JavaScript Array indexOf() Method

The indexOf() method returns the first index at which the element can be found in the array, or -1 if the element is not present. This method is case sensitive for string element.

const arr = [10, 20, 30, 40, 50];

console.log(arr.indexOf(10)); // Output: 0
console.log(arr.indexOf(30)); // Output: 2
console.log(arr.indexOf(50)); // Output: 4

Syntax

indexOf(searchElement) // or
indexOf(searchElement, fromIndex)

Parameters

Parameters	Descriptions
searchElement	The element whose index is to be find.
fromIndex (Optional)	Its default value is 0, which means search the element from beginning of the array. If fromIndex is greater than or equal to the array’s length, the method returns -1 as no search is performed. If fromIndex is negative, then the index counts back from the end of the array. if -array.length <= fromIndex < 0, fromIndex + array.length is used.

Return Value

It returns the first index of searchElement in the array, or -1 if the element is not found.

Example 1: Basic example of array indexOf() method.

const arr = ["apple", "banana", "cherry", "apple"];

console.log(arr.indexOf("apple"));  // Output: 0
console.log(arr.indexOf("cherry")); // Output: 2
console.log(arr.indexOf("mango"));  // Output: -1

Example 2: The indexOf() method with two parameters.

const arr = [10, 20, 30, 40, 50];

console.log(arr.indexOf(40, 2)); // Output: 3
console.log(arr.indexOf(10, 1)); // Output: -1

Example 3: Set the fromIndex value to Negative.

const arr = [10, 20, 30, 40, 50];

console.log(arr.indexOf(50, -1)); // Output: 3 (starts search at the last element)
console.log(arr.indexOf(40, -3)); // Output: 1 (starts search at index 1)

Example 4: Case Sensitivity

const arr = ["Apple", "banana", "Cherry"];

console.log(arr.indexOf("apple")); // Output: -1
console.log(arr.indexOf("Apple")); // Output: 0