Javascript (Arrays)

Question 1

What are arrays?

Answer 1

An array is a collection of elements. Arrays are JavaScript’s way of making lists. Arrays can store any data types (including strings, numbers, and booleans). Arrays are also ordered (each item has a numbered position)

Question 2

What are array literals?

Answer 2

Array literals create an array by wrapping items in square brackets [].

Question 3

What are the ways to create an array.

Answer 3

1. Using array literals

Question 4

What is the content inside of an array called?

Answer 4

An element, code block

Question 5

What is an index?

Answer 5

An index is a numbered position for an element. Each element in an array will have a numbered position.

Question 6

Arrays are zero-indexed. What does this mean?

Answer 6

Zero-indexed means that the positions start counting from 0 rather than 1. Therefore, the first item in an array will be at position 0.

Question 7

How can we update an element in an array?

Answer 7

We can update an element in an array by using the following syntax:

variableName[3] = ‘new value’;

Question 8

What is the difference between using let and const when working with arrays?

Answer 8

When we use the let variable when working with arrays we can reassign a new array and update an element in the array. With const variables we can update an element in an array but we can not reassign a new array.

Question 9

.length

Answer 9

The .length property is one of an array’s built in properties. It returns the number of items in the array.

Ex:
const newYearsResolutions = [‘Keep a journal’, ‘Take a falconry class’];

console.log(newYearsResolutions.length);
// Output: 2

Question 10

.push()

Answer 10

The .push() method allows you to add items to the end of an array. The .push() changes, or mutates, the array. This method is also called a destructive array method since it changes the initial array. When we add this method to an array we can call the array like we would a function.

Ex:
const itemTracker = [‘item 0’, ‘item 1’, ‘item 2’];

itemTracker.push(‘item 3’, ‘item 4’);

console.log(itemTracker);
// Output: [‘item 0’, ‘item 1’, ‘item 2’, ‘item 3’, ‘item 4’];

Question 11

.pop()

Answer 11

The .pop method removes the last item in an array. It does not take any arguments. This method also mutates the array. The .pop() method also returns the value of the last element. We can store this in a variable to use later in the code.

Ex:
const newItemTracker = [‘item 0’, ‘item 1’, ‘item 2’];

const removed = newItemTracker.pop();

console.log(newItemTracker);
// Output: [ ‘item 0’, ‘item 1’ ]
console.log(removed);
// Output: item 2

Question 12

.shift()

Answer 12

The .shift method removes and returns the first element of the array. All of the elements after it will shift down one place.

Question 13

.unshift()

Answer 13

Adds one or more elements to beginning of array and returns new length.

Question 14

.slice()

Answer 14

The .splice() method modifies an array by inserting, removing, and/or replacing array elements then returning an array of removed elements. The splice array is a shallow copy of the original array.

Question 15

.indexOf()

Answer 15

The .indexOf() method returns the first index at which an element can be found. Returns -1 if the element is not found.

Question 16

What happens if we try to change an array inside a function?

Answer 16

When you pass an array into a function, if the array is mutated inside the function, that change will be maintained outside the function as well. pass-by-reference)

Question 17

What are nested arrays?

Answer 17

Nested arrays are when an array contains another array.

Ex:
const nestedArr = [[1], [2, 3]];

Question 18

How do we access nested arrays?

Answer 18

To access the nested arrays we use bracket notation with the index value.

Ex:
const nestedArr = [[1], [2, 3]];

console.log(nestedArr[1]); // Output: [2, 3]

Question 19

How can access the elements within a nested array?

Answer 19

To access the elements within a nested array we can chain, or add on, more bracket notation with index values.

Ex:
const nestedArr = [[1], [2, 3]];

console.log(nestedArr[1]); // Output: [2, 3]
console.log(nestedArr[1][0]); // Output: 2

Question 20

Array.isArray()

Answer 20

Array.isArray() checks if the passed value is an Array.

Ex:
const starbucksMenu = [‘Cafe Latte’, ‘Strawberry Acai’, ‘Pink Drink’, ‘Caramel Machiato’, ‘Pumpkin Creme Cold Brew’, ‘Dragon Drink’, ‘Pumpkin Spic Latte’];

console.log(Array.isArray(starbucksMenu));
//prints true

Question 21

.join()

Answer 21

The join() method creates and returns a new string by concatenating all of the elements in an array (or an array-like object), separated by commas or a specified separator string.

Question 22

at()

Answer 22

The at() method returns an indexed element from an array.
E.g. Get the third element of fruits:

const fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
let index = 2;
let fruit = fruits.at(index);
Apple

Question 23

concat()

Answer 23

The concat() method concatenates (joins) two or more arrays and returns an array with the joined arrays
const arr1 = [“Cecilie”, “Lone”];
const arr2 = [“Emil”, “Tobias”, “Linus”];
const children = arr1.concat(arr2);
Cecilie,Lone,Emil,Tobias,Linus

Question 24

constructor

Answer 24

The constructor property returns the function that created the Array prototype.

For JavaScript arrays the constructor property returns:

function Array() { [native code] }

Question 25

copyWithin()

Answer 25

This method copies array elements within the array, to and from specified positions. The copyWithin() method is not supported in Internet Explorer.
const fruits = [“Banana”, “Orange”, “Apple”, “Mango”];
fruits.copyWithin(2, 0);
Banana,Orange,Banana,Orange

Question 26

every()

Answer 26

Checks if every element in an array pass a test.
Check if all values in ages[] are over 18:

const ages = [32, 33, 16, 40];

ages.every(checkAge)

function checkAge(age) {
return age > 18;
}

Question 27

Explain the simplest way to create a new array from an existing array

Answer 27

The simplest way is to use the spread operator:

const fruits = [‘banana’, ‘pear’, ‘apple’]

const morefruits = […fruits]

console.log(morefruits)
//[ ‘banana’, ‘pear’, ‘apple’ ]

Now we can modify the morefruits array without affecting in any way the original fruits array.

