JavaScript Program to Check Armstrong Number

An Armstrong number (also called a narcissistic number) is a number that equals the sum of its own digits, each raised to the power of the total number of digits. For example, 153 has 3 digits, and 1³ + 5³ + 3³ = 1 + 125 + 27 = 153 — so 153 is an Armstrong number.

Let’s write a JavaScript program that checks whether a given number is an Armstrong number, using a basic for loop to extract and sum the digit powers.

JavaScript Program to Check Armstrong Number (Using a Loop)

function isArmstrong(num) {
  const numStr = String(num);
  const power = numStr.length;
  let sum = 0;

  // add each digit raised to the power of total digit count
  for (let i = 0; i < numStr.length; i++) {
    const digit = Number(numStr[i]);
    sum += Math.pow(digit, power);
  }

  return sum === num;
}

const number = 153;
if (isArmstrong(number)) {
  console.log(`${number} is an Armstrong number`);
} else {
  console.log(`${number} is not an Armstrong number`);
}

Output:

153 is an Armstrong number

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JavaScript Program to Check Armstrong Number (Using reduce())

The same logic can be written more compactly with Array.prototype.reduce() once the number is split into an array of digits.

function isArmstrongReduce(num) {
  const digits = String(num).split("");
  const power = digits.length;

  const sum = digits.reduce((acc, digit) => acc + Math.pow(Number(digit), power), 0);

  return sum === num;
}

console.log(isArmstrongReduce(9474));  // 4 digits: 9^4 + 4^4 + 7^4 + 4^4 = 9474
console.log(isArmstrongReduce(123));

Output:

true
false

Both versions run in O(d) time, where d is the number of digits — effectively constant time for any normal-sized number. The loop version is easier to read for beginners; the reduce() version is the idiomatic one-liner you’ll see more often in real codebases.