JavaScript - Find Index of a Value in Array

Here are some effective methods to find the array index with a value in JavaScript.

Using indexOf() - Most Used

indexOf() returns the first index of a specified value in an array, or -1 if the value is not found.

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

// Find index of value 30
const index = a.indexOf(30);
console.log(index);

// Value not found
const notFound = a.indexOf(60);
console.log(notFound); 

2
-1

Using findIndex() for Complex Conditions

findIndex() is used for finding the index of an element based on complex conditions. It takes a callback function that can include any logic for locating an item within the array.

const a = [10, 15, 20, 25, 30];

// Find index of first value greater than 18
const index = a.findIndex(value => value > 18);
console.log(index);

2

Using for Loop for Custom Search

Using a for loop gives you complete control over how you search for items. You can stop the loop as soon as you find what you’re looking for, making it a flexible option for setting custom rules.

const a = [5, 10, 15, 20, 25];
let index = -1;

// Find index of value 20
for (let i = 0; i < a.length; i++) {
    if (a[i] === 20) {
        index = i;
        break;
    }
}
console.log(index); 

3

Using lastIndexOf() for Reverse Search

If you need the last occurrence of a value you can use lastIndexOf() method which returns the index of the last match, or -1 if the value is not found. This is useful what an arrays contain duplicate values.

const a = [5, 10, 15, 10, 5];

// Find last index of value 10
const index = a.lastIndexOf(10);
console.log(index);

3

Importance of Finding Array Indices

Finding array indices is essential for

• Data Manipulation: Helps identify and modify elements based on their position.
• Search Optimization: Enables targeted data retrieval for better performance.
• Conditional Processing: Allows for specific actions on elements that meet criteria.