JavaScript Program to Find Neon Number in a Range

A Neon number is a number where the sum of digits of the square of the number is equal to the number itself. For example, 9 is a Neon number because 9^2 = 81 and the sum of digits of 81 is 8 + 1 = 9, which is the number itself. We want to find all the Neon numbers within a given range.

Below are the approaches to find Neon Numbers in a Range:

Neon Number in a Range Using Iteration

In this approach, we iterate through each number in the given range and check if it’s a Neon number by calculating the sum of digits of the square of the number.

Example: The below code example Uses the simple iteration Method to find a Neon number in a Range in JavaScript.

Javascript

function isNeon(number) {
    let square = number * number;
    let sum = 0;
    while (square > 0) {
        sum += square % 10;
        square = Math.floor(square / 10);
    }
    return sum === number;
}

function findNeonNumbersInRange(start, end) {
    let neonNumbers = [];
    for (let i = start; i <= end; i++) {
        if (isNeon(i)) {
            neonNumbers.push(i);
        }
    }
    return neonNumbers;
}

console.log(findNeonNumbersInRange(1, 100));

Output

[ 1, 9 ]

Neon Number in a Range Using recursion

In this approach, the function will calculate the square of the number and then sum its digits. If the sum equals the original number, it’s a Neon number. Then, we’ll recursively call the function for the next number in the range until we reach the end.

Example: The below code example Uses the recursion Method to find a Neon number in a Range in JavaScript.

Javascript

function sumOfDigits(number) {
  if (number === 0) {
    return 0;
  }
  return (number % 10)
  + sumOfDigits(Math.floor(number / 10));
}

function isNeon(number) {
  let square = number * number;
  let sum = sumOfDigits(square);
  return sum === number;
}

function findNeonInRangeRecursive(start, end, result = []) {
  if (start > end) {
    return result;
  }
  if (isNeon(start)) {
    result.push(start);
  }
  return findNeonInRangeRecursive(start + 1, end, result);
}

console.log(findNeonInRangeRecursive(1, 100));

Output

[ 1, 9 ]

Neon Number in a Range Using JavaScript built-in methods

It generates an array of numbers within the given range using Array.from(), filters out the Neon numbers using the filter() method, and returns the resulting array.

Example: The below code example Uses the Array.from(), reduce() and filter() Method to find a Neon number in a Range in JavaScript.

Javascript

function sumOfDigits(number) {
  return number
    .toString()
    .split("")
    .reduce((acc, digit) => acc + parseInt(digit), 0);
}

function isNeon(number) {
  return sumOfDigits(number * number) === number;
}

function findNeonInRange(start, end) {
  return Array
    .from
    (
      { length: end - start + 1 },
      (_, index) => index + start,
    ).filter((num) => isNeon(num));
}

console.log(findNeonInRange(1, 100));

Output

[ 1, 9 ]