JS Array

Question 1

What is array in JS

Answer 1

Array is an ordered list of values /element whit index/

Question 2

Array Characteristics

Answer 2

1.Array can hold values of different typs
2.Array is auto-growing, the length is dinamicaly sized

Question 3

How create arr

Answer 3

let arr = newArray() //empty arr
let arr1 = newArray(10) //- array whit size 10
let arr2 = [el1, el2, el3]

Question 4

Find array length
let arr3 = [‘el1’, ‘el2’, ‘el3’]

Answer 4

let arr3 = [‘el1’, ‘el2’, ‘el3’]
console.log(arr3.length); //3

Question 5

Add el in the end of arr
let arr4 = [‘el1’, ‘el2’, ‘el3’]

Answer 5

let arr4 = [‘el1’, ‘el2’, ‘el3’]
arr4.push(‘el4’)
console.log(arr4); //[ ‘el1’, ‘el2’, ‘el3’, ‘el4’ ]

Question 6

add el in the begin of arr
let arr5 = [‘el1’, ‘el2’, ‘el3’]

Answer 6

let arr5 = [‘el1’, ‘el2’, ‘el3’]
arr5.unshift(‘el5’)
console.log(arr5); //[ ‘el5’, ‘el1’, ‘el2’, ‘el3’ ]

Question 7

// removing el in the end of arr
let arr6 = [‘el1’, ‘el2’, ‘el3’]

Answer 7

let arr6 = [‘el1’, ‘el2’, ‘el3’]
console.log(arr6.pop() //el3
console.log(arr6); //[ ‘el1’, ‘el2’ ]

Question 8

removing el in the begin of arr
let arr7 = [‘el1’, ‘el2’, ‘el3’]

Answer 8

let arr7 = [‘el1’, ‘el2’, ‘el3’]
console.log(arr7.shift() //el1
console.log(arr7); //[ ‘el2’, ‘el3’ ]

Question 9

// Find index el of arr
let arr8 = [‘el1’, ‘el2’, ‘el3’]
el2

Answer 9

console.log(arr8.indexOf(‘el2’)) //1

Question 10

Check is arr
let arr9 = [‘el1’, ‘el2’, ‘el3’]

Answer 10

console.log(Array.isArray(arr9)) //true

Question 11

What is the Stack?

Answer 11

The data structure - list whit elements. Use LIFO principe Last in First out. Operations is only at the top of the stack: push() add one or more el in top of stack /end arr/ and pop() remove element of top of the stack /end arr/.

Question 12

What is result?
let stack = [1,2,3]
stack.push(4)
console.log(stack);

Answer 12

push() add one or more el in top of stack /end/
console.log(stack); //[ 1, 2, 3, 4 ]

Question 13

What is result?
let stack1 = [1,2,3]
console.log(stack1.pop())
console.log(stack1);

Answer 13

pop() remove element of top of the stack /end arr/
console.log(stack1.pop()) //3
console.log(stack1);//[ 1, 2 ]

Question 14

What is result?
const reverse = (str) => {
let stack = [];
for (let i = 0; i < str.length; i++) {
stack.push(str[i]);
}

let reverseStr = “”;
while (stack.length > 0) {
reverseStr += stack.pop();
}
return reverseStr;
};
console.log(reverse(“Lia”));

Answer 14

reverse str with JS stack
console.log(reverse(“Lia”)); //aiL

Question 15

What is the Queue?

Answer 15

The data structure - ordered list whit elements. Use FIFO principe First in First out. Operations is
enqueue insert el of the end of queue, /add el in the end of arr with push()/
dequeue removal from the front of queue.//remove el of the front of arr with shift()
peek -returns el et the front, no modify queue

Question 16

Which method is used, what is the output?

let arr = [1, 2, 3, 4];
let delArr = arr.splice(0,3)
console.log(arr);
console.log(delArr);

Answer 16

use splice() for delete el from JS Arr two arguments 1. position to start 2. number of el for delete

console.log(arr); //[ 4 ]
console.log(delArr); //[ 1, 2, 3 ]

Question 17

Which method is used, what is the output?

let arr1 = [ 1, 2, 3 ]
// insertArr
arr1.splice(3,0, 4,5,6,7)
console.log(arr1);

Answer 17

use splice() for insert one or more el to JS Arr
three arguments 1. position for insert 2.0 zero -not delete el
3. other arguments is el for insert
console.log(arr1); [
1, 2, 3, 4,
5, 6, 7
]

Question 18

Which method is used, what is the output?
let arr2 = [ 1, 2, 3 ]
replaceArr
arr2.splice(1,1, 7)

console.log(arr2);

let arr3 = [ 1, 2, 3 ]
// replaceArr
arr3.splice(2,1, 8 ,11,12)

console.log(arr3);

