JavaScript

Question 1

What is the purpose of variables?

Answer 1

to store values/information
and we are able to refer/call this variable later!

i.e to store information for the future

Question 2

How do you declare a variable?

Answer 2

use keyword (“var”, “let”, “const”)

followed with the name of variable and end with semicolon “;”
ex) var name;

Question 3

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

Answer 3

single equal sign (assignment operator)

start with variable name following with an equal sign (=) and the value you want to assign to variable. end with semicolon

ex) Name = “Monique”;

• but usually will declare and assign variable in one step!!
  ex) var Name = “Monique”;

Question 4

What characters are allowed in variable names?

Answer 4

numbers, letters, dollar sign ($), underscore (_)

**but variables cannot start with numbers!!!

Question 5

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

Answer 5

lowercase and uppercase letters are different (even if its same letter)

(ex) Name, name –> are two different variable names

Question 6

What is the purpose of a string?

Answer 6

to store letters and other characters
a sequence of a set of characters

js wont care about what is in strings

Question 7

What is the purpose of a number?

Answer 7

to store numeric values
(that we usually do calculations with, mathematical operations)

financial information a good example to use numbers but not smt like a zipcode (b/c we arent usually doing mathematical operations with zipcode values)

Question 8

What is the purpose of a boolean?

Answer 8

purpose is to make decisions

it is able to represent if something is or isn’t (binary)

Question 9

What does the = operator mean in JavaScript?

Answer 9

assignment operator

used to assign a value to a variable
assign: to store information to a variable (name)

Question 10

How do you update the value of a variable?

Answer 10

don’t use keyword!! (if variable already exists)
write variable name with equal sign (assignment operator) and new variable value

Question 11

What is the difference between null and undefined?

Answer 11

both are value types

null: value that is assigned and purposely/intentionally left blank/empty
value that can only appear b/c DEVELOPER purposely assigned it to something
ex) optional inputs on a website from user (we still need to store data about user, so can use null in the optional inputs user did not fill out)

undefined: value that is assigned by BROWSER
value that can come from anywhere; if undefined pops up, need to investigate?

Question 12

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

Answer 12

to give clarity
to know if the correct value is being assigned to correct variables (debugging or double checking when you go back to code)

Question 13

Give five examples of JavaScript primitives.

Answer 13

string, number, boolean, null, undefined

Question 14

What data type is returned by an arithmetic operation?

Answer 14

number

Question 15

What is string concatenation?

Answer 15

adding two or more strings together to create one string

(addition but for strings)

*concatenation does not change the value of the individual strings”

Question 16

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

Answer 16

mathematical operations (summing), 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

shorthand operator
adds a value to an already existing variable and reassigns the resulting value to the same variable

ex) var name = “Monique”;
name += “ Chang”;

   name variable's value would now be Monique Chang, and not Monique

Question 19

What are objects used for?

Answer 19

to group together data, functions, etc that are related to each other

create multiple groups of organized data
(are able to use same property names for multiple objects)

Question 20

What are object properties?

Answer 20

individually named data inside objects
(are variables essentially)

Question 21

Describe object literal notation.

Answer 21

object name = { };

object name = {
property name: property value,
property name: property value,
property name: property value
};

*example of key/value pair

Question 22

How do you remove a property from an object?

Answer 22

use delete.”object name”

delete operator

Question 23

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

Answer 23

use dot notation or bracket notation

objectname.objectpropertyname/key

objectname[“objectpropertyname.key”]

*bracket notation wont work for methods!
*bracket notation useful b/c in dot notation, js perceives anything after dot as a string
so variables, numbers etc cant work with dot notation.

Question 24

. notation code reading
[ ] notation code reading

Answer 24

. = “of”
[ ] = “at”

Question 25

What are arrays used for?

Answer 25

storing/grouping a set of related values
(helpful when you don’t know how many values there will be)

allows us to group together like data types and put into “lists”
so that they are organized and we can deal with them one at a time

Question 26

Describe array literal notation.

Answer 26

array name = [ ];

array name = [
“value”,
“value”,
value
];

*value can be any data type!

Question 27

How are arrays different from “plain” objects?

Answer 27

arrays have a order (index numbers; numerically indexed)
no property names (property names are basically the index numbers)
are able to know how many pieces of data is being stored (length property)
need to use method (push) to add data to array (not assign directly like objects)

Question 28

What number represents the first index of an array?

