JavaScript

Question 1

What is the purpose of variables?

Answer 1

Variables store data!

Question 2

How do you declare a variable?

Answer 2

Use the var, let, or const keyword

Question 3

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

Answer 3

Follow the declaration with an “=“ and the value you’d like to assign

Question 4

What characters are allowed in variable names?

Answer 4

Start with letter, underscore, or dollarsign. Afterwards — numbers are okay. No keywords! No dashes or periods~!

Question 5

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

Answer 5

the two variables lovesDogs and lovesdogs are different due to the D / d.

Question 6

What is the purpose of a string?

Answer 6

Stores text-data, good for manipulation.

Question 7

What is the purpose of a number?

Answer 7

Stores number-data, good for mathematics

Question 8

What is the purpose of a Boolean?

Answer 8

Stores true/false data, good for logic

Question 9

What does the = operator mean in Javascript?

Answer 9

Assignment operator — assigns right value to the left operand

Question 10

How do you update the value of a variable?

Answer 10

Use the = operator with the variable on the left, and the new value on the right.

Question 11

What is the difference between null and undefined?

Answer 11

Null is an intentionally empty value, undefined means no value was assigned

Question 12

Why is it a good habit to include labels when logging values to the console?

Answer 12

When you add multiple console logs it gets confusing pretty quickly what you logging.

Question 13

Give five examples of JavaScript primitives.

Answer 13

Number, string, Boolean, null, undefined

Question 14

What is a JavaScript primitive?

Answer 14

A data that is not an object, and hence has no methods or properties.

Question 15

What data type is retuned by an arithmetic operation?

Answer 15

Number is returned

Question 16

What is string concatenation?

Answer 16

Joining together two strings and returning a new string

Question 17

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

Answer 17

Adds two operands (not necessarily nums!)

Question 18

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

Answer 18

Boolean (t / f)

Question 19

What do the += “plus-equals’ operator do?

Answer 19

Shorthand — adds the value of right side to the left side variable and returns the result

Question 20

How are reference data types and primitive data types stored differently?

Answer 20

Reference data types only store the memory pointer to that specific data

Question 21

The two reference data types are…

Answer 21

Objects and arrays

Question 22

What are objects used for?

Answer 22

To group together variables and functions in order to model something from the real world

Question 23

What are object properties?

Answer 23

The variables of the object are its properties (its functions are known methods)

Question 24

Describe object literal notation.

Answer 24

Declare variable and assign it the value of empty curly braces

Question 25

How do you remove a property from an object

Answer 25

Use the delete keyword followed by the property and value

Question 26

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

Answer 26

Dot & bracket notation

Question 27

Dot notation vs bracket notation differences?

Answer 27

Dot notation requires a legal variable name —> bracket notation can also be used with variables

Question 28

What are arrays used for?

Answer 28

Storing related data of unknown length — sort of like lists

Question 29

Describe array literal notation.

Answer 29

var array = [data seperated by commas

Question 30

How are arrays different from plain objects?

Answer 30

array keys are index numbers, object keys are assigned by us

Question 31

What number represents the first index of an array?

Answer 31

Arrays are 0-indexed

Question 32

What is the length property of an array?

Answer 32

It holds a number with the amount of items in the array

Question 33

How do you calculate the last index of an array?

Answer 33

Use array.length - 1

Question 34

What is a function in JavaScript?

Answer 34

A set of statements that performs a task — often with an input and output.

Question 35

Describe the parts of a function definition.

Answer 35

Function keyword, function name (opt), (paramteter list), {code body, return(opt)}

Question 36

Describe the parts of a function call.

Answer 36

Function name, (arguments)

Question 37

How are function calls and function definitions different?

Answer 37

Definitions are stored in memory, calls look in memory and run the stored function

Question 38

What is the difference between a parameter and an argument?

Answer 38

Parameters are for definitions, no concrete value —> arguments are for calling, has value

Question 39

Why are function parameters useful?

Answer 39

They allow us to take advantage of a functions functionality by passing in specific arguments

Question 40

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

Answer 40

1. kicks you out of that execution context
2. Specifies a value to be returned to the function caller

Question 41

Why do we log things to the console?

Answer 41

It helps with debugging —> we can see what is stored in memory at that point

Question 42

What is a method?

Answer 42

A property on objects with a function stored as the value

Question 43

How is a method different from any other function?

