 123

Which object is built into the browser for making HTTP requests in JavaScript?

Answer 123

XMLHttpRequest

Question 124

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

Answer 124

load

Question 125

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

Answer 125

prototype, inherits

Question 126

What is Array.prototype.filter useful for?

Answer 126

array.filter copies, doesnt modify and only values you want

Question 127

What is Array.prototype.map useful for?

Answer 127

modifying all values in a copy

Question 128

What is Array.prototype.reduce useful for?

Answer 128

combining all the elements of a big array to one