Answer 28

zero

Question 29

What is the length property of an array?

Answer 29

tells you the length of the array (how many values/items are in the array)
different from index numbers!!

arrayname.length

Question 30

How do you calculate the last index of an array?

Answer 30

can use length property to find length of the array and subtract 1 to get last index num of the array
(b/c index numbers start at 0 so the last index number will be length of array minus one)

Question 31

data models

Answer 31

objects in arrays
arrays in objects

organizing large chunks of data?

Question 32

What is a function in JavaScript?

Answer 32

a block of code that contains set of instructions that gets executed when the function is called

take in values –> spit out values based on values provided
make change to something, somewhere

Question 33

Describe the parts of a function definition.

Answer 33

function keyword, optional name, optional parameter(s), code block, optional return word (in code block)

Question 34

Describe the parts of a function call.

Answer 34

function name with arguments(may or may not have) in parentheses
*if no arguments, still need ( )

Question 35

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

Answer 35

function call: function name, arguments

function definition: function keyword, parameters, code block,

Question 36

What is the difference between a parameter and an argument?

Answer 36

parameter: placeholder for arguments

argument: what you actually put into function to execute the code

Question 37

Why are function parameters useful?

Answer 37

parameters give us the ability to have mutability based on data

don’t have to write multiple functions

gives us functionality

Question 38

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

Answer 38

Causes the function to produce a value we can use

Prevents any more code in the function’s code block from being run entirely.
(even if theres more code after return statement in the code block)

Question 39

an expression in JS

Answer 39

needs to be evaluated to reach a value

Question 40

Why do we log things to the console?

Answer 40

debugging tool

double check code, but soley to check

Question 41

What is a method?

Answer 41

a type of function

Question 42

How is a method different from any other function?

Answer 42

methods are attached to objects

Question 43

How do you remove the last element from an array?

Answer 43

pop( ) method

*will return that element you removed

Question 44

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

Answer 44

Math.floor ( )

Question 45

How do you generate a random number?

Answer 45

Math.random( );

0 to 1 (but 1 isn’t included)

Question 46

How do you delete an element from an array?

Answer 46

splice method

Question 47

How do you append an element to an array?

Answer 47

push method

*returns length of array
*append means to add at the end

Question 48

How do you break a string up into an array?

Answer 48

split method

*returns an array

Question 49

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

Answer 49

no they do not

can use console.log(“the string”) to check

Question 50

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

Answer 50

a lot!

Question 51

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

Answer 51

not always

Question 52

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

Answer 52

a lot!

Question 53

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

Answer 53

MDN

Question 54

Give 6 examples of comparison operators.

Answer 54

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

Question 55

What data type do comparison expressions evaluate to?

Answer 55

booleans

Question 56

What is the purpose of an if statement?

Answer 56

to check condition to decide whether to run a certain code block or not

Question 57

Is else required in order to use an if statement?

Answer 57

no

*if else is included means something will always happen
we dont necessarily want something to occur always

Question 58

Describe the syntax (structure) of an if statement.

Answer 58

if keyword, condition, code block

Question 59

What are the three logical operators?

Answer 59

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

Question 60

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

Answer 60

use “and” or “or”

Question 61

truthy values and falsey values (for conditionals)

Answer 61

truthy: assumed to have values
*infinte amount of truthy values (everything that is not truthy)

falsey: emptiness

*(not that many)
false, 0, empty strings, null, undefined, NaN

Question 62

What is the purpose of a loop?

Answer 62

a set of code can be run over and over again

gives us ability to repeat functionality

Question 63

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

Answer 63

if condition is met: loop continues
if condition is not met: loop stops

what decides when to stop a loop

Question 64

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

Answer 64

one time a loop code block is run

Question 65

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

Answer 65

before code block is executed (before each itieration)

Question 66

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

Answer 66

one time before everything

Question 67

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

Answer 67

during first iteration: after initialization
during every following iteration: right at start of loop (before code block runs) / after final expression

Question 68

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

Answer 68

before the condition runs again
after the code block runs

Question 69

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

Answer 69

break

Question 70

What does the ++ increment operator do?

