JAVASCRIPT

How to Count the Occurrence of a Number in an Array Using JavaScript?

By Arun Kumar

Jun 03, 2025 2 Min Read 420 Views

(Last Updated)

In programming, one of the most common tasks is working with arrays, especially searching for elements and to count the occurrence of a number. Whether you’re preparing for coding interviews or solving real-world problems, mastering array operations is essential.

In this article, we tackle a simple yet practical challenge: to count the occurrence of a number in an Array using JavaScript

We’ll walk through a clear problem statement, analyze the input and output requirements, and finally explore a clean solution using built-in JavaScript functions.

How to Count the Occurrence of a Number in an Array Using JavaScript?

Problem Description
Your task:
Input Format
Output Format

Sample Test Case
JavaScript Solution

How the Code Works

Conclusion

How to Count the Occurrence of a Number in an Array Using JavaScript?

Given a number K and an array of N elements, how often does K appear in the array? If the number is not present at all, return -1.

Problem Description

You are given:

Two integers:
- N — the number of elements in the array
- K — the number whose repetitions you want to count
A list of N integers.

Your task:

Count how many times K appears in the array.
If K is not found at all, output -1.

Input Format

First line: Two space-separated integers N and K.
Second line: N space-separated integers representing the array.

Output Format

Print a single number:
- The count of how many times K appears in the array.
- If K is not present at all, print -1.

Sample Test Case

Input

6 2
1 2 3 5 7 8

Output

Explanation:
In this example, K is 2, and the array is [1, 2, 3, 5, 7, 8]. The number 2 appears once in the array, so the output is 1.

JavaScript Solution

If you’re using a coding platform to code JavaScript that accepts standard input/output (such as HackerRank or CodeChef), the following solution will work as expected:

const readline = require(‘readline’);

const rl = readline.createInterface({
input: process.stdin,
output: process.stdout
});

let inputLines = [];

rl.on(‘line’, function(line) {
inputLines.push(line);

if (inputLines.length === 2) {
let [n, k] = inputLines[0].split(‘ ‘).map(Number);
let arr = inputLines[1].split(‘ ‘).map(Number);

let count = arr.filter(x => x === k).length;

if (count === 0) {
console.log(‘-1’);
} else {
console.log(count);
}

rl.close();
}
});

JavaScript Solution

How the Code Works

Reading Input
The program reads two lines of input:
- The first line contains N and K.
- The second line contains the array elements.
Processing
The array is processed using Array.prototype.filter() to count how often K appears.
Output
- If the count is zero, it prints -1.
- Otherwise, it prints the number of occurrences of K.

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Conclusion

This problem is an excellent exercise for developing a deeper understanding of:

Reading and parsing structured input
Filtering and processing arrays
Handling edge cases effectively

Success Stories

About the Author

Arun Kumar

As an autodidact Full Stack Mentor, I specialize in developing innovative and scalable web applications with extensive expertise in both front-end and back-end technologies. I am dedicated to advancing the developer community through insightful blog posts and tutorials that foster technical mastery and encourage ongoing professional growth.

View all posts by Arun Kumar