How To Use Array Methods in JavaScript: Mutator Methods

This article was originally written for DigitalOcean.

Arrays in JavaScript consist of a list of elements. JavaScript has many useful built-in methods, which we will review in this article.

In order to get the most out of this tutorial, you should have some familiarity with creating, indexing, modifying, and looping through arrays, which you can review in the tutorial Understanding Arrays in JavaScript.

Arrays are similar to strings, in that they both consist of a sequence of elements that can be accessed via index number. However, it is important to remember that strings are an immutable datatype, meaning they cannot be changed. Arrays, on the other hand, are mutable, which means that many array methods will affect the original array, not a copy of the array.

Methods that modify the original array are known as mutator methods, and methods that return a new value or representation are known as accessor methods.

In this article, we will learn about adding and removing elements, reversing, replacing, merging and otherwise modifying elements in an array.

Note: Array methods are properly written out as Array.prototype.method(), as Array.prototype refers to the Array object itself. For simplicity, we will simply list the name as method().

Accessor Methods


The concat() method merges two or more arrays together to form a new array. It does not mutate or affect any of the original arrays.

In the below example, we will create two arrays of types of shellfish and combine them into one new array.

// Create arrays of monovalves and bivalves
let monovalves = ['abalone', 'conch']
let bivalves = ['oyster', 'mussel', 'clam']

// Concatenate them together into shellfish variable
let shellfish = monovalves.concat(bivalves)

[ 'abalone', 'conch', 'oyster', 'mussel', 'clam' ]

The concat() method can take multiple arguments, effectively allowing you to concatenate many arrays together with a single method.


The join() method converts all the elements of an array into a new string.

let fish = ['piranha', 'barracuda', 'koi', 'eel']

If no argument is given, the output of join() will be a comma separated string with no extra whitespace.

// Join the elements of an array into a string
let fishString = fish.join()


The parameter of the join() function will contain the separator you would like between each array element.

// Join the elements of an array into a string
let fishString = fish.join(', ')

'piranha, barracuda, koi, eel'

In the above example, writing ', ' with whitespace separated the array items in a more readable fashion. An empty string provided as an argument will remove the default commas completely.


The slice() method copies a portion of an array to a new array.

let fish = ['piranha', 'barracuda', 'koi', 'eel']

Supposed we wanted to copy the last two items in the array to a new array. We would start with the index number of the first element we want, which is 2 for koi. We would end with the index number following the last element we want. eel is 4, so we would put 5.

// Slice a new array from 2 to 5
let fishWithShortNames = fish.slice(2, 5)

[ 'koi', 'eel' ]

In this particular case, since eel is the last item in the array, the second argument is actually unnecessary. slice() will start at the first index and stop at the end of the array if no second argument is provided.

// Slice a new array from 2 to the end of the array
let fishWithShortNames = fish.slice(2)

[ 'koi', 'eel' ]

slice() is an accessor method, and will not modify the original array. slice() is not to be confused with splice(), which can add or delete items from the original array.


The indexOf() method returns the index number of the first instance of an element.

In the below example, we have a string in which barracuda is listed twice.

let fish = ['piranha', 'barracuda', 'koi', 'barracuda']

We will use indexOf() to find the first instance.

// Find the first instance of an element

If the given argument is a value that does not exist in the array, the console will return -1.


The lastIndexOf() method returns the index number of the last instance of an element.

We can test on the same example from indexOf(), which includes barracuda twice.

let fish = ['piranha', 'barracuda', 'koi', 'barracuda']

// Find the last instance of an element

lastIndexOf() will search the array starting from the end and return the first index number it finds.

Iteration Methods


The forEach() method calls a function for each element in an array.

We can use forEach() to print each item in the fish array to the console.

let fish = ['piranha', 'barracuda', 'koi', 'eel']

// Print out each item in the array
fish.forEach(individualFish => {

Another way to do this is using the for loop keyword and testing it against the length property of the array.

// Loop through the length of the array
for (let i = 0; i < fish.length; i++) {

The above code will have the same output as using the forEach() method. forEach() is more concise and straightforward for this particular task. forEach() is an iteration method.


The map() method creates a new array with the results of a function call on each element in the array.

Just like forEach(), map() is an iteration method and as an example we can print each iteration of a loop to the console. map() does not mutate the original array, and it returns a new array value. map() must be placed into a new variable, unlike forEach().

let fish = ['piranha', 'barracuda', 'koi', 'eel']

// Print out each item in the array
let printFish = => {


We can use map() to change the values of each item in an array. For example, we will add an s to the end of each item in the fish array to pluralize them.

// Pluralize all items in the fish array
let pluralFish = => {
  return `${individualFish}s`

[ 'piranhas', 'barracudas', 'kois', 'eels' ]

The original fish variable is unchanged, but pluralFish now contains a modified version of the original variable.


The filter() method creates a new array with the elements that pass the result of a given test.

We could use filter() to return a new array containing only the items in a list that start with a specific letter.

let seaCreatures = ['shark', 'whale', 'squid', 'starfish', 'narwhal']

// Filter all creatures that start with "s" into a new list
let filteredList = seaCreatures.filter(creature => {
  return creature[0] === 's'

[ 'shark', 'squid', 'starfish' ]

We tested which items in the array have an s at the 0 index, and assigned the result into a new variable.

filter() is an iteration method, and does not mutate the original array.


The reduce() method will reduce an array to a single value.

This is seen commonly with numbers, such as finding the sum of all the numbers in an array.

let numbers = [42, 23, 16, 15, 4, 8]

// Get the sum of all numerical values
let sum = reduce((a, b) => {
  return a + b


reduce() can also be used with strings and other datatypes. The value returned by reduce() can be a number, string, array, or other datatype. reduce() is an iteration method that does not mutate the original array.


The find() method returns the first value in an array that passes a given test.

As an example, we will create an array of sea creatures.

let seaCreatures = ['whale', 'octopus', 'shark', 'cuttlefish', 'flounder']

Then we will use the find() method to test if any of the creatures in the array are cephalopods.

// Check if a given value is a cephalopod
const isCephalopod = cephalopod => {
  return ['cuttlefish', 'octopus'].includes(cephalopod)


Since octopus was the first entry in the array to satisfy the test in the isCephalopod() function, it is the first value to be returned.


The find() method returns the first index in an array that passes a given test.

We can use the same seaCreatures example from the find() method.

let seaCreatures = ['whale', 'octopus', 'shark', 'cuttlefish', 'flounder']

Using the isCephalopod test, we will find the index number instead of the value of the first match.

// Check if a given value is a cephalopod
const isCephalopod = cephalopod => {
  return ['cuttlefish', 'octopus'].includes(cephalopod)


octopus is the first item to match the test and has an index of 1, therefore it is the index number that is returned. If the test is not satisfied, findIndex() will return -1.


In this lesson, we reviewed the major built-in accessor and iteration array methods in JavaScript. Array methods are extremely diverse and useful, allowing you to add, remove, insert, mutate, modify, and loop through arrays.

To review the basics of arrays, read Understanding Arrays in JavaScript. To see a complete list of all array methods, view the Array reference on Mozilla Developer Network.