Write JavaScript to accept an input from user and check whether number is an Armstrong in nature or not.

An Armstrong number equals the sum of its digits, each raised to the power of the number of digits (e.g. \(153 = 1^3 + 5^3 + 3^3\)). The script below reads a number from the user and checks this.

<!DOCTYPE html>
<html>
<body>
<input type="number" id="num" placeholder="Enter a number">
<button onclick="checkArmstrong()">Check</button>
<p id="result"></p>

<script>
function checkArmstrong() {
  let str = document.getElementById("num").value;
  let n = parseInt(str);
  if (isNaN(n) || n < 0) {
    document.getElementById("result").innerHTML = "Enter a valid number";
    return;
  }
  let digits = String(n).length;
  let sum = 0, temp = n;
  while (temp > 0) {
    let d = temp % 10;
    sum += Math.pow(d, digits);
    temp = Math.floor(temp / 10);
  }
  let msg = (sum === n) ? n + " is an Armstrong number"
                        : n + " is not an Armstrong number";
  document.getElementById("result").innerHTML = msg;
}
</script>
</body>
</html>

How it works

1. The value typed in the input box is read and converted to an integer.
2. digits stores the number of digits (3 for 153).
3. The loop takes the last digit with % 10, adds its power to sum, and removes it with Math.floor(temp / 10).
4. If sum equals the original number, it is an Armstrong number.

Test: 153 → \(1 + 125 + 27 = 153\) (Armstrong); 370, 371, 407 and 9474 are also Armstrong numbers; 123 → \(1 + 8 + 27 = 36\) (not). A simpler version can use let n = parseInt(prompt("Enter a number")); and alert(msg).