JavaScript

Question 1

What is the purpose of variables?

Answer 1

To use as an identifier in JavaScript and the variable must be identified with unique names.

Question 2

How do you declare a variable?

Answer 2

Use var, let (variable with restricted scope), or const (a variable that can’t be reassigned) keyword.

Question 3

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

Answer 3

Put = operator. var a = 1.

Question 4

What characters are allowed in variable names?

Answer 4

The first character must start with a letter, underscore, or a dollar sign ($), and don’t forget variable names are case-sensitive and don’t use JavaScript’s reserved keywords.

Question 5

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

Answer 5

This means that any identifiers should always be typed with consistent character capitalization within acceptable rules.

Question 6

What is the purpose of a string?

Answer 6

It is used for storing and manipulating text in JavaScript.

Question 7

What is the purpose of a number?

Answer 7

It is a primitive wrapper object used to represent and manipulate numbers.

Question 8

What is the purpose of a boolean?

Answer 8

It is used to represent one of two values in a value, such as True or False, Yes or No, and On or Off.

Question 9

What does the = operator mean in JavaScript?

Answer 9

It performs some operation on single or multiple operands (data values) and produces a result.

Question 10

How do you update the value of a variable?

Answer 10

Just update the new value under the same variable name or use temporary values.

Question 11

What is the difference between null and undefined?

Answer 11

They are equal in value but different in type.

Question 12

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

Answer 12

To identify easier, faster, and more directly when you work on some big project or work with coworkers.

Question 13

Give five examples of JavaScript primitives.

Answer 13

String, number, boolean, null, and undefined. Everything else is an object.

Question 14

What data type is returned by an arithmetic operation?

Answer 14

It returns by numerical values.

Question 15

What is string concatenation?

Answer 15

It is the process of appending one string to the end of another string.

Question 16

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

Answer 16

It produces the sum of numeric operands or string concatenation.

Question 17

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

Answer 17

It returns a boolean value.

Question 18

What does the plus-equals (+=) operator do?

Answer 18

It 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

It is a standalone entity that holds multiple values in terms of properties and methods.

Question 20

What are object properties?

Answer 20

It’s a variable that is attached to the object. It is sorted as the same as an ordinary variable except for the attachment to the object.

Question 21

Describe object literal notation.

Answer 21

var object name = {property name: value of it, another property name: value of it}.

Question 22

How do you remove a property from an object?

Answer 22

Use keyword delete the object name.property name or keyword delete object name[‘property name’].

Question 23

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

Answer 23

Object name.property name = value that you want to update or object name[‘property name’] = value that you want to update.

Question 24

What are arrays used for?

Answer 24

It is used when you store a collection of variables of the same type.

Question 25

Describe array literal notation.

Answer 25

var array object name = [‘’, ‘’, ‘’ ];

Question 26

How are arrays different from “plain” objects?

Answer 26

Objects represent a special data type that is mutable and can be used to store a collection of data. On the other hand, arrays are a special type of variable that is also mutable and can also be used to store a list of values.

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

The length property sets or returns the number of elements in that array.

Question 29

How do you calculate the last index of an array?

Answer 29

the length of the array - 1.

Question 30

What is a function in JavaScript?

Answer 30

It is a code to perform a particular task or calculate a value.

Question 31

Describe the parts of a function definition.

Answer 31

keyword function, name of the function, parameter, but it should take some input and return output and there is some obvious relationship between them

Question 32

Describe the parts of a function call.

Answer 32

Keyword function and give the name of the function and put parenthesis (give parameter or not). You should define the function in the scope from which you wish to call it.

Question 33

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

Answer 33

A function definition’s meaning is to remember the method of it but stays still. On the other hand, a function call instructs JavaScript to execute the code of the function.

Question 34

What is the difference between a parameter and an argument?

Answer 34

The parameters are listed inside of the parenthesis. The arguments are the values received by the function when it is invoked.

Question 35

Why are function parameters useful?

Answer 35

Because those are expected to be the same as variables except that the values of these variables are defined when we call the function.

Question 36

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

Answer 36

It ends the execution of a function and returns control to the calling function. Execution resumes in the calling function at the point immediately following the call. A return statement can return a value to the calling function.

Question 37

Why do we log things to the console?

Answer 37

It prints the output to the console, so we can see what is going on, and be able to fix the code if something is wrong.

Question 38

What is a method?

Answer 38

It is a function that is a property of an object.

Question 39

How is a method different from any other function?

Answer 39

A method is a function that belongs to an object.

Question 40

How do you remove the last element from an array?

Answer 40

Use array pop() method. name of array.pop();

Question 41

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

Answer 41

Use Math.floor() function.

Question 42

How do you generate a random number?

Answer 42

Use Math.random() function.

Question 43

How do you delete an element from an array?

Answer 43

Use splice() method, pop() method, or shift() method (removes the first array).

Question 44

How do you append an element to an array?

Answer 44

Use unshift() method to add a new element at the beginning, push() method to add a new element at the end, or name of array[index number].

Question 45

How do you break a string up into an array?

Answer 45

Use the split(‘ ‘) method.

Question 46

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

Answer 46

No. Use console.log() to check.

Question 47

Roughly how many string methods are there according to the MDN Web docs?

Answer 47

split(), split([separator[, limit]]),

Question 48

Roughly how many array methods are there according to the MDN Web docs?

Answer 48

38 methods on there.

Question 49

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

Answer 49

MDN

Question 50

Give 6 examples of comparison operators.

