Variables, Types, and Collections

Question 1

8 data types

Answer 1

string, bigint, symbol, object, null, and undefined

Question 2

let and const

Answer 2

block scoped

block = anything inside curly braces

Question 3

when we don’t assign a value to a variable

Answer 3

undefined

except with const (you will get SyntaxError: missing initializer in const declaration)

Question 4

check typeof

Answer 4

if (typeof role == ‘undefined’)

Question 5

which three data types have wrapper functions?

Answer 5

Boolean, Number, String

Question 6

which two datatypes represent absence of data?

Answer 6

null, undefined

Question 7

typecasting methods

Answer 7

Number()
String()
Boolean()
toString() - used to covered a number to a string
parseInt() - parse a string argument to an integer or NaN
parseFloat() - method used to parse an argument to floating point

Question 8

array construction

Answer 8

var brands = ["Fender", "VOX", "Line6"]
var brands = new Array("Fender", "VOX", "Line6")

Question 9

Number type in JS

Answer 9

represents any number between -(2^53 - 1) and (2^53 - 1)

Question 10

most common string methods

Answer 10

toLowerCase()
toUpperCase()

concat(str [,…str]) - combines two or more strings and returns a new string
includes(searchString, [,position])
indexOf(searchValue [, fromIndex]) = returns the index of a value or -1 if not found
replace(searchFor, replaceWith)
substring(indexStart [, indexEnd]) - the substring() method returns a part of the string between the start and end indexes or the start index to the end of the string
search()
padStart() & padEnd()
trim()

Question 11

common methods for Number object

Answer 11

isNaN() - static method can be used to determine whether a value is NaN

isInteger()

parseFloat(str) & parseInt(str [,radix])

toFixed(digits) - returns number that has the specified number of digits after the decimal point

toString([radix])

valueOf() - returns primitive value of specified number object

Question 12

common date methods

Answer 12

now() = returns current time in milliseconds
parse() = parses a date string and returns milliseconds
UTC() = returns milliseconds in UTC
Get Methods (getDay, getMonth, getHours, etc)
Set Methods (setFullYear, setMinutes, setSeconds, etc)
UTC Methods (getUTCDate, setUTCdate)

toString() - returns a string value of a date object
toDateString() - returns the date value of the date object without the time in a string format
toUTCString() - converts a date to a string using the UTC time zone
toISOString() - convert UTC time into a simplified ISO-8601 format
toLocaleDateString() - return a date value or parts of date in a specified locale
getDate / setDate

Question 13

type coercion with different operators

Answer 13

== or !=
coerces primitives to numbers except for null and undefined

comparison operators
coerces primitives to numbers

logical operators
coerce primitives to boolean

Question 14

type coercion with other primitives

Answer 14

symbol: evaluated as true in boolean coercion / cannot be implicitly coerced to a string or number
null: evaluated as false / 0 / null (string coercion)
undefined: false / NaN in numeric / undefined in string

NaN: false / NaN in string coercion

Question 15

falsy values

Answer 15

false
0
-0
0n (BigInt 0)
''
null
undefined
NaN

Question 16

double bang

Answer 16

coerces to the equivalent boolean value

Question 17

for in loop
vs.
for of loop

Answer 17

for in loop - when concerned with index

for of loop - for simplicity when you are just trying to access the values

Question 18

adding elements to arrays

Answer 18

push() - add one or more element to the end of an array; returns the new length of the array

unshift() - add one or more elements to the beginning of array; returns new length

splice() - allows modifying the contents of an array; used to add elements starting from a specific condition

Question 19

removing array elements

Answer 19

pop() - can be used to remove the last element from an array, returns removed element

shift() - remove first element; returns removed element

splice() - used to remove or replace, returns an array containing the removed elements

Question 20

SLICE()

Answer 20

used to make a copy of an array or a portion of an array; does not change original array

Question 21

finding array elements

Answer 21

includes()
indexOf()
lastIndexOf() - returns the last index of the given element in an array
findIndex() - returns first index that satisfies the given condition
find() - returns the value of the first element that satisfies the condition; undefined otherwise

Question 22

advanced data manipulation

Answer 22

map() - creates a new array based on a callback function that is invoked for every element of the original array

filter() - create a new array that contains elements which pass the given condition in a callback function

reduce() - uses the specified reducer function to return a single output value based on the reduction

flat() - used to create a new array containing all the sub-array elements concatenated into the original array recursively up to the specified path

every() - used to check whether all the elements of an array pass the given condition

some() - used to check if at least one element of an array passes the given condition in a callback function

Question 23

spread / rest operator

Answer 23

…

spreads the elements of array instead of the array itself

Question 24

in json…properties and any strings….

Answer 24

are enclosed within double quotes

Question 25

how do we convert a js object to json

Answer 25

stringify

SERIALIZED

Question 26

how to access JSON object like a JS object

Answer 26

parse()

DESERIALIZE

Question 27

how to iterate over data received

Answer 27

for in loop to iterative over properties

or object.entries (gets an array of key:value pairs)

Question 28

functions can be…

Answer 28

assigned to a variable
stored in an array
passed and returned
property of an object (method)

Question 29

6 types of functions

Answer 29

named functions
anonymous function
recursive function
outer function
inner function
immediately invoked (IIFE)

Question 30

IIFE

Answer 30

put parenthesis immediately after defining it

Question 31

arrow function considerations

Answer 31

this is treated differently

traditional functions: the this keyword is set to the object that invoked the function; otherwise, the this keyword defaults to the global variable

arrow functions: does not default to the global object; the value of this is determined by the context of where the function is called

Question 32

closure

Answer 32

allows code to access parent scope even after code is finished

Question 33

apply, call, bind

Answer 33

apply() - invokes a function and assigns the object passed in to the keyword ‘this’. arguments for the function are passed in as an array.

call() - invokes a function and assigns the object passed in to “this”. arguments are passed in as a comma separated list.

bind() - returns a new function; the object passed in is this when the function is invoked; arguments may be bound to the function as well

Question 34

object class methods

Answer 34

assign() - used to copy all the enumerable own properties from one or more source objects to a target object

entries() - can be used to get an array of all the key-value pairs of an object’s own enumerable string properties

values() - get array of all obj’s values

defineProperties() - define new or modify existing (also, defineProperty for just one)

freeze() - changes can no longer be made

preventExtensions() - prevents the addition of new properties

seal() - prevents new properties and deletions

is() - used to determine whether two values are the same

Question 35

object instance methods

Answer 35

hasOwnPropertyOf()
isPrototypeOf()
toString()
valueOf()