JavaScript Array includes() Method

The JavaScript Array.includes() method is used to check if a given array contains a specified element. It returns a Boolean (true or false) based on whether the element is found in the array. This method is case-sensitive and works with both primitive types (like numbers and strings) and object references.

For example, array.includes(valueToFind) is useful when you need to verify the existence of an element in an array.

Syntax

The syntax for the Array.includes() method is:

plaintext
array.includes(valueToFind, fromIndex)

Parameters

The Array.includes() method accepts two parameters:

  • valueToFind (Required): The element to search for in the array.
  • fromIndex (Optional): The index at which to start the search. The default is 0. If a negative index is provided, it starts from the end of the array.

Return Value

The method returns a Boolean value:

  • true if the array contains the specified element.
  • false if the element is not found.

Examples of JavaScript Array.includes() Method

Example 1: Checking for a Value in an Array

You can use includes() to check if a value exists in an array.

javascript
const fruits = ['apple', 'banana', 'mango'];
const hasApple = fruits.includes('apple');
console.log(hasApple);  // Output: true

Explanation: The includes() method checks if the array fruits contains the element 'apple' and returns true because 'apple' exists in the array.

Example 2: Checking for a Non-Existent Value

If the value does not exist in the array, includes() returns false.

javascript
const fruits = ['apple', 'banana', 'mango'];
const hasOrange = fruits.includes('orange');
console.log(hasOrange);  // Output: false

Explanation: The method returns false because 'orange' is not present in the fruits array.

Example 3: Using includes() with a Number Array

The includes() method works with numbers as well.

javascript
const numbers = [1, 2, 3, 4, 5];
const hasNumber = numbers.includes(3);
console.log(hasNumber);  // Output: true

Explanation: The method checks if 3 exists in the numbers array and returns true.

Example 4: Case-Sensitivity in includes()

The method is case-sensitive when working with strings.

javascript
const fruits = ['Apple', 'Banana', 'Mango'];
const hasAppleLowerCase = fruits.includes('apple');
console.log(hasAppleLowerCase);  // Output: false

Explanation: The method returns false because 'apple' (in lowercase) does not match 'Apple' (with an uppercase “A”) in the fruits array.

Example 5: Using includes() with fromIndex

You can specify the index from which to start searching using the fromIndex parameter.

javascript
const numbers = [1, 2, 3, 4, 5];
const hasNumberFromIndex = numbers.includes(3, 3);
console.log(hasNumberFromIndex);  // Output: false

Explanation: The method starts searching from index 3, but 3 is located before that, so it returns false.

Example 6: Using includes() with Negative fromIndex

You can provide a negative fromIndex to start the search from the end of the array.

javascript
const numbers = [1, 2, 3, 4, 5];
const hasNumberFromNegativeIndex = numbers.includes(4, -2);
console.log(hasNumberFromNegativeIndex);  // Output: true

Explanation: The fromIndex of -2 means the method starts searching from the second-to-last element. Since 4 is present at index 3, it returns true.

Example 7: Using includes() with NaN

The includes() method correctly detects NaN (Not-a-Number) in an array, unlike some other methods such as indexOf().

javascript
const array = [NaN, 2, 3];
const hasNaN = array.includes(NaN);
console.log(hasNaN);  // Output: true

Explanation: The method returns true because NaN is included in the array.

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