4 - Data Structures: Objects and Arrays

Question 1

How to access a property of an object?

Answer 1

Using the bracket, or a dot notation.

Question 2

What do you retrieve by accessing a property?

Answer 2

Value that property references.

Question 3

Which objects don’t have properties?

Answer 3

‘null’ and ‘undefined’

Question 4

What is the return value of typeof null?

Answer 4

‘object’

Question 5

What is a requirement for a property name so that you can access it with a dot notation?

Answer 5

It must follow the rules of the bindings.

Question 6

What property names do you use to retrieve values from an array?

Answer 6

Numbers. They start with ‘0’.

Question 7

What is a method?

Answer 7

Property on an object that references a function value.

Question 8

Conceptually, what kind of a data structure is an array?

not in the JS context

Answer 8

Stack. LIFO

Question 9

What is a JS object in regard to what it contains?

Answer 9

Arbitrary collections of properties.

Question 10

How to syntactically write a property name of an object which doesn’t adhere to the rules of naming a binding?

Answer 10

Using quotes.

Question 11

What purposes have braces in general?

Answer 11

To use for a set of statements and a notation for an object literal.

Question 12

What will be returned when you access a property that doesn’t exist on the object?

Answer 12

‘undefined’

Question 13

How to remove a property from an object?

Answer 13

delete obj.foo

Question 14

How to check if an object has a property?

Answer 14

‘a’ in object

Question 15

How to find all properties of an object?

Answer 15

Object.keys(obj)

Question 16

How to copy all properties from one object to another?

Answer 16

Object.assign(toObj, fromObj)

Question 17

What is the return value of ‘typeof []’

Answer 17

object

Question 18

Which data types are not mutable?

Answer 18

Strings, numbers, booleans, null and undefined

Question 19

How to compare if two bindings have reference to the same object?

Answer 19

let o1 = { foo: 1 }
let o2 = o1;
o1 == o2 // true

Question 20

Is this valid const o = { a: 1 }; o.b = 2? Why?

Answer 20

Yes.

It mutates the elements of an object and not the assignment.

Question 21

How is the comparison with == of two different objects called?

Answer 21

Identity comparison.

Question 22

How to write classical for loop in a easier way?

Answer 22

for (let elem of collection) {

for…of

Question 23

What other objects can you loop over with for..of loop?

Answer 23

Strings, objects, but iterables in general.

Question 24

What do ‘unshift’ and ‘push’ do?

Answer 24

unshift -> add element from the left
push -> add element from the right

Question 25

What do 'shift' and 'pop' do?

Answer 25

shift -> remove element on the left pop -> remove element on the right

Question 26

What does indexOf do?

Answer 26

Finds the index of the first matching element. [1,2,3].indexOf(2) => 3

Question 27

What do indexOf and lastIndexOf return when they can't find an element?

Answer 27

-1

Question 28

What does lastIndexOf do?

Answer 28

Finds the index of the first matching element searching from the right. [1,2,3].indexOf(2) => 1

Question 29

How Array.slice behaves?

Answer 29

It creates a new array from the one it was called on. [1, 2, 3, 4, 5].slice(2) => [3, 4, 5] [1, 2, 3, 4, 5].slice(2, 4) => [3, 4]

Question 30

How many elements Array.slice takes? Which element is inclusive, and which is exclusive?

Answer 30

Two elements. First one is inclusive, the second one is exclusive.

Question 31

Will this work '[1, 2, 3, 4, 5].concat(3)'?

Answer 31

Yes. | It can concatenate non-array values.

Question 32

How to join two arrays using a Array function?

Answer 32

Array.concat.

Question 33

Why can't you add a new property to a string?

Answer 33

Because strings are not objects.

Question 34

Why do strings have properties even though they are immutable?

Answer 34

Because they inherit properties from String.prototype.

Question 35

Do strings have .slice and .indexOf properties?

Answer 35

Yes.

Question 36

What does String.trim do?

Answer 36

Removes whitespace from the start and end of the string.

Question 37

How does String.padStart work?

Answer 37

'6'.padStart(4, '1') // => '1116'

Question 38

How does String.split work?

Answer 38

It splits the string on a given char.

Question 39

How does String.join work?

Answer 39

It doesn't exist.

Question 40

How to repeat a string 'foo' three times?

Answer 40

With String.repeat => 'foo'.repeat(3)

Question 41

How can you access letter 'x' in a string '000x00'?

Answer 41

'000x00'[3]

Question 42

How can you "collect" multiple arguments to a function to a single parameter?

Answer 42

With rest param. function foo(...args)

Question 43

If you have an array and want to pass its elements to a function as arguments, how do you do it?

Answer 43

foo(...ary)

Question 44

Why is spread operator called so?

Answer 44

Because it "spreads" elements of an array in place as separate values.

Question 45

What is the Math object by itself technically? Why is it needed?

Answer 45

It acts as a namespace for Math.sum, Math.sqrt etc so that the global namespace isn't polluted by those functions.

Question 46

What does it mean to "pollute" a global namespace?

Answer 46

Global namespace is polluted when bindings are defined in the outer most scope. It increases the likelihood of overriding some binding.

Question 47

How does Javascript warn you when you redeclare bindings created with var and function keyword?

Answer 47

It doesn't.

Question 48

What keywords do you have to use so that Javascript warns you when you redeclare a binding?

Answer 48

let and const

Question 49

What is the return value of Math.random?

Answer 49

A pseudorandom number between (inclusive) zero and (exclusive) one.

Question 50

How are random numbers conceptually created considering that computers are deterministic machines?

Answer 50

By creating a hidden value which gets modified every time a new random number gets created.

Question 51

What is the return value of Math.floor(8.23)?

Answer 51

8

Question 52

What is the opposite of Math.floor?

Answer 52

Math.ceil

Question 53

Which function will round the given fractional number up or down?

Answer 53

Math.round

Question 54

What is the return value of Math.round(0.50)?

Answer 54

1

Question 55

How do you ensure you always get a positive number from any given number input?

Answer 55

Math.abs

Question 56

If the function accepts an array with two elements as an argument, how do you assign those elements in the function signature?

Answer 56

function foo([firstElemt, secondElem])

Question 57

What is the value of binding z here 'let [z] = null'

Answer 57

Type error: null is not iterable

Question 58

How many memory addresses are created with 'let o = { a: [1, 2] }'

Answer 58

Two. One for the object "o" and the other for the object referenced by the "a" property.

Question 59

How do you send Javascript literal object over the network?

Answer 59

You can't. You have to serialize the object.

Question 60

How do you serialize the Javascript object?

Answer 60

JSON.stringify

Question 61

How do you deserialize JSON object?

Answer 61

There are no JSON objects. JSON is a string which is deserialized with JSON.parse.

Question 62

What is the syntax for putting comments within JSON files?

Answer 62

JSON doesn't support comments.

Question 63

Which quotes are available in JSON format?

Answer 63

Double quotes.