Answer 43

Methods are linked directly to their related object —> otherwise they’re the exact same

Question 44

How do you remove the last element from an array?

Answer 44

The pop method (no args)

Question 45

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

Answer 45

The floor method (number to execute on)

Question 46

How do you generate a random number?

Answer 46

Use the random method to get a num between 0 -> 1 (no args)

Question 47

How do you delete an element from an array?

Answer 47

The splice method (takes starting index and number of elements to remove)

Question 48

How do you append an element to an array?

Answer 48

The push method (element to push)

Question 49

How do you break a string up into an array?

Answer 49

The split method (takes the character to split with)

Question 50

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

Answer 50

Nope —> console.log(original string) to check for changes!

Question 51

Roughly how many string methods are there according to MDN?

Answer 51

~50!

Question 52

Roughly how many array methods are there?

Answer 52

~80!

Question 53

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

Answer 53

Nope! Some methods alter the original array (like pop) & return undefined

Question 54

What three-letter acronym should you always include when searching Google for answers?

Answer 54

MDN!

Question 55

Give 6 examples of comparison operators.

Answer 55

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

Question 56

What data type do comparison expressions evaluate to?

Answer 56

Boolean t/f

Question 57

What is the purpose of an if statement?

Answer 57

To evaluate a condition and run code specific to that condition

Question 58

Is “else” required in order to use an if statement?

Answer 58

Nope! (But it’s good practice if you’re trying to catch edge cases)

Question 59

Describe the syntax of an if statement.

Answer 59

If keyword (condition) {code block to run if condition is satisified}

Question 60

What are the three logical operators?

Answer 60

&& (and) || (or) ! (Not)

Question 61

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

Answer 61

Write the conditions in the same parenthesis, seperated by the desired logical operator.

Question 62

What is the purpose of a loop?

Answer 62

To repeat code!

Question 63

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

Answer 63

It tells the loop when to stop!

Question 64

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

Answer 64

Repeating some code over some data

Question 65

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

Answer 65

1. Before the loop starts
2. After each iteration

Question 66

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

Answer 66

The first time the loop runs

Question 67

When does the condition of a for loop get evaluated?

Answer 67

1. After the initialization statement
2. After each final expression

Question 68

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

Answer 68

After each iteration of the loop

Question 69

Besides the return statement, what keyword exits the loop before it evaluates to false?

Answer 69

Break statement will break the loop, but not the function

Question 70

What does the ++ increment operator do?

Answer 70

1. It increases the value of the affected variable by 1
2. It substitutes first and THEN increments

Question 71

How do you iterate through the keys of an object?

Answer 71

Use a for / in loop!

Question 72

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

Answer 72

The focus event

Question 73

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

Answer 73

The blur event

Question 74

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

Answer 74

The input event

Question 75

What event is fired when a user clicks the “submit” button in a form?

Answer 75

The submit event

Question 76

What does the event.preventDefault() method do?

Answer 76

It prevents the page from reloading after submitting a form (default behavior)

Question 77

What property of a form element object contains all of the form controls?

Answer 77

The form.elements property —> make sure you check legal variable names for {.notation}

Question 78

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

Answer 78

Form.elements.(element-name).value

Question 79

What is one risk of writing a lot of code without checking if it works?

Answer 79

When you inevitably run into a bug, you have no idea what part of the code it came from

Question 80

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

Answer 80

You can use it alongside console.logs to make sure your code does what you think it does

Question 81

Pretty much all of the time you create a form, the first line will be…

Answer 81

event.preventDefault( );

Question 82

What is the event.target?

Answer 82

The element that dispatched the event (the one the user interacted with)

Question 83

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

Answer 83

It hides all content within that element from the user.

Question 84

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

Answer 84

Takes CSS selectors as the argument, returns true if the element matches the selector(s), otherwise returns false

Question 85

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

Answer 85

Use the getAttribute( ) method with the HTML attribute you are looking for.

Question 86

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

Answer 86

You would have to create all those elements, give them the appropriate classes, append them as children, and give each child the appropriate event listener to hide/show on click.

Question 87

If you didn’t use a loop to conditionally show or hide the views, how would your JS code be written instead?

Answer 87

You would have to add event listeners for each view item, and when that view item is clicked bring it into view, while hiding the other view items.

Question 88

What is JSON?

