JavaScript & TypeScript

Question 1

add an array to an existing array in JS

Answer 1

«array1».push(…«array2»)

Question 2

deep copy a string to another const in JS

Answer 2

const «variable name» = (‘ ‘ + «string»).slice(1)

Question 3

what are the primitive types in TS?

Answer 3

» undefined
» null
» any
» boolean
» number
» string

Question 4

declare a variable’s type in TS

Answer 4

«variable»: «type»

Question 5

what are the complex types in TS?

Answer 5

» void
» never
» Array
» object
» enum
» tuple

Question 6

declare an enum in TS

Answer 6

enum Color {
  Red _= 1,
  Green _= 2,
  Blue _= 4
}

let c: Color = Color.Green
console.log( Color[2] )

» numbering starts at 0 as a default, set first number to change

Question 7

declare an array in TS

Answer 7

let fruits: string[] = [‘apple’, ‘orange’, ‘banana’]

let fruits: Array = [‘apple’, ‘orange’, ‘banana’]

Question 8

declare a tuple in TS

Answer 8

let coordinates: [number, number]

Question 9

when to declare type void in TS?

Answer 9

function warnUser(): void {
  console.log('This is my warning message');
}

» for functions that don’t return a value

Question 10

when to declare type never in TS?

Answer 10

function error(message: string): never {
  throw new Error(message);
}

» for functions that always throw errors
------------------------
function infiniteLoop(): never {
  while (true) {}
}

» for functions that never return

Question 11

cast variable as type in TS

Answer 11

«variable» as «type»

» only AS syntax is valid in JSX

Question 12

what is an interface in TS?

Answer 12

» a name for a data structure

Question 13

declare an interface in TS

Answer 13

interface «object interface name» {
  «prop1»: «type»
  «prop2»?: «type»   » optional
  readonly «prop3»: «type»   » readonly
}

» readonly value is set only in assignment

Question 14

declare a function interface in TS

Answer 14

interface «function name» {
(«param1»: «type», «param2»: «type»): «return type»
}

» parameter names do not need to match the implementation names

Question 15

declare an indexable object interface in TS

Answer 15

interface «object name» {
_readonly [«index name»: number | string]: «index value type»
«_another prop1»: «property type»
}

» property type must be subset of index value type
» readonly value is only set in assignment

Question 16

interpolate string in JS

Answer 16

I am ${«age»} years old

Question 17

parse JSON string in JS

Answer 17

JSON.parse(«string»)

Question 18

transform JSON object to string in JS

Answer 18

JSON.stringify(«json»)

Question 19

difference between normal functions and arrow functions in JS

Answer 19

» arrow functions don’t bind their «this» value

Question 20

get first array element that matches a condition in JS

Answer 20

«array».find(«condition»)

» stops on finding the first matching element

Question 21

execute function with a delay in JS

Answer 21

setTimeout(«callback», «milliseconds»)

Question 22

set object property using object property shorthand in JS

Answer 22

const name = 'Edvard'
const years = '32'

const user = {
name
age: years
}

Question 23

destructure JS object property

Answer 23

const { «property» } = «object»

Question 24

destructure and name JS object property

Answer 24

const { «property»: «new name» } = «object»

Question 25

destructure JS object property and set default value

Answer 25

const { «property» = «default value» } = «object»

Question 26

destructure function parameter(s) from a JS object

Answer 26

function «name»({ «param1», «_param2» }) {
  ...
}

Question 27

extract keys of object into an array in JS

Answer 27

Object.keys(«object»)

Question 28

check if an array contains a specific element in JS

Answer 28

«array».includes(«element»)

Question 29

check if a string contains a specific substring in JS

Answer 29

«string».includes(«substring»)

Question 30

check if every array element passes a specific test in JS

Answer 30

«array».every((«element») => «test result»)

Question 31

replace part of string in JS

Answer 31

«string».replace(«to replace», «replacement»)

» replaces only first match

Question 32

remove property from object in JS

Answer 32

delete «object».«property»

Question 33

manipulate JSON.stringify() output of object in JS

Answer 33

«object».toJSON(() => {
  const value = this
  ...
  return «return value»
})

Question 34

convert string to number in JS

Answer 34

parseInt(«string»)

parseFloat(«string»)

Question 35

naming convention for DOM elements in JS

Answer 35

$«name»

» $button = document.querySelector(‘button’)

Question 36

disable DOM element in JS

Answer 36

«element».setAttribute(‘disabled’, ‘disabled’)

Question 37

enable DOM element in JS

Answer 37

«element».removeAttribute(‘disabled’)

Question 38

focus DOM element in JS

Answer 38

«element».focus()

Question 39

remove leading an tailing whitspaces and newlines from string in JS

Answer 39

«string».trim()

Question 40

remove element from array by its index in JS

Answer 40

«array».splice(«index», «count»)

» returns array of removed objects

Question 41

get array index of element that complies a criteria in JS

Answer 41

«array».findIndex(«check function»)

Question 42

combine array elements in JS

Answer 42

«array».reduce((accumulator, currentValue) => {
…
return «value»
})

» accumulator initiates as first array element
» currentValue initiates as second array element

Question 43

check if one or more array elements comply a criteria in JS

Answer 43

«array».some(«check function»)

» returns true or false

Question 44

destructure array value(s) to variables in JS

Answer 44

const [«var0», «_var1»] = «array»

Question 45

get array index of specific element in JS

Answer 45

«array».indexOf(«element»)

Question 46

ignore specific TS error

Answer 46

//@ts-ignore
«line of code with error»

Question 47

round number in JS

Answer 47

Math.round(«number»)

Question 48

format date to German date string in JS

Answer 48

«date».toLocaleDateString(‘de-DE’)