JavaScript

Question 1

What is the purpose of variables?

Answer 1

to store data that can later be used when called upon.
create permanence in data

Question 2

How do you declare a variable?

Answer 2

var variableName

Question 3

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

Answer 3

variableName = value;

Question 4

What characters are allowed in variable names?

Answer 4

letters
numbers (can’t start the variable name)
underscore
dollar sign

Question 5

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

Answer 5

fullName and FullName are two different variables

Question 6

What is the purpose of a string?

Answer 6

used when working with any kind of text
a set of characters in a row that javascript won’t care about

Question 7

What is the purpose of a number?

Answer 7

mathematical operations

Question 8

What is the purpose of a boolean?

Answer 8

hold true/false data types
helpful when determining which part of a script should run

Question 9

What does the = operator mean in JavaScript?

Answer 9

assignment operator
stores value to a variable

Question 10

How do you update the value of a variable?

Answer 10

variableName = new value;

Question 11

What is the difference between null and undefined?

Answer 11

undefined is a primitive value automatically assigned to variables that have just been declared, or to formal arguments for which there are no actual arguments. (automatic assignment from the browser)

null value represents a reference that points, generally intentionally, to a nonexistent or invalid object or address. (null only exists because its been ASSIGNED–> purposeful)

Question 12

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

Answer 12

So you know what data you’re looking at.
Sometimes, it’s helpful to put more than one console.log() in your code to help you debug.

Question 13

Give five examples of JavaScript primitives.

Answer 13

string
number
boolean
undefined
null

Question 14

What data type is returned by an arithmetic operation?

Answer 14

numbers

Question 15

What is string concatenation?

Answer 15

adding two strings together

Question 16

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

Answer 16

addition
string concatenation

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

objects group together a set of variables and functions that are related to each other

Question 20

What are object properties?

Answer 20

variables that are part of objects. They tell us about the object

Question 21

Describe object literal notation.

Answer 21

you create an object by declaring the variable and assigning its properties and their corresponding values inside of curly brackets or instead use empty curly brackets and add properties later
ex)
var hotel = {
name: ‘Quay’,
rooms: 40,
booked: 25,
}

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.propertyName = propertyValue;

square bracket notation
object[‘propertyName’] = propertyValue;

Question 24

What are arrays used for?

Answer 24

group similar data types together by storing values in a list

Question 25

Describe array literal notation.

Answer 25

var arrayName = [‘item 1’, item2, ‘item 3’, ‘item 4’, item5];

Question 26

How are arrays different from “plain” objects?

Answer 26

The key for each value is its index number (starting at 0)
Don’t have individually named pieces of data, but rather are numerically indexed.
You can determine the number of items in an array (length) but not in a plain object.
Use a method (push) to add stuff to an array, use dot notation or bracket notation with assignment operator to add stuff to a plain object.
Arrays have an order, objects do not

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

arrayName.length

returns the number of items stored in the array

Question 29

How do you calculate the last index of an array?

Answer 29

subtract the length of the array by 1
lastIndex = arrayName.length - 1;

Question 30

What is a function in JavaScript?

Answer 30

a block of code that gives step by step instructions to accomplish a specific task

Question 31

Describe the parts of a function definition.

Answer 31

// [1] [2] [3]
function example(parameter1, parameter2, parameter3, …) { // [4]
// …more JavaScript code…
// [5]
return;
} // [6]

1. The function keyword to begin the creation of a new function.
2. An optional name. (Our function’s name is sayHello.)
3. A comma-separated list of zero or more parameters, surrounded by () parentheses. (Our sayHello function doesn’t have any parameters.)
4. The start of the function’s code block, as indicated by an { opening curly brace.
5. An optional return statement. (Our sayHello function doesn’t have a return statement.)
   The end of the function’s code block, as indicated by a } closing curly brace.

Question 32

Describe the parts of a function call.

Answer 32

// [1] [2]
example(arg1, arg2, arg3);

1. The function’s name. Again, our function’s name is sayHello.
2. A comma-separated list of zero or more arguments surrounded by () parentheses.
   Our sayHello function does not take any arguments.

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 contains the steps on what to do with the data (function code block) (parameter)
the function call invokes the function and passes through the data that’s to be worked on (argument)

Question 34

What is the difference between a parameter and an argument?

Answer 34

parameter –> placeholder (function definition)
argument –> actual data (function call)

Question 35

Why are function parameters useful?

Answer 35

give us the ability to have mutability in what occurs when we call a function (diff parameters allow for the return of different values)

Question 36

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

What is a method?

Answer 37

A function that’s assigned to the property of an object

Question 38

Why do we log things to the console?