Answer 70

adds one to the variable it is applied to (and saves that value in that variable)

*count++ : using count and then incrementing
*++count : incrementing count value before using

Question 71

How do you iterate through the keys of an object?

Answer 71

use for in loop

Question 72

Why do we log things to the console?

Answer 72

debugging, checking if certain code works properly. etc.

Question 73

What is a “model”?

Answer 73

representation of something

Question 74

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

Answer 74

the html document (document that represents the entire page)

Question 75

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

Answer 75

JS objects (b/c DOM tree is made of objects)

Document Object Model: is a JavaScript object modeling the HTML document’s content

Question 76

What is a DOM Tree?

Answer 76

form of organization of data

all data necessary to make a single element and all of it’s content (child elements, attributes, etc)

Question 77

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

Answer 77

getElementById( ), querySelector( )

*querySelector( ) use this!!!!!!

Question 78

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

Answer 78

getElementsByClassName( ), getElementsByTagName( ), querySelectorAll( )

Question 79

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

Answer 79

if you are going to use the return value of the DOM query again (multiple times)
don’t have to keep searching for it each time u use it

Question 80

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

Answer 80

console.dir( )

dir = directory

Question 81

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

Answer 81

HTML document loads code from top to bottom, so need to put at bottom so body element needs to load first

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

Question 82

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

Answer 82

argument: css selector

returns: first element with the argument css selector

Question 83

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

Answer 83

argument: css selector

returns: all elements with the argument css selector (in a NodeList)

Question 84

NodeList

Answer 84

looks like an array but is not the same

its an array-like object (but cant adjust like an array)

Question 85

What is the purpose of events and event handling?

Answer 85

events: to be aware when certain action happens

event handling: to allow us to do smt in response to event (interactiveness)

Question 86

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

Answer 86

no

Question 87

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

Answer 87

addEventListener

Question 88

What is a callback function?

Answer 88

a function passed as an argument in another function

Question 89

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

Answer 89

event object

a object js makes that holds all relevant info about that certain event (when it is fired)

Question 90

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

Answer 90

the place event occurred from

use inspect or mdn

Question 91

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

Answer 91

handleClick executes when event happens

handleClick( ) is being called right when page loads
code in function code block isn’t correct
and when you click button, returns undefined (so nothing happens)

Question 92

What is the className property of element objects?

Answer 92

property that contains value of class attribute of that element

Question 93

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

Answer 93

use className property to change value of the class attribute of that element

get element -> assign new value (using assignment operator)

Question 94

What is the textContent property of element objects?

Answer 94

property that contains text content of certain element

Question 95

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

Answer 95

use textContent property to change text content (using assignment operator)

Question 96

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

Answer 96

no

Question 97

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

Answer 97

more complicated

Question 98

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

Answer 98

easy access to elements w/o having to query/search for it everytime

data is readily accessible in the language we are writing
better to use data w/in language you are working with

Question 99

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

Answer 99

focus

Question 100

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

Answer 100

blur

Question 101

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

Answer 101

input

Question 102

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

Answer 102

submit

submit event is an event on FORM element, NOT submit button(submit button triggers event)

Question 103

What does the event.preventDefault() method do?

Answer 103

prevents default action for an event from being taken as it normally would be.

Question 104

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

Answer 104

automatically reloads page

preventDefault will always be present when using submit events

Question 105

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

Answer 105

elements

Question 106

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

Answer 106

value

Question 107

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

Answer 107

missing errors and having to now go through a lot of code to fix errors

not catching errors early and not being able to see clearly where error is from

Question 108

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

Answer 108

can see how certain code affects webpage/browser in live time
can check code right away

Question 109

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

Answer 109

no

Question 110

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

Answer 110

appendChild( ) method

Question 111

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

Answer 111

the attribute name and value

example: (“class”, “column-full”)

Question 112

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

Answer 112

create element, append element to parent element u want to using appendChild( )

Question 113

What is the textContent property of an element object for?

Answer 113

add/set text to element
check what text content of element is of an element

Question 114

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

Answer 114

element.className( )
element.setAttribute( )

Question 115

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

Answer 115

can reuse function:
call function again to make same structure
use function in multiple different contexts/situation

Question 116

What is the event.target?

