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Find Index with indexOf()

Published at25 Jul 2026
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JavaScript Collections Easy 3 views
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Write a JavaScript function that takes an array and a value, and returns the first index where the value appears using Array.indexOf(), or -1 if the value is not present.

Array.prototype.indexOf() is a JavaScript method that returns the first index at which a given element can be found in the array. It uses strict equality (===) to compare elements. If the element is not present, it returns -1. An optional second parameter specifies the starting index for the search.

The indexOf() method is the standard way to check if an element exists in an array and find its position. The pattern arr.indexOf(val) !== -1 is the traditional way to check for existence before Array.includes() was introduced in ES6. It remains widely used for finding positions rather than just checking existence.

Time complexity is O(n) where n is the array length, as it performs a linear search. Space complexity is O(1). The search starts from index 0 and proceeds from left to right, stopping at the first match.

Edge cases include empty arrays (returns -1), NaN values (indexOf uses === which considers NaN !== NaN, so NaN is never found), multiple occurrences (returns the first one only), and searching for undefined or null values.

Example Input & Output

Example 1
Input
[],5
Output
-1
Explanation

Empty array returns -1

Example 2
Input
[42],42
Output
0
Explanation

Single element at index 0

Example 3
Input
[1,2,3,2,1],2
Output
1
Explanation

Returns first occurrence (index 1)

Example 4
Input
[10,20,30,40],30
Output
2
Explanation

30 is at index 2

Example 5
Input
[10,20,30],25
Output
-1
Explanation

25 not found, returns -1

Algorithm Flow

Recommendation Algorithm Flow for Find Index with indexOf()

Solution Approach

Find the first index of a specific value in an array using indexOf(). Returns the index if found, or -1 if the value is not present. The method uses strict equality (===) for comparison, meaning it distinguishes between 0 and '0' or false and 0.

function solution(arr, v) {
  return arr.indexOf(v);
}

indexOf() searches from the beginning of the array. Use lastIndexOf() to search from the end, which returns the last occurrence. For finding objects by a property condition, use findIndex() which accepts a callback function.

Time complexity is O(n), space complexity is O(1).

Best Answers

javascript - Approach 1
function solution(arr, val) {
  return arr.indexOf(val);
}