Answer 18

use splice() for replace one or more el to JS Arr
three arguments 1. position for start 2.how much element for replace if 0 we only insert el
3. arguments is new el
\
console.log(arr2); //[ 1, 7, 3 ]

console.log(arr3); //[ 1, 2, 8, 11, 12 ]

Question 19

Which method is used, what is the output?

let arr4 = [ 1, 2, 3 ]
let arr41 = arr4.slice()
console.log(arr41);
console.log(arr4);

Answer 19

// slice() return new arr have 2 parameters
// 1. start 2.stop
use slice for clone arr
console.log(arr41); //[ 1, 2, 3 ]
console.log(arr4); //[ 1, 2, 3 ]

Question 20

Which method is used, what is the output?
let arr5 = [ 1, 2, 3, 4, 5, 6, 7 ]
let arr51 = arr5.slice(1,3)
console.log(arr51);

Answer 20

use slice for copy part el of arr
console.log(arr51);[ 2, 3 ]

Question 21

Which method is used, what is the output?

let arr6 = [ 1, 2, 3, 4, 5, 6, 7 ]
console.log(arr6.indexOf(3))

Answer 21

IndexOf() to find index of el
let arr6 = [ 1, 2, 3, 4, 5, 6, 7 ]
console.log(arr6.indexOf(3));//2

Question 22

Which method is used, what is the output?

let arr7 = [1, 2, 6, 3, 4, 5, 6, 7];
const find = (el, arr) => {
let results = [];
let idx = arr.indexOf(el);
while (idx !== -1) {
results.push(idx);
idx = arr.indexOf(el, idx + 1);
}
return results;
};
console.log(find(6, arr7));

Answer 22

implement find with indexOf - every position of el -return arr with position
console.log(find(6, arr7)); [ 2, 6 ]

Question 23

Which method is used, what is the output?

let arr8 = [1, 2, 3, 4, 5, 6, 7];
let result = arr8.every(e => e > 0);
console.log(result);

Answer 23

use every to check el of arr
console.log(result); //true

Question 24

Which method is used, what is the output?

let arr9 = [1, 2, 3, 4];
let range = {
min: 0,
max: 10,
};

let isInRange = arr9.every(function (e) {
return e >= this.min && e <= this.max;
}, range);

console.log(isInRange);

Answer 24

check with every() is every el of arr is in range
console.log(isInRange); true

Question 25

Which method is used, what is the output?

let arrSome = [1, 2, 3, 4, 5, 6, 7];

const lessFive = arrSome.some((e) => e < 5);
console.log(lessFive);

Answer 25

// check with some() have el less then 5

//true

Question 26

Which method is used, what is the output?

let arrSome1 = [1, 2, 3, 4, 5, 6, 7];

const exist = (val, arr) => arr.some(e => e===val)
console.log(exist(4, arrSome1));

Answer 26

function exist with some()
true

Question 27

Which method is used, what is the output?
let arrSort = [11, 22, 53, 4, 5, 6, 7];
console.log(arrSort.sort((a,b) => a - b));

Answer 27

// To sort el in arr we use sort() for numbers and

console.log(arrSort.sort((a,b) => a - b)); //[4, 5, 6, 7, 11, 22, 53]

Question 28

Which method is used, what is the output?

let arrSort1 = [‘cat’, ‘been’, ‘ant’];
console.log(arrSort1.sort((a,b) => a.localeCompare(b)));
let arrSort3 = [‘cat’, ‘Been’, ‘ant’, ‘Dog’];
console.log(arrSort3.sort((a,b) => a.localeCompare(b)));

Answer 28

To sort el in arr we use
sort() and locateCompare for str in arr
console.log(arrSort1.sort((a,b) => a.localeCompare(b))); [ ‘ant’, ‘been’, ‘cat’ ]

console.log(arrSort3.sort((a,b) => a.localeCompare(b))); [ ‘ant’, ‘Been’, ‘cat’, ‘Dog’ ]

Question 29

Which method is used, what is the output?

let arrObj = [
{name:’John’, salary: 590000},
{name:’John1’, salary: 9190000},
{name:’John2’, salary: 290000},
{name:’John3’, salary: 1390000}
]

console.log(arrObj.sort((x,y) => x.salary - y.salary));

Answer 29

Use sort() for arr with objects for numerical property
console.log(arrObj.sort((x,y) => x.salary - y.salary));
[
{ name: ‘John2’, salary: 290000 },
{ name: ‘John’, salary: 590000 },
{ name: ‘John3’, salary: 1390000 },
{ name: ‘John1’, salary: 9190000 }
]

Question 30

Which method is used, what is the output?
let arrObj = [
{name:’John’, salary: 590000},
{name:’John1’, salary: 9190000},
{name:’John2’, salary: 290000},
{name:’Anton’, salary: 1390000}
]
console.log(arrObj.sort((x,y) => x.name.localeCompare(y.name)));