Answer 116

property on event object that tells the event originated from (whereever the event occured)

Question 117

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

Answer 117

event bubbling

Question 118

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

Answer 118

tagName

nodeName(less specific)

Question 119

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

Answer 119

argument: css selector
return: closest ancestor that matches the argument css selector

*opposite of querySelector (b/c closest searches outward while querySelector searches within)

Question 120

How can you remove an element from the DOM?

Answer 120

remove( )

on dom object we want to remove

Question 121

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 121

add event listener to parent

Question 122

What is the event.target?

Answer 122

property on the event object which stores a ref to the dom element where event originated from

Question 123

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

Answer 123

element is no longer visible

removes element from document flow

Question 124

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

Answer 124

argument: css selector(s) as a string

return: boolean stating whether the argument selector(s) match the ones of the element (true)
(false) if the argument selectors do not match those of the element

Question 125

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

Answer 125

element.getAttribute( )

attribute name as a string goes in ( ) and the return is the value of that attribute

Question 126

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

Answer 126

as often as possible!

anytime u need to check anything

Question 127

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 127

need to add individual event listeners to each tab and view

Question 128

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 128

long conditional block
conditional blocks for every single view

Question 129

What is JSON?

Answer 129

JavaScript Object Notation

format for rendering data that follows norms of a JS object

*cant store objects or array long term but u can w/ strings so we utilize this

Question 130

What are serialization and deserialization?

Answer 130

serialization: get data and turn into one piece of data to store easily (ease of analysis) (packing stuff in one suitcase for trip)

deserialization: get data that is serialized and turn back into more complicated pieces of data that is easily accessible and can grow (might not all be in once place)
(unpacking everything after trip (so that everything is more accessible than keeping in suitcase)

Question 131

Why are serialization and deserialization useful?

Answer 131

serialization: data good for long term storage and transmission

deserialization: good for interaction with data

Question 132

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

Answer 132

JSON.stringify( )

Question 133

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

Answer 133

JSON.parse( )

Question 134

How do you store data in localStorage?

Answer 134

localStorage.setItem( )

argument: keyname , keyvalue

Question 135

How do you retrieve data from localStorage?

Answer 135

localStorage.getItem( )

arguement: is the keyname as a string

Question 136

What data type can localStorage save in the browser?

Answer 136

strings (thats why we learned JSON)

Question 137

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

Answer 137

right before window document is about to be unloaded

close tab, refresh tab, close browser, browser crashes

Question 138

What is a method?

Answer 138

function that is stored as a a property of an object

Question 139

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

Answer 139

method def:: has function code block, has colon, and ‘function’ keyword

method call: object name, arguments, property name

Question 140

Describe method definition syntax (structure).

Answer 140

object name {
property name: function def(can have name or not) (parameter(s)) {
//function code block
}
};

Question 141

Describe method call syntax (structure).

Answer 141

object.method(arguments);

Question 142

How is a method different from any other function?

Answer 142

methods are attatched to an object
need to state object to use methods

Question 143

What is the defining characteristic of Object-Oriented Programming?

Answer 143

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

Question 144

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

Answer 144

abstraction, encapsulation, inheritance, polymorphism

Question 145

What is “abstraction”?

Answer 145

to being able to work with (possibly) complex things in simple ways
process of reducing complexity

Question 146

What does API stand for?

Answer 146

application programming interface

Question 147

What is the purpose of an API?

Answer 147

to give programmers a way to interact with a system in a simplified, consistent fashion: aka, an abstraction

Question 148

What is this in JavaScript?

Answer 148

implicit parameter that allows us to view object from where this method is being used

Question 149

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

Answer 149

always there, even if you don’t write it in parameter list
are able to get value
implied value that is always present

Question 150

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

Answer 150

call time!!!

Question 151

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 151

value of this: nothing!

greet function has not been called yet, ‘this’ is determined during call time, NOT definition time

Question 152

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

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

Answer 152

greet function is now being called, so ‘this’ is:
It’s-a-me, Mario!

Question 153

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

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

Answer 153

It’s-a-me, undefined!

hello() is a free floating function with no object to the left, so the object is the window.
window object does not have a property named firstName, so firstName is undefined.

Question 154

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

Answer 154

No, w/in definition, cant guarantee ‘this’ will be anything (this is a parameter/placeholder)

Question 155

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

Answer 155

whatever is to the left of the dot of method is value of ‘this’
*no dot, then being executed in global scope: so object is window!

