Arrays

Question 1

How do we create an array?

Answer 1

let arrayName = []

Question 2

What can we store in an array?

Answer 2

• Any valid data type
• objects
• arrays
• functions

Question 3

How do we find the value of an item in an array?

Answer 3

we query the array with arrayName[n]

n is the index of the item in the array

Question 4

What happens if we query an array with an index larger than the array holds

Answer 4

we get ‘undefined’ as the result

Question 5

Can we get items from the end of an array?

Answer 5

Yes, by using negative indices we can query from the end of an array.

Question 6

How could target the end of an array?

Answer 6

arrayName[arrayName.length - 1]

Question 7

How can we assign a value to an item in an array

Answer 7

arrayName[n] = someValue

will create or reassign the value of an item in an array

Question 8

if you insert a item value deep into an array, what will the intervening values be set to?

Answer 8

undefined

(or empty in google chrome)

Question 9

What method could we use to find the index of a value in an array?

Answer 9

We could use the .indexOf method to find the index of a value.

Question 10

Describe the indexOf method syntax on an array

Answer 10

arrayName.indexOf(value)

Question 11

What data types will indexOf work on?

Answer 11

indexOf works on primitive data types

Question 12

If a value is not present in an array, what does indexOf return

Answer 12

indexOf will return -1 if a value does not exist in an array.

Question 13

How can we add a value to the front of an array?

Answer 13

we can use the unshift method to add a value to the front of an array.

Question 14

How can we add a value to the end of an array?

Answer 14

We can use the push method to add a value to the end of an array.

Question 15

What are two methods used for adding values to an array

Answer 15

1. .unshift - to the front of an array
2. .push - to the end of an array

Question 16

How can we remove a value from the start of an array?

Answer 16

We can use the .shift method to remove a value from the start of an array

Question 17

What is the syntax for using the shift method on an array

Answer 17

arrayName.shift()

Question 18

What method can we use to remove a value from the end of an array

Answer 18

We can use the pop method to remove a value from the end of an array

Question 19

Name two methods we can use to remove values from an array

Answer 19

1. .shift - remove from front of array
2. .pop - remove from end of array

Question 20

What does the splice method allow us to do?

Answer 20

It allows us to add or remove values from any position in an array

Question 21

What is the splice method syntax on an array?

Answer 21

arrayName.splice(index, deleteCount, itemsToAdd)

Question 22

When using the splice method, where does the editing begin?

Answer 22

It begins with the index value given as the first argument in the method.

Question 23

If we want to only add, and not delete, items using the splice method, how do we do so?

Answer 23

We set the the deleteCount argument, the second argument to the splice method, to zero.

.splice(index, 0, itemsToAdd)

Question 24

If we are removing items from an array using splice, which index will it remove first

Answer 24

splice will remove the length starting with the index passed as the first argument.

Question 25

If we are adding items to an array using splice, which value will be the first to change.

Answer 25

the splice method begins editing at the index given as its first argument, so:

• splice(0,0,1) makes index 0 = 1
• splice(3,0,2) makes index 3 = 2

Question 26

If we are adding items to an array using splice, which value will be the first to change.

Answer 26

the splice method begins editing at the index given as its first argument, so:

• splice(0,0,1) makes index 0 = 1
• splice(3,0,2) makes index 3 = 2

Question 27

How do we use the slice method on an array?

Answer 27

The slice method copies a section of an array, without changing the original

We generally save this new array to a variable

Question 28

How do the splice and slice methods on arrays differ

Answer 28

Splice modifies the original array

Slice copies a slice of the original, but does not modify the original array

Question 29

Give the syntax for the slice method on arrays

Answer 29

arrayName.slice(start, end)

start = includes start index

end = excludes end index

Question 30

Given:

array = [1, 2, 3, 4, 5, 6]

newarray = array.slice(1, 4)

What does newarray contain?

Answer 30

newarray = [2, 3, 4]

Question 31

Do we need to pass parameters to the shift method.

Answer 31

No, the shift method remove only the first element in an Array

Question 32

If an array is mutated in a function, does the mutation persist outside the function scope?

Answer 32

Yes, if the mutation was destructive, then the mutation persists.

Question 33

What do we mean by passed-by-reference?

Answer 33

passed-by-reference refers to the fact that when we pass an array into a function, we are passing a reference to that array in memory.

Question 34

How do we use the .find method in JavaScript?

Answer 34

The .find method is used to check an array of objects for the presence of an object with a specific condition.

Question 35

How many arguments are typically used in the .find method?

Answer 35

We typically only use one argument in the .find method, the item argument (the others are index, array

Question 36

What is the syntax for using a .find method on an array of objects in JavaScript?

Answer 36

array.find(item => item.id == 1);

This would return the object with the presence of id: 1

If we saved the result to a variable, we could then access other key:value pairs within the object

let user = users.find(item => item.id == 1)

console.log(user.name) - would log the value of the name key from the object with id: 1

Question 37

When does the method .find stop iterating over an array?

Answer 37

.find stops iterating over an array when the first truthy value is found.

If nothing is found it returns undefined.

Question 38

What method should we use on an array if we want all of a certain value returned?

Answer 38

We should use the .filter method on an array to return all the occurrences of a certain value.

Question 39

What is the syntax of a .filter method call?

Answer 39

let results = array.filter(item => item > 10)

results would then be an array with any value > 10

Question 40

Can we use the .filter method on an array of objects?

Answer 40

Yes, .filter can be used on an array of objects to create an array of all objects matching certain criteria.

Question 41

Give the syntax of using the .filter method on an array of objects.

Answer 41

let results = arr.filter(item => item.id < 3)

for(let result of results){

console.log(result.name)

}

Question 42

How is the .sort method used on arrays?

Answer 42

The .sort method with sort an array alphabetically (even if the contents are intergers), unless we provide a function as an argument to .sort

Question 43

What is a typical .sort syntax used to sort items numerically?

Answer 43

arr.sort((a,b) => a - b)

when a - b produces a positive result the order needs to be reversed (i.e. a > b)

for this reason, to sort in reverse we use b - a, when this produces a positive result the order is kept (i.e. b > a)

Question 44

What method might we pass to the function inside the .sort method to sort strings alphabetically?

Answer 44

We would use the localeCompare method because, unlike sort a - b, it compares the letters alphabetically instead of based on their character code value.

arr.sort((a,b) => a.localeCompare(b)

Question 45

Instead of sorting using b - a in our .sort function, what might we do to reverse an array?

Answer 45

We could use the .reverse method to sort from higher to lower (in descending order)

Question 46

Why is it good practice to include the initial value when using the .reduce method?

Answer 46

It is good practice, because if the array is empty the .reduce method will throw an error.

Question 47

What is the syntax of a reduce method call?

Answer 47

let arrSum = arr.reduce((sum, current) => sum + current, 0)

Where the 0 is the initial value to start with.

Question 48

What does the method .reduceRight do for us in JavaScript?

Answer 48

.reduceRight begins it’s iteration from the right side of the array

Question 49

What does the syntax Array.isArray(x) do for us in JavaScript and why would we use it?

Answer 49

Array.isArray(x) checks to see if x is an array, and returns true if it is.

We use it to determine if a data structure is an array or not, as in JavaScript, arrays are object-based data-structures.