Answer 38

So we can check our code along the way to make sure it’s doing what it’s supposed to do. helps with debugging

Question 39

What is a method?

Answer 39

A method is a function stored in the property of an object.

Question 40

How is a method different from any other function?

Answer 40

located within an object
called with the object

Question 41

How do you remove the last element from an array?

Answer 41

pop() method
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()

Question 44

How do you delete an element from an array?

Answer 44

array.pop() removes last element from the array
array.shift() removes first element of the array
array.splice() removes indicated item(s) from the array

Question 45

How do you append an element to an array?

Answer 45

array.push() appends an element to the end of the array

Question 46

How do you break a string up into an array?

Answer 46

element.split()

Question 47

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

Answer 47

No, strings are immutable

Use console.log on the original variable

Question 48

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

Answer 48

50

Question 49

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

Answer 49

no

Question 50

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

Answer 50

40

Question 51

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

Answer 51

MDN

Question 52

Give 6 examples of comparison operators.

Answer 52

=== strictly equals to
!== strictly is not equal to
> greater than
< less than
>= greater than or equal to
<= less than or equal to

Question 53

What data type do comparison expressions evaluate to?

Answer 53

boolean (true / false)

Question 54

What is the purpose of an if statement?

Answer 54

to set up a condition to compare to. If the condition evaluates to true, any statements in the subsequent code block are executed

Question 55

Is else required in order to use an if statement?

Answer 55

nope

Question 56

Describe the syntax (structure) of an if statement.

Answer 56

keyword + condition + opening curly brace + code to execute if value is true + closing curly brace
if (score >=50) { congratulate(); }

Question 57

What are the three logical operators?

Answer 57

&& and
|| or
! not (inverts boolean value)

Question 58

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

Answer 58

Use the logical and operator (&&)

Question 59

What is the purpose of a loop?

Answer 59

offer the ability to repeat functionality

Question 60

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

Answer 60

Evaluate a condition to determine whether or not the loop should run

Question 61

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

Answer 61

one time that the loop code block gets run

Question 62

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

Answer 62

before each iteration of the code block executes

Question 63

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

Answer 63

Before anything runs (just once)

Question 64

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

Answer 64

After initialization on the first iteration
before the loops code block and after the final expression

Question 65

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

Answer 65

after the code block runs, before the next iteration

Question 66

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

Answer 66

break

Question 67

What does the ++ increment operator do?

Answer 67

adds one to the counter variable and saves it to that variable

Question 68

What is a “model”?

Answer 68

A representation of something
a copy of the webpage loaded in the browser

Question 69

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

Answer 69

The HTML code / the webpage that’s loaded in the browser

Question 70

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

Answer 70

The node objects within the HTML code/ webpage

Question 71

What is a DOM Tree?

Answer 71

Is a kind of tree whose nodes represent an HTML document’s contents.
All the necessary info to give the characteristics of a single element

Question 72

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

Answer 72

document.querySelector()
document.getElementById()

Question 73

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

Answer 73

document.querySelectorAll()

Question 74

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

Answer 74

If your script needs to use the same element(s) more than once

Question 75

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

Answer 75

console.dir()

Question 76

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

Answer 76

The browser needs to parse all of the elements in the HTML page before the JavaScript code can access them.

Question 77

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

Answer 77

Argument: A CSS selector as a string
Returns: the first matching element

Question 78

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

Answer 78

Argument: a CSS Selector as a string
Returns: All matching elements in a node list

Question 79

What are the different nodes of the DOM tree?

Answer 79

Document node
Element nodes (ex: h1, p, h2, li, etc)
Attribute nodes (part of the attribute node that carries them, not a child)
Text nodes (cannot have children)

Question 80

What is the DOM?

Answer 80

A javascript object that’s modeling the HTML documents objects

Question 81

What is a node list

Answer 81

an array-like object that can’t be changed/messed with

Question 82

What is the purpose of events and event handling?

Answer 82

Events – be aware when a certain action happens
event handling– allows us as the developer to do something when the event happens.
interactivity

Question 83

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

Answer 83

no

Question 84

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

Answer 84

.addEventListener()

Question 85

What is a callback function?

Answer 85

A function passed as an argument to another function

Question 86

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

Answer 86

the event object -

Question 87

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

Answer 87

The place where the event originated from.
console.log(object.event) + check the MDN.

Question 88

What is the difference between these two snippets of code?
1. element.addEventListener(‘click’, handleClick)
2. element.addEventListener(‘click’, handleClick())

Answer 88

1. Calls the function when the click event happens
2. The function will be called when the code is run (probably when the page loads)

Question 89

Do event handler functions return a value?