Question 156

What kind of inheritance does the JavaScript programming language use?

Answer 156

prototype-based (or prototypal) inheritance.

Question 157

What is a prototype in JavaScript?

Answer 157

object that contains properties that can be used by other objects

a tool that can be passed to many sources to use

Question 158

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 158

they have a prototype that has all these methods (where shared behavior is stored)

Question 159

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

Answer 159

prototype (if multiple prototypes (prototypal chain), will search from closest to farthest (linear))

Question 160

What does the new operator do?

Answer 160

create a empty js object, takes (constructor) function being called’s prototype (prototype property) and adds to empty object( __proto__ or [[prototype]] ), makes empty obj the value of ‘this’ & function codeblock is run, returns object if no other return is specified

constructor functions dont have return values
allows us to invoke a function as a constructor function

Question 161

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

Answer 161

prototype

Question 162

What does the instanceof operator do?

Answer 162

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

*object instanceof constructor

Question 163

What is a “callback” function?

Answer 163

fucntion that is passed as an argument to another function

Question 164

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 164

setTimeout( )

Question 165

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

Answer 165

setInterval( )

Question 166

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

Answer 166

0

does not mean action happens immediately, is put in queue immediately

Question 167

What do setTimeout() and setInterval() return?

Answer 167

interval ID

Question 168

What is AJAX?

Answer 168

programming practice of building complex, dynamic webpages using a technology known as XMLHttpRequest.
Ajax allows you to update parts of the DOM of an HTML page without the need for a full page refresh. Ajax also lets you work asynchronously, meaning your code continues to run while the targeted part of your web page is trying to reload (compared to synchronously, which blocks your code from running until that part of your page is done reloading).

technique for loading data into a webpage w/o having to reload entire webpage (just reload parts of webpage)
don’t have to go to other pages

Question 169

What does the AJAX acronym stand for?

Answer 169

Asynchronous JavaScript And XML

Question 170

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

Answer 170

XMLHttpRequest

Question 171

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

Answer 171

load event

Question 172

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

Answer 172

XMLHttpRequest is a JS object so addEventListener method can be applied

Question 173

ES6 ↓

Question 174

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

Answer 174

block of code written inbetween curly braces

examples: loops, functions, conditionals

Question 175

What does block scope mean?

Answer 175

only exists within a certain code block

so cant access variables declared in block, outside of block

Question 176

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

Answer 176

block scope
so variables only exist w/in that code block

*var is function scope, not block scope! (lives/exists past the code block)

Question 177

What is the difference between let and const?

Answer 177

let can be reassigned (mutable)
const cannot be reassigned (read only reference; immutable)

Question 178

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

Answer 178

the actual value to which the const variable reference is not immutable
even though the Array variable is a constant, you can change its values.
however, you cannot reassign a different value to the Array constant itself

Question 179

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

Answer 179

prefer const
(use const until u cant!)

if variable is not being reassigned at anytime in code, should use const
if not, should use let

Question 180

What is the syntax for writing a template literal?

Answer 180

same as strings but w/ backticks and not ‘ ‘ or “ “
assign w/ const variable
${expression} if u want to add JS expression into template literal

Question 181

What is “string interpolation”?

Answer 181

to create strings by doing substitution of placeholders (syntax: ${})

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

Question 182

What is destructuring, conceptually?

Answer 182

assigning properties of an object or array into individual variables.

extracting property values from objects and array element values into new variables

Question 183

What is the syntax for Object destructuring?

Answer 183

var/let/const { property1: variable1, property2, etc } = object;

if u want to assign a property value to a variable thats not the same name as property key name, use colon, if not can just put the property key name (like example above)

Question 184

What is the syntax for Array destructuring?

Answer 184

const [variable1, variable2, variable3, etc] = array;

variable names correspond to elements in array

Question 185

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

Answer 185

curly braces or square bracket will be on left side of = if its destructuring

object/array name we are destructuring from is on right side of assignment operator and not on the left

Question 186

What is the syntax for defining an arrow function?

