Array.indexOf()

The Array.indexOf() method in JavaScript is used to find the first occurrence of a specified element in an array. It returns the index of the element if found, or -1 if the element is not present.

Syntax

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indexOf(searchElement)
indexOf(searchElement, fromIndex)

Parameters

ParameterDescription
searchElementThe element to locate in the array.
fromIndex (optional)The index to start the search from. Defaults to 0. If fromIndex is negative, it is treated as array.length + fromIndex.

Return Value

The indexOf() method returns the first index at which the specified searchElement is found, or -1 if the element is not found in the array.

Examples

1. Finding the Index of an Element

This example demonstrates how to find the first occurrence of an element in an array.

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const fruits = ['apple', 'banana', 'cherry', 'apple'];

console.log(fruits.indexOf('apple'));
console.log(fruits.indexOf('cherry'));

Output

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  1. fruits.indexOf('apple') returns 0 as the first occurrence of 'apple' is at index 0.
  2. fruits.indexOf('cherry') returns 2 as 'cherry' is located at index 2.

2. Using fromIndex to Start the Search

Here, the fromIndex parameter specifies where the search should start.

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const fruits = ['apple', 'banana', 'cherry', 'apple'];

console.log(fruits.indexOf('apple', 1));
console.log(fruits.indexOf('banana', 2));

Output

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  1. fruits.indexOf('apple', 1) starts the search from index 1 and finds 'apple' at index 3.
  2. fruits.indexOf('banana', 2) starts the search from index 2. Since 'banana' is not present after index 2, it returns -1.

3. Searching with Negative fromIndex

A negative fromIndex indicates the offset from the end of the array.

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const fruits = ['apple', 'banana', 'cherry', 'apple'];

console.log(fruits.indexOf('apple', -2));
console.log(fruits.indexOf('cherry', -3));

Output

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In the first example, fruits.indexOf('apple', -2) starts searching from the second-to-last element and finds 'apple' at index 3.

In the second example, fruits.indexOf('cherry', -3) starts searching from the third-to-last element and finds 'cherry' at index 2.

4. Element Not Found

When the searchElement is not found, the indexOf() method returns -1.

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const fruits = ['apple', 'banana', 'cherry'];

console.log(fruits.indexOf('grape'));

Output

-1

Since 'grape' is not in the fruits array, the method returns -1.