A JavaScript function to check if a given number is an Armstrong number

isArmstrong.js

function isArmstrong(num){
  var numStr = num.toString();
  var isArmstrongNum = false;
  var sum = 0;
  if(numStr.length === 3){
    for(var i=0; i < numStr.length; i++) {
      var n = parseInt(numStr.charAt(i));
      sum += Math.pow(n, 3);
    }
    if(sum === num) {
      isArmstrongNum = true;
    }
  }

  return isArmstrongNum;
}