Answer 30

Use sort() for arr with objects for string property
console.log(arrObj.sort((x,y) => x.name.localeCompare(y.name)));

[
{ name: ‘Anton’, salary: 1390000 },
{ name: ‘John’, salary: 590000 },
{ name: ‘John1’, salary: 9190000 },
{ name: ‘John2’, salary: 290000 }
]

Question 31

Which method is used, what is the output?
let arrObj = [
{name:’John’, salary: 590000},
{name:’John1’, salary: 9190000},
{name:’Jo’, salary: 290000},
{name:’Ant’, salary: 1390000}
]
console.log(arrObj.sort((x,y) => x.name.length -y.name.length));

Answer 31

Use sort() for arr with objects for longest string property
console.log(arrObj.sort((x,y) => x.name.length -y.name.length));
[
{ name: ‘Jo’, salary: 290000 },
{ name: ‘Ant’, salary: 1390000 },
{ name: ‘John’, salary: 590000 },
{ name: ‘John1’, salary: 9190000 }
]

Question 32

Which method is used, what is the output?

let city = [
{name:’Los Angeles’, population: 3800000},
{name:’New York’, population: 90190000},
{name:’Chicago’, population: 3290000},
{name:’Montana’, population: 15390000}
]

let bigCity = city.filter(e => e.population > 4000000 )

console.log(bigCity);

Answer 32

Use filter() for arr with objects cityes propertis is biggest then 4 000 000

console.log(bigCity);

[
{ name: ‘New York’, population: 90190000 },
{ name: ‘Montana’, population: 15390000 }
]

Question 33

Which method is used, what is the output?

let cities = [
{name:’Los Angeles’, population: 3800000},
{name:’New York’, population: 90190000},
{name:’Chicago’, population: 3290000},
{name:’Montana’, population: 15390000}
]
cities
.filter(e => e.population > 4000000)
.sort((a,b) => a.population - b.population)
.map(c => console.log(${c.name}:${c.population}) )

Answer 33

Use filter() sort() map() for arr with objects cityes propertis is biggest then 4 000 000 sort them and log in special format

cities
.filter(e => e.population > 4000000)
.sort((a,b) => a.population - b.population)
.map(c => console.log(${c.name}:${c.population}) )

Montana:15390000
New York:90190000

Question 34

Which method is used, what is the output?

let data = [11, 22, 53, 4, 5, 6, 7, “cat”, “been”, “ant”];

function isInRang(val) {
if (typeof val !== “number”) {
return false;
}
return val >= this.min && val <= this.max;
}

let rang = {
min: 1,
max: 10,
};

let numInRang = data.filter(isInRang, rang);
console.log(numInRang);

Answer 34

Use filter() and function isIn Rang to check only number in rang

console.log(numInRang);
[ 4, 5, 6, 7 ]

Question 35

Which method is used, what is the output?

let circle = [10, 30, 50]
const circleArea =(r) => Math.floor(Math.PIrr)
let areas = circle.map(circleArea)
console.log(areas);

other way the same task

let circle = [10, 30, 50]
let areas = circle.map((r) => Math.floor(Math.PIrr))
console.log(areas);

Answer 35

Use map() we have r of circle and must calculate area for each circle

console.log(areas); //[ 314, 2827, 7853 ]

Question 36

Which method is used, what is the output?

let num = [16, 25, 64]
const numSq = num.map(Math.sqrt)
console.log(numSq);

Answer 36

use map()
console.log(numSq); [ 4, 5, 8 ]

Question 37

Which method is used, what is the output?
let letter = [‘A’,’B’,’C’]
letter.forEach(e => console.log(e))

Answer 37

forEach() iterates over el in arr and execute a predefined func ones for el
console.log(e))
A
B
C

Question 38

Which method is used, what is the output?

let num1 = [20,30, 40]
const sum = num1.reduce((acc,currN) => acc + currN)
console.log(sum);

Answer 38

reduce() method have 2 arguments 1.reducer function 2.initial value

reduce method call reduce function for every el of arr

reducer func returns value which is an accumolate result, this result is base for next call
console.log(sum); //90

Question 39

Which method is used, what is the output?
let shopCart = [
{
product:’phone’,
qty:1,
price:699
},
{
product:’Protector’,
qty:1,
price:9.98
},
{
product:’Memory’,
qty:2,
price:20.99
},
]

const total = shopCart.reduce((acc, curr) => {
return acc + curr.qty*curr.price
},0)
console.log(total);

Answer 39

Use reduce() to calculate total amount for shoppingCart
console.log(total); //750.96

Question 40

Which method is used, what is the output?