Answer 89

No

Question 90

What is the className property of element objects?

Answer 90

Contains the current value of the class attribute on that element

Question 91

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

Answer 91

Use the className property to assign a new class to the element object you want to change
elementObject.className = newClass

Question 92

What is the textContent property of element objects?

Answer 92

A property that contains the text content of a DOM element

Question 93

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

Answer 93

No

Question 94

Would this assignment be simpler or more complicated if we didn’t use a variable to keep track of the number of clicks?

Answer 94

more difficult

Question 95

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

Answer 95

You want the data readily accessible in the language you’re writing

Question 96

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

Answer 96

Use the textContent property to assign a new class to the element object you want to change
elementObject.textContent = newText

Question 97

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

Answer 97

focus function
focus ()

Question 98

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

Answer 98

Blur function
blur()

Question 99

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

Answer 99

input

Question 100

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

Answer 100

submit

Question 101

What does the event.preventDefault() method do?

Answer 101

Prevents the default for any event that would’ve been take
(Default event = what the browser is preconfigured to do)

Question 102

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

Answer 102

elements
document.forms.elements

Question 103

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

Answer 103

value
document.forms.elements.value

Question 104

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

Answer 104

If you run into an error, it’s hard to identify where that error originated

Question 105

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

Answer 105

It reloads the page and sends the information back to the page, which is useless (because we don’t specify an action in the HTML code).
So, we always use event.preventDefault() with submitting forms

Question 106

Does the document.createElement() method insert a new element into the page?

Answer 106

Nope.

Question 107

How do you add an element as a child to another element?

Answer 107

appendChild method
parent.appendChild(child)

Question 108

What do you pass as the arguments to the element.setAttribute() method?

Answer 108

the attribute type and value as strings or variables
ex) setAttribute(name, value)

Question 109

What steps do you need to take in order to insert a new element into the page?

Answer 109

1. Create the element
2. Configure the element
3. Append the element to the page at the parent node (the parent node needs to be queried first)

Question 110

What is the textContent property of an element object for?

Answer 110

To add text content to the element that you’re creating

Question 111

What are two advantages of defining a function to do create something (like the work of creating a DOM tree)?

Answer 111

1. You can reuse it in multiple places
2. You can use it with different sets of data

Question 112

Name two ways to set the class attribute of a DOM element.

Answer 112

1. setAttribute (‘class’, value)
2. setClassName()

Question 113

What is the event.target?

Answer 113

A property on the event object that stores a reference to the DOM element that the event fired on

Question 114

Why is it possible to listen for events on one element that actually happen its descendent elements?

Answer 114

event bubbling

Question 115

What DOM element property tells you what type of element it is?

Answer 115

tagName

For example, if the element is an <img></img>, its tagName property is “IMG”

For DOM trees which represent HTML documents, the returned tag name is always in the canonical upper-case form. For example, tagName called on a <div> element returns “DIV”.</div>

Question 116

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

Answer 116

The argument is the CSS selector
The closest ancestor element
(opposite of query selector)

Question 117

How can you remove an element from the DOM?

Answer 117

Call the remove method on the object of the Dom element that you want to remove
element.remove()

Question 118

If you wanted to insert new clickable DOM elements into the page using JavaScript, how could you avoid adding an event listener to every new element individually?

Answer 118

Add it to the parent

Question 119

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

Answer 119

It’s hidden from view / removed from the document flow

Question 120

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

Answer 120

argument: A css selector as a string
returns: boolean

Question 121

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

Answer 121

getAttribute() method
argument is a string of the type of attribute we want to retrieve

Question 122

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

Answer 122

Every step where your code is doing something/making a change

Question 123

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 123

Use individual event listeners for each tab

Question 124

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 124

A lot of if statements

Question 125

What is JSON?

Answer 125

Javascript Object notation
JavaScript Object Notation (JSON) is a standard text-based format for representing structured data based on JavaScript object syntax.
JSON is an extremely common data interchange format used to send and store information in computer systems.

Question 126

What are serialization and deserialization?

Answer 126

Serialization is the process of turning an object in memory into a string representation so that it can be stored in memory. All of the data is one contiguous stream (stream of bytes)

Deserialization is the reverse process: turning a stream of bytes into an object in memory. Bringing back the complexity of how the data is stored

Question 127

Why are serialization and deserialization useful?

Answer 127

Let’s you move data from one place to another (storing and transmitting data)
Deserialized data is easy to work with

Question 128

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

Answer 128

JSON.stringify (data)

Question 129

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

Answer 129

JSON.parse(data)

Question 130

How do you store data in localStorage?