Answer 186

parameter(s) => code;
*if more than one parameter, need ( )
* if more than one statement in code block, need { }

(param1, param2, etc) => {
return;
};

param => expression;

( ) => expression;

Question 187

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

Answer 187

evaluated as an expression rather than a statement and has implicit return

Question 188

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

Answer 188

lexical scope, takes “this” value from the enclosing function (“parent” function, function that encloses the arrow function)

“this” value is defined when arrow function is DEFINED, and not CALLED like es5!!
opposites!!

Question 189

What is a CLI?

Answer 189

command line interface
text based system to interact w/ program

Question 190

What is a GUI?

Answer 190

graphical user interface
interact w/ program w/ icons instead of text

Question 191

Give at least one use case for each of the commands listed in this exercise.
man
cat
ls
pwd
echo
touch
mkdir
mv
rm
cp

Answer 191

man: look up manuals for command line commands to see how they work and what options they have
cat: concatenate files
ls: list contents of directories
pwd: prints full file name of current/working directory (where am i right now in my system?)
echo: displays line of text (console.log for terminal kinda)
touch: change/update files’ timestamps (but use mostly to create a file)
mkdir: make a directory
mv: move or rename a file or directory
rm: remove a file or directory (be careful!!)
cp: copy a file or directory

Question 192

What are the three virtues of a great programmer?

Answer 192

laziness, impatience, hubris

Question 193

What is Node.js?

Answer 193

a program that allows JavaScript to be run outside of a web browser

ex: you have some JS in a file, can use node.js to run that JS in file

Question 194

What can Node.js be used for?

Answer 194

used to build back ends for Web applications, command-line programs, or any kind of automation that developers wish to perform.

Question 195

What is a REPL?

Answer 195

read–eval–print loop
a simple interactive computer programming environment that takes single user inputs, executes them, and returns the result to the user

ex: browser console, bash shell

Question 196

When was Node.js created?

Answer 196

2009

Question 197

What back end languages have you heard of?

Answer 197

scripting languages (interpreted on-the-fly, garbage collection, dynamic, safer): python, ruby (jit), php, javascript (jit), perl, elixir, erlang
*jit: just in time compilation (interpreter is very sophisticated)

memory managed languages (compiled ahead of time, has garbage collection): java, kotlin c#, golang, haskell, julia, f#, fortan

manual memory management (compiled ahead of time, no garbage collection, low-level, hard but fast, dangerous if done wrong): c++, c, swift, rust, d, zig

sql (for databases)

Question 198

What is the process object in a Node.js program?

Answer 198

a global obj that provides information about, and control over, the current Node.js process

Question 199

How do you access the process object in a Node.js program?

Answer 199

‘process’
always available to Node.js applications

Question 200

What is the data type of process.argv in Node.js?

Answer 200

array of strings

Question 201

What is a JavaScript module?

Answer 201

a single JS file (in JS context)

Question 202

What values are passed into a Node.js module’s local scope?

Answer 202

require, module, exports, _filename, _dirname

Question 203

Give two examples of truly global variables in a Node.js program.

Answer 203

process, console

Question 204

What is the purpose of module.exports in a Node.js module?

Answer 204

require () returns content in module.exports object. So need to put content we want to use in other modules into module.exports in order to access

tells node.js which code to export and use in other modules

Question 205

How do you import functionality into a Node.js module from another Node.js module?

Answer 205

use require( ) (pass in relative path to other module we want to access)
assign to variable

return value is value that is assigned to module.exports

Question 206

What is the JavaScript Event Loop?

Answer 206

grabs tasks from task queue and pushes onto call stack when call stack is empty
collecting and processing events that is in queue

watches task queue and call stack, when call stack is clear, pushes tasks from task queue onto call stack

Question 207

What is different between “blocking” and “non-blocking” with respect to how code is executed?

Answer 207

blocking: stops future code from executing (synchronous)
occupies call stack (nothing can run while that function is being run (is on call stack))

non-blocking: asynchronous

Question 208

What is a directory?

Answer 208

file system object that lists other files and directories
computer “folder”

Question 209

What is a relative file path?

Answer 209

path to a file from where you are currently

Question 210

What is an absolute file path?

Answer 210

path from the root file system to the target file path

root starts with a /

Question 211

What module does Node.js include for manipulating the file system?

Answer 211

fs (file system)

Question 212

What method is available in the Node.js fs module for writing data to a file?

