Array.prototype.includes()

Summary: In this tutorial, you will learn how to use the JavaScript Array includes() method to check if an array includes an element.

Introduction to the JavaScript Array includes() method

The Array includes() method returns true if an array contains an element or false otherwise.

Here’s the syntax of the includes() method:

array.includes(searchElement, fromIndex)Code language: CSS (css)

The includes() method accepts two arguments:

• searchElement is the array element you want to search.
• fromIndex is a zero-based index at which the method starts searching for searchElement. The default is zero.

The fromIndex can be positive or negative.

If fromIndex >= array.length, then the includes() will not search for the searchElement and returns false.

When 0 <= fromIndex < array.length , then the includes() searches from the fromIndex to the end of the array.

In case -array.length <= fromIndex < 0, fromIndex + array.length , the includes() method searches entires array for searchElement.

JavaScript Array includes() method examples

Let’s take some examples of using the Array includes() method.

1) Basic Array includes() method examples

The following example uses the includes() method to check if a number is included in an array:

const numbers = [1, 2, 3];
const result = numbers.includes(1);

console.log({ result });Code language: JavaScript (javascript)

Output:

{ result: true }Code language: CSS (css)

How it works.

First, define an array of numbers:

const numbers = [1, 2, 3];Code language: JavaScript (javascript)

Second, check if the number 1 is included in the array using the includes() method:

const result = numbers.includes(1);Code language: JavaScript (javascript)

Third, display the result in the console:

console.log({ result });Code language: JavaScript (javascript)

Since the number 1 is included in the array, the includes() method returns true.

The following example uses the includes() method to check if the number 3 is included in the numbers array:

const numbers = [1, 2, 3];
const result = numbers.includes(4);

console.log({ result });Code language: JavaScript (javascript)

Output:

{ result: false }Code language: CSS (css)

The includes() method returns false because the numbers array does not contain the number 4.

2) Using the fromIndex argument

The following example uses the includes() method with the fromIndex to check if an array contains the number 1 from the index 1:

const numbers = [1, 2, 3];
const result = numbers.includes(1, 1);

console.log({ result });Code language: JavaScript (javascript)

Output:

{ result: false }Code language: CSS (css)

The includes() returns false because there is no number 1 starting from the index 1.

3) Using Array includes() method with arrays of objects

The following example uses the includes() method to check if an object is in an array.

const bmw = { name: 'BMW' };
const toyota = { name: 'Toyota' };
const ford = { name: 'Ford' };

const cars = [ford, toyota];

console.log(cars.includes(ford)); 
console.log(cars.includes(bmw));Code language: JavaScript (javascript)

How it works.

First, create some objects that have the name property with different values:

const bmw = { name: 'BMW' };
const toyota = { name: 'Toyota' };
const ford = { name: 'Ford' };Code language: JavaScript (javascript)

Second, initialize the cars array with two objects ford and toyota:

const cars = [ford, toyota];Code language: JavaScript (javascript)

Third, check if the cars array contains the  ford object using the includes() method and display the result to the console:

console.log(cars.includes(ford));Code language: CSS (css)

It returns true because the cars object contains the ford object.

Finally, check ifbmw object is included in the  cars array:

console.log(cars.includes(bmw));Code language: CSS (css)

It returns false.

includes vs. indexOf

In JavaScript, NaN is not equal to NaN, so both expressions below return false:

let result =  NaN == NaN;
console.log({result});

result =  NaN === NaN;
console.log({result});Code language: JavaScript (javascript)

The indexOf() method returns -1 when you check if an array contains NaN, but the include() method returns true. For example:

const arr = [1, NaN, 2, 3];

let result = arr.indexOf(NaN);
console.log({ result }); // -1

result = arr.includes(NaN);
console.log({ result }); // trueCode language: JavaScript (javascript)

Output:

{ result: -1 }
{ result: true }Code language: CSS (css)

Summary

• Use the JavaScript Array includes() method to check if an element is included in an array.