Answer 130

localStorage.setItem(keyName, keyValue)

Question 131

How do you retrieve data from localStorage?

Answer 131

localStorage.getItem(KeyName)

Question 132

How do you retrieve data from localStorage?

Answer 132

strings

Question 133

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

Answer 133

The beforeunload event is fired when the window, the document and its resources are about to be unloaded. The document is still visible and the event is still cancelable at this point.

Question 134

What is a method?

Answer 134

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

Question 135

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

Answer 135

Method definition has a function code block and is assigned to a property (:), the function keyword

Method call passes the arguments.
object.method (argument(s))

Question 136

Describe method definition syntax (structure).

Answer 136

object {
method: function() {
code block}

object
method name
function keyword and optional name (most often anonymous)
parameters, if there are any
function code block

Question 137

Describe method call syntax (structure).

Answer 137

// [1] [2] [3]
object.method(arg1, arg2, arg3);

1. The object
2. The method’s name.
3. A comma-separated list of zero or more arguments surrounded by () parentheses.

Question 138

How is a method different from any other function?

Answer 138

It’s the property of an object

Question 139

What is the defining characteristic of Object-Oriented Programming?

Answer 139

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

Question 140

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

Answer 140

Abstraction
Encapsulation
Inheritance
Polymorphism

Question 141

What does API stand for?

Answer 141

application programming interface (API)

Question 142

What is the purpose of an API?

Answer 142

The purpose of every software API is to give programmers a way to interact with a system in a simplified, consistent fashion: aka, an abstraction.

Question 143

What is “abstraction”?

Answer 143

Being able to work with (possibly) complex things in simple ways.

Question 144

What is this in JavaScript?

Answer 144

This is an implicit parameter whose value is determined when the function is called, not when it is defined.

Question 145

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

Answer 145

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.
It’s a value that’s given to each function–it’s’ always present

Question 146

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

Answer 146

Call time

Question 147

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 147

nothing (the function hasn’t been called yet)

Question 148

Given the below 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 148

It’s a-me, Mario!

this refers to the character object. character.firstName = Mario

Question 149

Given the below 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 149

It’s a-me, undefined!

this is referring to the default Window object. window doesn’t have a property named firstName

Question 150

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

Answer 150

We wouldn’t know because it’s value is assigned upon function call
Parameters are placeholders for future values

Question 151

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

Answer 151

Whatever is to the left of the dot

Question 152

What kind of inheritance does the JavaScript programming language use?

Answer 152

prototype-based (or prototypal) inheritance. – JavaScript objects give certain behaviors (methods) or data (properties) to other objects.

Question 153

What is a prototype in JavaScript?

Answer 153

Simply an object that contains properties and (predominantly) methods that can be used by other objects.

Question 154

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 154

prototypal inheritance

Question 155

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

Answer 155

on its prototype

Question 156

What does the new operator do?

Answer 156

The new operator lets developers create an instance of a user-defined object type or of one of the built-in object types that has a constructor function.
1. Makes an empty object
2. Takes the prototype property from the function and adds it to the new property
3. Runs the function block and makes the value of that new object the value of this
4. It returns the object if no other return is specified

Question 157

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

Answer 157

prototype

Question 158

What does the instanceof operator do?

Answer 158

The instanceof operator tests the presence of constructor.prototype in object’s prototype chain. This usually means object was constructed with constructor.

obj instanceof class/constructor function

Question 159

What is a “callback” function?

Answer 159

A callback function is a function passed into another function as an argument, which is then invoked inside the outer function to complete some kind of routine or action.

Question 160

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 160

setTimeout()

Question 161

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

Answer 161

setInterval()

Question 162

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

Answer 162

0 - gets put in the queue immediately

Question 163

What do setTimeout() and setInterval() return?

Answer 163

timeout id and Interval id, respectively

Question 164

What is AJAX?

Answer 164

A technique for loading data into part of a page without having to refresh the entire page.

Question 165

What does the AJAX acronym stand for?

Answer 165

Asynchronous JavaScript and XML

Question 166

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

Answer 166

XMLHttpRequest (xhr)

Question 167

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

Answer 167

load

Question 168

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

Answer 168

They are objects that can receive events
They share a prototype

Question 169

What is a code block? What are some examples of a code block?

Answer 169

A group of statements of code to be executed together. Usually contained within curly braces
function code block
conditional (if statement) code block
loop code block

Question 170

What does block scope mean?

Answer 170

the value of the variable is dependent/available based on the block of code its located within.

A block scoped variable means that the variable defined within a block will not be accessible from outside the block. A block can reside inside a function, and a block scoped variable will not be available outside the block even if the block is inside a function.

Question 171

What is the scope of a variable declared with const or let?