Answer 212

writeFile

Question 213

Are file operations using the fs module synchronous or asynchronous?

Answer 213

both!

Question 214

What is NPM?

Answer 214

npm is the world’s largest software registry.
npm consists of three distinct components:

the website
the Command Line Interface (CLI)
the registry

Question 215

What is a package?

Answer 215

reusable code + package.json file

directory w/ one of more files that also includes a package.json file

Question 216

How can you create a package.json with npm?

Answer 216

npm init (can include –yes at end if you dont want survey; want default package.json)

Question 217

What is a dependency and how to you add one to a package?

Answer 217

a package your project relies on in order to function properly
third party code or your code in another file etc. that your application requires in order to function

npm install command

Question 218

What happens when you add a dependency to a package with npm?

Answer 218

downloads package from npm registry and puts into node_modules directory
dependencies property of package json is added

node_modules directory is installed

Question 219

How do you add express to your package dependencies?

Answer 219

npm install express

Question 220

What Express application method starts the server and binds it to a network PORT?

Answer 220

listen()

Question 221

How do you mount a middleware with an Express application?

Answer 221

app.use( )

what is used to tell express what to do when you receive request or response

Question 222

Which objects does an Express application pass to your middleware to manage the request/response lifecycle of the server?

Answer 222

request obj: data model of http request message (FROM the client)
response obj: data model of the http response message (TO the client)

next

Question 223

What does the express.json() middleware do and when would you need it?

Answer 223

returns a middleware that takes in request bodies in json format and parses it and puts in on request body as an obj

Question 224

What is the significance of an HTTP request’s method?

Answer 224

indicates desired action

a rest api
get, post, delete, updating (put, casche)

Question 225

What are the three states a Promise can be in?

Answer 225

pending, fulfilled, rejected

Question 226

How do you handle the fulfillment of a Promise?

Answer 226

using .then()

passing the return value of promise into then

Question 227

How do you handle the rejection of a Promise?

Answer 227

using .catch()
passing rejection callback function into catch

Question 228

What is Array.prototype.filter useful for?

Answer 228

filters out specific elements from one array (that fit a certain condition) into a separate array

Question 229

What is Array.prototype.map useful for

Answer 229

when you want to change all values in an entire array
transformation of an entire array

formatting strings

Question 230

What is Array.prototype.reduce useful for?

Answer 230

incrementally reduces a whole array of values into a single value

kinda like a js aggreagate function
can create an object w/ array of values
can get array of values and create a lookup obj

Question 231

What is “syntactic sugar”?

Answer 231

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
Syntactic sugar is usually a shorthand for a common operation that could also be expressed in an alternate, more verbose, form

Question 232

What is the typeof an ES6 class?

Answer 232

function! not an object

Question 233

Describe ES6 class syntax.

Answer 233

classkeyword nameofclass {

constructor(parameters) {
code block;
this.smt = “smt”;
}

prototypemethodname (parameters) {
code block;
}
etc.

}

*constructor is optional

Question 234

What is “refactoring”?

Answer 234

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.

changing code structure w/o changing behavior

Question 235

class {}

Answer 235

the {} is the class body!

Question 236

class constructor method

Answer 236

is what runs when you use “new” to create object!!

only necessary when you want to assign properties during object initialization
when you want to use “this”?

Question 237

What is Webpack?

Answer 237

static module bundler for modern JavaScript applications.

webpack is a module bundler:
many modules ==> one module that can be loaded in the browser(usually main.js?)

*used for frontend dev?

Question 238

How do you add a devDependency to a package?

Answer 238

npm install –save-dev packagenames
npm i -D packagenames

*need the –save-dev or -D

dev dependencies help devs write/deliver the code but are not required to run the code (in production)
need during development time!!

Question 239

What is an NPM script?

Answer 239

property in package.json where the property name is any name you want, and value is a command line command

a convenient way to bundle common shell commands for your project.
They are typically commands, or a string of commands, which would normally be entered at the command line in order to do something with your application.

Question 240

How do you execute Webpack with npm run?

Answer 240

npm run propetynamewherewebpackisavalue

for webpack-intro exercise, it would be npm run build

Question 241

How are ES Modules different from CommonJS modules?