Question 28

Explain the simplest way to add an item to the end of an existing array

Answer 28

The simplest way is to use the push() method:

const fruits = [‘banana’, ‘pear’, ‘apple’]
fruits.push(‘mango’)
push() appends the item to the end of the array.

This method accepts multiple items.

const fruits = [‘banana’, ‘pear’, ‘apple’]
fruits.push(‘mango’, ‘orange’)

//[ ‘banana’, ‘pear’, ‘apple’, ‘mango’, ‘orange’ ]

Question 29

Explain the simplest way to add an item to the beginning of an existing array

Answer 29

In this case you can use the unshift() method:

const fruits = [‘banana’, ‘pear’, ‘apple’]
fruits.unshift(‘orange’)

fruits //[ ‘orange’, ‘banana’, ‘pear’, ‘apple’ ]

This method accepts multiple items.

const fruits = [‘banana’, ‘pear’, ‘apple’]
fruits.unshift(‘mango’, ‘orange’)

//[ ‘mango’, ‘orange’, ‘banana’, ‘pear’, ‘apple’ ]

Question 30

Name two main ways of removing an item from an array

Answer 30

We can alter the original array,
or we can create a new array without the item we want to remove.

Question 31

Name the main ways of removing items from an array while altering the original array.

Answer 31

1. We can use pop() to remove the last item from the array,

const fruits = [‘banana’, ‘pear’, ‘apple’]
fruits.pop() //apple
fruits //[ ‘banana’, ‘pear’ ]

2. We can use shift() to remove the first item of an array,

const fruits = [‘banana’, ‘pear’, ‘apple’]
fruits.shift() //banana
fruits //[ ‘pear’, ‘apple’ ]

3. We can use splice() to remove elements at a specific positions in the middle of the array,

const fruits = [‘banana’, ‘pear’, ‘apple’]

//remove the item at index 2
fruits.splice(2, 1)

fruits //[ ‘banana’, ‘pear’ ]

Question 32

Give one way to create a new array without one specific item

Answer 32

If we want to create a new array without one specific item, we can first clone the array, or you can use slice().

Suppose you have an array, and you want to remove an item in position i. You can do it in this way:

const items = [‘a’, ‘b’, ‘c’, ‘d’, ‘e’, ‘f’]
const i = 3

const filteredItems = items.slice(0, i-1).concat(items.slice(i, items.length))

console.log(filteredItems)
// [“a”, “b”, “d”, “e”, “f”]

slice() creates a new array with the indexes it receives. We create a new array, from start to the index we want to remove, and concatenate another array from the first position following the one we removed to the end of the array.

Question 33

Give the best way to add an item to the array without mutating its content (i.e. by creating a new array)

Answer 33

The best way to add an item to the array without mutating its content, i.e. by creating a new array, is to use the spread operator.

1. adding a new item to the array in the first position

const fruits = [‘banana’, ‘pear’, ‘apple’]

const morefruits = [‘orange’, …fruits ]

console.log(morefruits)
//[ ‘orange’, ‘banana’, ‘pear’, ‘apple’ ]

When we use …fruits we basically expand the content of fruits. Since we enclosed that in the array literal syntax [], we’ll end up creating a new array morefruits, with the original content of fruits, plus anything we add to it.

2. adding a new item to the array in the last position

In the same way as p.1, we can append to the end of the array using […fruits, ‘orange’]:

const fruits = [‘banana’, ‘pear’, ‘apple’]

const morefruits = […fruits, ‘orange’]

console.log(morefruits)
//[ ‘banana’, ‘pear’, ‘apple’, ‘orange’ ]

Question 34

How to initialise a new array with a set of values?

Answer 34

We can initialize a new array with a set of values using this syntax, which first initializes an array of 12 elements, and fills each element with the number 0:

const b = Array(12).fill(0)

b is now an array of 12 items that all have the value 0.

Question 35

Give one alternative way to define an array from the array literal syntax (const b = [])

Answer 35

const b = Array() is another way to define an array, which is equivalent to using the array literal syntax const b = []

Question 36

What is array destructuring?

Answer 36

Array destructuring is a way to extract variables from the items contained into an array.

Example:

const list = [1, 2]
const [first, second] = list

console.log(first) //1
console.log(second) //2

We had numbers into an array, now we have two variables, first and second, which contains the values of the 2 items of the array.

We can also destructure the array to more variables:

const list = [1, 2, 3, 4, 5]
const [first, second, third, fourth, fifth] = list

The value assigned to the variables depends on the order listed.

Question 37

Give an example of using the spread operator with array destructuring

Answer 37

const list = [1, 2, 3, 4, 5]
const [first, second, …others] = list
others is an array containing [3, 4, 5].

Question 38

Explain how we can check if an array contains a specific value

Answer 38

You can check if an array contains a value using the includes() method.

Example:

const list = [1, 2, 3, 4]

list.includes(1) //true
list.includes(5) //false

Question 39

What happens if we try to change an array inside a function?

Answer 39

When you pass an array into a function, if the array is mutated inside the function, that change will be maintained outside the function as well. pass-by-reference)