Answer 171

block-scope

Question 172

What is the difference between let and const?

Answer 172

block-scoped variables declared by the const keyword can’t be reassigned. read-only reference

The variables declared by the let keyword are mutable. It means that you can change their values anytime you want

Question 173

Why is it possible to .push() a new value into a const variable that points to an Array?

Answer 173

The actual value to which the const variable reference is mutable.

Question 174

How should you decide on which type of declaration to use?

Answer 174

If the variable is being reassigned, use let. If it’s not, use const

Question 175

What is the syntax for writing a template literal?

Answer 175

you create a template literal by wrapping your text in backticks (`)

let simple = This is a template literal;

Question 176

What is “string interpolation”?

Answer 176

Embedding variables and expressions in a string. The JavaScript engine will automatically replace these variables and expressions with their values.

${variable_name}

Question 177

What is destructuring, conceptually?

Answer 177

Taking property values or array objects and taking them from their respective objects and arrays and assigning them to a variable

Question 178

What is the syntax for Object destructuring?

Answer 178

let { property1: variable1, property2: variable2 } = object;

If the variables have the same names as the properties of the object, you can make the code more concise as follows:
let { firstName, lastName } = person;

Question 179

What is the syntax for Array destructuring?

Answer 179

const [first, second, third ] = theArray

Question 180

How can you tell the difference between destructuring and creating Object/Array literals?

Answer 180

Look at what’s on the left and right of the assigment operator
if the variable name that holds the object or array is on the right, it’s destructuring. If it’s on the left, its creating

Question 181

What is the syntax for defining an arrow function?

Answer 181

(param 1, param2) => { statements }

don’t need the brackets if the codeblock is multiline
if it’s multiline, a return statement is required

param => expression;

() => exp;

Question 182

When an arrow function’s body is left without curly braces, what changes in its functionality?

Answer 182

The function body gets evaluated as an expression and the result of the expression is implicitly returned

Question 183

How is the value of this determined within an arrow function?

Answer 183

Arrow functions have lexical this, meaning the value of this is determined by the surrounding scope (the lexical environment).

Question 184

What are the three states a Promise can be in?

Answer 184

Pending, fulfilled, rejected

can’t change state once it’s fulfilled or rejected

Question 185

How do you handle the fulfillment of a Promise?

Answer 185

then () method

pass a callback function that handles the successful promise

Question 186

How do you handle the rejection of a Promise?

Answer 186

catch()

Question 187

What is Array.prototype.filter useful for?

Answer 187

to extract certain kinds of data form a list

Question 188

What is Array.prototype.map useful for?

Answer 188

Creates a new array populated with the results of calling a provided function on every element in the calling array.

Question 189

What is Array.prototype.reduce useful for?

Answer 189

executes a user-supplied “reducer” callback function on each element of the array, in order, passing in the return value from the calculation on the preceding element. The final result of running the reducer across all elements of the array is a single value.

JS general purpose aggregate function

Question 190

What is “syntactic sugar”?

Answer 190

syntax within a programming language that is designed to make things easier to read or to express. It makes the language “sweeter” for human use: things can be expressed more clearly, more concisely, or in an alternative style that some may prefer.
Syntactic sugar is usually a shorthand for a common operation that could also be expressed in an alternate, more verbose, form:

Question 191

What is the typeof an ES6 class?

Answer 191

function

Question 192

Describe ES6 class syntax.

Answer 192

class keyword Person {
constructor (param1, param2, ….rest) {
this.something= ‘a thing’
}
another method ()
return value
}
}

Question 193

What is “refactoring”?

Answer 193

The process of restructuring existing computer code—changing the factoring—without changing its external behavior. Refactoring is intended to improve the design, structure, and/or implementation of the software (its non-functional attributes), while preserving its functionality.

Question 194

How are ES Modules different from CommonJS modules?

Answer 194

CommonJS Modules:
The dominant implementation of this standard is in Node.js (Node.js modules
have a few features that go beyond CommonJS). Characteristics:
Compact syntax
Designed for synchronous loading
Main use: server

Asynchronous Module Definition (AMD):
The most popular implementation of this standard is RequireJS.
Characteristics:
Slightly more complicated syntax, enabling AMD to work without eval()
(or a compilation step).
Designed for asynchronous loading
Main use: browsers

Question 195

What kind of modules can Webpack support?

Answer 195

ECMA Script Modules, Common Modules, and AMD modules

Question 196

What is the purpose of state in React?

Answer 196

Keep track of values that change over time

Think of it as a data model for a component

Question 197

How to you pass an event handler to a React element?

Answer 197

provide a listener when the element is initially rendered.