Answer 241

es: standardized module system for JS, import/export syntax

commonJS; default for node.js, require/module.export syntax

Question 242

What kind of modules can Webpack support?

Answer 242

ECMAScript, commonJS, AMD

Question 243

“default” in ES Modules

Answer 243

can only be used once per module

export default ‘myValue’

dont need curly braces

Question 244

named exports

Answer 244

many per module

grouped variables in curly braces
(like this)
export {
noop,
toArray,
createElement
}

exported declarations
like functions, variables, classes

export const foo = ‘bar’
export function myFunction() {}
export class Student {}

*imported by their exact declared name and curly braces are mandatory

Question 245

node module resolution

Answer 245

if you put a directory as file name for export or import, node.js system will automatically look for a index file to look into

es6-modules exercise: import { createElement, noop } from ‘../lib
same as import { createElement, noop } from ‘../lib/index

Question 246

inheritance extends and super in classes

Answer 246

need to use super() in constructor function to state parent class’s constructor
so watever was in parent class constructor function, you state in super() before adding more things to “this”

think to person and instructor example

Question 247

What does express.static() return?

Answer 247

a function that serves static files

Question 248

What is the local __dirname variable in a Node.js module?

Answer 248

tells us the absolute path of the current module’s directory

Question 249

What does the join() method of Node’s path module do?

Answer 249

joins all given path segments together using the platform-specific separator(/ or ) as a delimiter, then normalizes the resulting path.

Question 250

What does fetch() return?

Answer 250

a promise that will be fulfilled once response is avaliable

Then the promise resolves to the Response object representing the response to your request.

Question 251

What is the default request method used by fetch()?

Answer 251

get

Question 252

How do you specify the request method (GET, POST, etc.) when calling fetch?

Answer 252

fetch (‘url’, {method: writethemethod});

second argument for fetch() is where you specify

Question 253

response.json()

Answer 253

the result is not JSON!!!
parses JSON into a JS object!

Question 254

When does React call a component’s componentDidMount method?

Answer 254

after the first successful render (lifecycle diagram shows this)

Question 255

Name three React.Component lifecycle methods.

Answer 255

componentDIdMount, componentDidUpdate, componentWillUnmount, constructor, render

Question 256

How do you pass data to a child component?

Answer 256

through props

Question 257

for asynchronous errors in express

Answer 257

need to catch w/ next(err)

Question 258

What does the acronym LIFO mean?

(data structures stacks)

Answer 258

last in, first out

Question 259

What methods are available on a Stack data structure?

Answer 259

pop(), push(value) (most important)

peek(),

print() (for debugging purposes)

Question 260

What must you do to access the value at an arbitrary point in a stack (not just the “top”)?

Answer 260

pop() each value off of stack one by one until you reach the point you want to reach

Question 261

What does the acronym FIFO mean?

data structures (queue)

Answer 261

first in, first out

Question 262

What methods are available on a Queue data structure?

Answer 262

dequeue, enqueue

Question 263

What must you do to access the value at an arbitrary point in a queue (not just the “front”)?

Answer 263

dequeue values until the pt u want to access is at the front

Question 264

How are linked lists different from an array?

Answer 264

arrays: random access (in order to go to a specific place in the list, you DONT have to start at the beginning, then jump from node to node until you get there)

linked lists: sequential access (in order to go to a specific place in the list, you have to start at the beginning, then jump from node to node until you get there), doesn’t need to be mutated to scan its contents

Question 265

How would you access an arbitrary node in a linked list (not just the “head”)?

Answer 265

traverse the list (start at head and use next property until to get to where you want)

Question 266

linked lists

Answer 266

way of organizing list of data (not the most convenient)

why they exist: due to a problem low level programmers have to deal with?

Question 267

What must the return value of myFunction be if the following expression is possible?

myFunction( ) ( );

Answer 267

a function
myFunction get called, returns a function which gets called with second parentheses

Question 268

What does this code do?

const wrap = value => ( ) => value;

Answer 268

function wrap(value) {
return function ( ) {
return value;
}
}

this is what is happening above (arrow function that returns another arrow function)

Question 269

In JavaScript, when is a function’s scope determined; when it is called or when it is defined?

Answer 269

when it is defined (called lexical scope; ‘where it was written’)

Question 270

What allows JavaScript functions to “remember” values from their surroundings?