const cssClasses =[‘btn’, ‘btn-primary’, ‘btn-active’]
const btnClass = cssClasses.join(‘ ‘)
console.log(btnClass);

Answer 40

Join() concatenate all el of arr and return new str

in this case we use join() - to join css classes
console.log(btnClass);
btn btn-primary btn-active

Question 41

Which method is used, what is the output?

const title = [‘A’,’B’,’C’]
const url = title.join(‘-‘).toLowerCase()
console.log(url);

Answer 41

//join() to replace all occurrences of a str and replace whit small letter

console.log(url); //a-b-c

Question 42

What is represented in this task?

let activities = [
[‘Work’, 9],
[‘Eat’, 1],
[‘Commute’, 2],
[‘Play game’, 1],
[‘Sleep’, 7],
]

console.table(activities)

Answer 42

Multidimensional Array
console.table(activities)
┌─────────┬─────────────┬───┐
│ (index) │ 0 │ 1 │
├─────────┼─────────────┼───┤
│ 0 │ ‘Work’ │ 9 │
│ 1 │ ‘Eat’ │ 1 │
│ 2 │ ‘Commute’ │ 2 │
│ 3 │ ‘Play game’ │ 1 │
│ 4 │ ‘Sleep’ │ 7 │
└─────────┴─────────────┴───┘

Question 43

What is represented in this task?

let activities = [
[‘Work’, 9],
[‘Eat’, 1],
[‘Commute’, 2],
[‘Play game’, 1],
[‘Sleep’, 7],
]

activities.push([‘Study’,2])
console.table(activities)

Answer 43

Multidimensional Array with push

console.table(activities)

┌─────────┬─────────────┬───┐
│ (index) │ 0 │ 1 │
├─────────┼─────────────┼───┤
│ 0 │ ‘Work’ │ 9 │
│ 1 │ ‘Eat’ │ 1 │
│ 2 │ ‘Commute’ │ 2 │
│ 3 │ ‘Play game’ │ 1 │
│ 4 │ ‘Sleep’ │ 7 │
│ 5 │ ‘Study’ │ 2 │

Question 44

What is represented in this task?

let activities = [
[‘Work’, 9],
[‘Eat’, 1],
[‘Commute’, 2],
[‘Play game’, 1],
[‘Sleep’, 7],
]

activities.splice(2,0,[‘Programming’,4])
console.table(activities)

Answer 44

Multidimensional Array with splice

console.table(activities)

┌─────────┬───────────────┬───┐
│ (index) │ 0 │ 1 │
├─────────┼───────────────┼───┤
│ 0 │ ‘Work’ │ 9 │
│ 1 │ ‘Eat’ │ 1 │
│ 2 │ ‘Programming’ │ 4 │
│ 3 │ ‘Commute’ │ 2 │
│ 4 │ ‘Play game’ │ 1 │
│ 5 │ ‘Sleep’ │ 7 │
│ 6 │ ‘Study’ │ 2 │
└─────────┴───────────────┴───┘

Question 45

What is represented in this task?

let activities = [
[‘Work’, 9],
[‘Eat’, 1],
[‘Commute’, 2],
[‘Play game’, 1],
[‘Sleep’, 7],
]
activities.forEach(a=>{
let percentage = ((a[1]/24)*100).toFixed()
a[2] = percentage + ‘%’
})
console.table(activities)

Answer 45

with forEach calculate % of the hours spend to each activity and appends % to the inner arr

console.table(activities)

┌─────────┬───────────────┬───┬───────┐
│ (index) │ 0 │ 1 │ 2 │
├─────────┼───────────────┼───┼───────┤
│ 0 │ ‘Work’ │ 9 │ ‘38%’ │
│ 1 │ ‘Eat’ │ 1 │ ‘4%’ │
│ 2 │ ‘Programming’ │ 4 │ ‘17%’ │
│ 3 │ ‘Commute’ │ 2 │ ‘8%’ │
│ 4 │ ‘Play game’ │ 1 │ ‘4%’ │
│ 5 │ ‘Sleep’ │ 7 │ ‘29%’ │
│ 6 │ ‘Study’ │ 2 │ ‘8%’ │
└─────────┴───────────────┴───┴───────┘

Question 46

What is represented in this task?

let activity = [
[‘Work’, 9],
[‘Eat’, 1],
[‘Commute’, 2],
[‘Play game’, 1],
[‘Sleep’, 7],
]

activity.pop()
console.table(activity)

Answer 46

Remove el from Multidimensional Array
console.table(activity)
┌─────────┬─────────────┬───┐
│ (index) │ 0 │ 1 │
├─────────┼─────────────┼───┤
│ 0 │ ‘Work’ │ 9 │
│ 1 │ ‘Eat’ │ 1 │
│ 2 │ ‘Commute’ │ 2 │
│ 3 │ ‘Play game’ │ 1 │