Mid Term study Arrays

Question 1

Three ways to declare an Array?

Answer 1

let numbers = Array();
// OR
let numbers = new Array();
// OR
let numbers = [];

Question 2

How to create an empty array with 3 elements

Answer 2

let numbers = Array(3);

Question 3

How to add the number 10 to the Array below?

let numbers = [2, 4, 6, 8];

Answer 3

numbers[4] = 10;

Question 4

What is output to the console from below:

let numbers = [2, 4, 6, 8];
console.log(numbers);

Answer 4

(4) [2, 4, 6 ,8]

(4) -> this is the array length
[2, 4, 6, 8] -> these are the array elements

Question 5

What is the output to the console from below:

let numbers = [2, 4, 6, 8];
numbers.length = 2;
console.log(numbers.length);
console.log(numbers);

Answer 5

console.log(numbers.length);
outputs: 2

console.log(numbers);
outputs: [2, 4]

Question 6

Is the below possible?:

let things = [24, “cat”, 8.9, true];
console.log(things);

Answer 6

Yes because Javascript can hold any data type

Question 7

How to return index[2] from this array?

let numbers = [2, 4, 6, 8];

Answer 7

numbers.at(2)

.at() method will return the value at the specified index.

Question 8

What is the console output:

let numbers = [2, 4, 6, 8];
console.log(numbers.at(-2))

Answer 8

output would be 6

Question 9

Why are the .push() and .pop() array methods faster than the .shift() and .unshift() methods?

Answer 9

.pop() and .push() are faster than .shift()/.unshift() because .shift/unshift require you to re-index every other array element. While pop and push just remove or add to the end of the array.

Question 10

What happens below:

let numbers = [2, 4, 6, 8];
console.log(numbers.pop()):
numbers.push(5);

Answer 10

console.log(numbers.pop()): - > pops the last element from the array(8 in this case)

numbers.push(5); - > pushes 5 to the end of the array

Question 11

What happens below:

let numbers = [2, 4, 6, 8];
console.log(numbers.shift()):
numbers.unshift(5);

Answer 11

console.log(numbers.shift()): ->shifts the first element from the array(2 in this case)

numbers.unshift(5); - > shifts 5 to the start of the array

Question 12

How do you Join the arrays below?

let numbers = [2, 4, 6, 8];
let more = [1, 2, 3];

Answer 12

numbers = numbers.concat(more);

Array is now:
[2, 4, 6, 8, 1, 2, 3]

Question 13

How would you join all the elements in the array below into a string with each element spaced by “:”

let numbers = [2, 4, 6, 8];

Answer 13

let new = numbers.join(“:”):

new would equal the string
“2:4:6:8”

Question 14

How would you copy part of an array, and copy from indexes 4, 5, 6 from the array below:

let numbers = [2, 4, 6, 8, 10, 12, 14, 16, 18, 20];

Answer 14

let new = numbers.slice(4, 7);

.slice(start, end)

.slice is inclusive of the start but exclusive of the end.

Question 15

Explain what the Array.splice() below does?

let numbers = [2, 4, 6, 8, 10, 12, 14, 16, 18, 20];
let new = numbers.splice(4, 5);

Answer 15

Array.splice(start index, how much elements);

will start at index 4(value 10) and take from 5 indexes from there(inclusive). So new will equal [10, 12, 14, 16, 18]

Question 16

Reverse the array below

let numbers = [2, 4, 6, 8, 10, 12, 14, 16, 18, 20];

Answer 16

numbers.reverse();

Question 17

How to sort the array below?

let numbers = [10, -1, 20, 15, -20]

Answer 17

numbers.sort();

Question 18

What is the output below and explain:

let numbers = [8, -2, 1, 17, 18, -9, 10];
console.log(numbers.indexOf(17));
console.log(numbers.indexOf(99));
console.log(numbers.indexOf(17, 2));
console.log(numbers.indexOf(17, 4));

Answer 18

console.log(numbers.indexOf(17)); = returns index of 17 which would be 3

console.log(numbers.indexOf(99)); = 99 is not in array so returns -1

console.log(numbers.indexOf(17, 2)); = searches for index of 17 starting from index 2, will find it so returns index 3

console.log(numbers.indexOf(17, 4)); = searches for index of 17 starting from index 4, will not find it so returns -1

Question 19

What is array.lastindexOf() and how is it different from array.indexOf()?

Answer 19

array.lastindexOf() is the same except instead of moving forwards in indexes, it searches the array backwards

Question 20

Write syntax for a for:of loop of the array below

let numbers = [2, 4, 6];

Answer 20

for(const number of numbers){
// Use const if u dont wanna change element values
}
//OR
for(let number of numbers){
//Use if u want to change element values
}

Question 21

Write syntax for a for:in loop of the array below

let numbers = [2, 4, 6];

Answer 21

for (const index in numbers) {
console.log(numbers[index]);
}