Answer 50

Strict equal to (===), greater than (>), greater than or equal to (>=), less than or equal to (<=), less than (

Question 51

What data type do comparison expressions evaluate to?

Answer 51

Boolean

Question 52

What is the purpose of an if statement?

Answer 52

To perform different actions based on different conditions. Use if statement to specify a code to be executed if a specified condition is true.

Question 53

Is else required in order to use an if statement?

Answer 53

No, it’s not required to write the else part for the if statement.

Question 54

Describe the syntax (structure) of an if statement.

Answer 54

Keyword if (variable name, comparison, value) { }.

Question 55

What are the three logical operators?

Answer 55

And (&&), or (||), not (!).

Question 56

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

Answer 56

Using logical and or or operators.

Question 57

What is the purpose of a loop?

Answer 57

It used to repeatedly run a block of code until a certain condition is met.

Question 58

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

Answer 58

It is an expression that is evaluated once before every iteration.

Question 59

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

Answer 59

It describes going through a set of operations that deal with the code conditions.

Question 60

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

Answer 60

When the while loop is created.

Question 61

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

Answer 61

When the loop is created.

Question 62

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

Answer 62

Right after the initialization.

Question 63

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

Answer 63

at the end of each loop iteration.

Question 64

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

Answer 64

Break.

Question 65

What does the ++ increment operator do?

Answer 65

Added 1 to the value of the variable.

Question 66

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

Answer 66

Focus.

Question 67

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

Answer 67

Blur.

Question 68

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

Answer 68

Input.

Question 69

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

Answer 69

Submit.

Question 70

What does the event.preventDefault() method do?

Answer 70

Its default action should not be taken as it normally would be.

Question 71

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

Answer 71

Refresh the page and refresh.

Question 72

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

Answer 72

Elements.

Question 73

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

Answer 73

Value property.

Question 74

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

Answer 74

Hard to debug when an error occurs.

Question 75

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

Answer 75

Checking and debugging very easily.

Question 76

What is the event.target?

Answer 76

The target property of the event interface is a reference to the object onto which the event was dispatched.

Question 77

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

Answer 77

It makes it invisible and it takes off from the document.

Question 78

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

Answer 78

It returns a boolean if the value of the CSS selector and the element is matched.

Question 79

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

Answer 79

Whenever I set up the new variable or after creating a loop or condition.

Question 80

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 80

I have to write so many conditional statements. So much work has to do.

Question 81

What is a method?

Answer 81

It’s a function that is a property of an object.

Question 82

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

Answer 82

The instance methods are built-in tasks performed by an object instance. The static methods are tasks that are called directly on an object constructor.

Question 83

Describe method definition syntax (structure).

Answer 83

Put property name: function definition with parameter and opening curly brace for the function code block.

Question 84

Describe method call syntax (structure).

Answer 84

Put property name of the object name and put values in parentheses.

Question 85

How is a method different from any other function?

Answer 85

A method is associated with an object, but a function is not.

Question 86

What is the defining characteristic of Object-Oriented Programming?

Answer 86

It’s a programming paradigm based on the concept of objects that can contain data and code.

Question 87

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

Answer 87

Abstraction, Encapsulation, Inheritance, Polymorphism.

Question 88

What is “abstraction”?

Answer 88

It’s one of the key concepts of OOP. The main goal is to handle complexity by hiding unnecessary details from the user.

Question 89

What does API stand for?

Answer 89

It’s the acronym Application Programming Interface, which is a software intermediary that allows two applications to talk to each other.

Question 90

What is the purpose of an API?

Answer 90

It simplifies programming by abstracting the underlying implementation and only exposing objects or actions the developer needs.

Question 91

What is “this” in JavaScript?

Answer 91

This refers to the object it belongs to, but it has different values depending on where it is used. This refers to the owner object in a method, but This refers to the global object in a function.

Question 92

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

Answer 92

Because This keyword is available in a function’s code block even though it was never included in the function’s parameter or declared as a variable.

Question 93

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

Answer 93

The value of this is determined at call time.

Question 94

``` What does “this” refer to in the following code snippet? var character = { firstName: 'Mario', greet: function () { var message = 'It's me, ' + this.firstName + '!'; console.log(message); } }; ```

Answer 94

This refers to the variable character.

Question 95

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

Answer 95

It will get the value of the variable message because that is a value of the greet property of the character object.

Question 96

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

Answer 96

It will get an error because there is no function named hello is exist.

Question 97

What kind of inheritance does the JavaScript programming language use?

Answer 97

It uses prototypal inheritance.

Question 98

What is a prototype in JavaScript?

Answer 98

It’s a mechanism that JavaScript objects inherit features from one another.

Question 99

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

Answer 99

Because of the prototype chain. It explains why different objects have properties and methods defined on other objects available to them.

Question 100

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

Answer 100

Look it up to the proto objects.

Question 101

What does the “new” operator do?

Answer 101

It creates a new object and adds a property to the new object (__proto__) that links to the constructor function’s prototype object. Bind the newly created object instance as “this” context. Return “this” if the function doesn’t return an object.

Question 102

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

Answer 102

The prototype property.

Question 103

What does the “instanceof” operator do?

Answer 103

It determines whether the prototype property of the constructor exists somewhere in the prototype chain of the object.

Question 104

What is a “callback” function?

Answer 104

It’s a function passed into another function as an argument. It used to be invoked inside the outer function to complete some other action.

Question 105

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 105

setTimeout() method.

Question 106

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

Answer 106

0.

Question 107

What do setTimeout() and setInterval() return?

Answer 107

It returns a number of IDs.