Answer 88

JavaScript Object Notation —> text based format for represented structured data in objects

Question 89

What are serialization and deserialization?

Answer 89

S: process of turning an object into a stream of bytes so you can send it / store it.
D: turnings stream of sent / stored information into an object in memory

Question 90

Why are serialization and deserialization useful?

Answer 90

Whenever we want to send or store JS objects, we need a way to do it!

Question 91

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

Answer 91

JSON.stringify( ); parameters: value, replacer(opt.), space(opt.)

Question 92

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

Answer 92

JSON.parse( ); parameters: text, reviver(opt.)

Question 93

How do you store data in localStorage?

Answer 93

Use the setItem method and pass in the key and value of what you are storing (JSON value!)

Question 94

How do you retrieve data from localStorage?

Answer 94

Use the getItem method and pass in the key of the JSON item you are pulling

Question 95

What data type can localStorage save in the browser?

Answer 95

JSON

Question 96

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

Answer 96

It happens before the page gets rid of all the data — before you leave the page, typically.

Question 97

What is ‘this’ in JavaScript?

Answer 97

It’s an implicit parameter defined when the function is called

Question 98

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

Answer 98

It’s available in the code block even though it was never explicitly declared.

Question 99

When is the value of ‘this’ determined in a function?

Answer 99

When the function is called.

Question 100

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

Answer 100

You can’t — this is not determined until that function is called. Look for where it’s called

Question 101

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

Answer 101

Look for the object to the left of the dot. If there’s no value there, ‘this’ is the global window object

Question 102

What kind of inheritance does the JavaScript programming language use?

Answer 102

Prototypical inheritance

Question 103

What is a prototype in JavaScript?

Answer 103

A mechanism that JS objects use to inherit features from one another

Question 104

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

Answer 104

Through prototypical inheritance! The factory that makes strings attaches a prototype chain to strings that allows them to access the methods we are familiar with.

Question 105

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

Answer 105

The prototype chain!

Question 106

What does the new operator do?

Answer 106

The new operator creates an instance of an object. The type of instance is determined by the constructor following the new keyword.

Question 107

Specifically, the 4 things the new operator does are…

Answer 107

1. Creates a blank, plain JS object.
2. Points newInstance’s [[prototype]] to the constructor function’s prototype property.
3. It binds the ‘this’ keyword to that new object.
4. If the constructor returns a non-primitive, the return value becomes the result of the entire new expression (but we never return anything from these constructors!)

Question 108

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

Answer 108

The prototype property allows shared behavior between instances created with new.

Question 109

What does the instanceof operator do?

Answer 109

The instance of operator returns a bool (t / f) whether the object to the right is an instanceof the left.

Question 110

What is a “callback” function?

Answer 110

A function that passed into another function as an argument.

Question 111

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

Answer 111

Use setTimeout(cb fx, time(Ms))

Question 112

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

Answer 112

Use setInterval (cb fx, time(Ms) —> stop interval using clearInterval

Question 113

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

Answer 113

0 —> the code will execute immediately (or more accurately, the next event cycle)

Question 114

What do setTimeout() and setInterval() return?

Answer 114

They both return an ID —> can be passed to clearTimeout()/Interval() to cancel the timeout / interval.

Question 115

What is array.prototype.filter useful for?

Answer 115

It’s good when you need to grab elements from an array that satisfy a certain condition

Question 116

What is array.prototype.map useful for?

Answer 116

It’s good for when you want to do something to every element of the array

Question 117

What is array.prototype.reduce useful for?

Answer 117

It’s good for when you want to reduce an array down to one value.

Question 118

If your goal is to return an object from reduce, don’t forget to…

Answer 118

Start the recursion with a new empty object to prevent reference errors

Question 119

What is “synatic sugar”

Answer 119

Syntatic sugar makes the code easier to read and understand

Question 120

What is the type of an ES6 class?

Answer 120

It’s a function!

Question 121

Describe ES6 class syntax?

Answer 121

Class keyword, name of class, optionally a constructor, optionally a method

Question 122

What is refactoring?

Answer 122

Changing code structure without affecting it’s functionality.

Question 123

Why is prototyping beneficial?

Answer 123

If we define a function on the constructor, each instance of that object gets a copy of that function (which is super overkill —> takes up a lot of memory). Defining it on the prototype means that all instances point to that same function (which saves memory)