01. Sub Sum

function subSum(arr, startIndex, endIndex) {
  if (!Array.isArray(arr)) {
    return NaN;
  }
  if (Number(startIndex) < 0) {
    startIndex = 0;
  }
  if (Number(endIndex) > arr.length - 1) {
    endIndex = Number(arr.length - 1);
  }

  const inputArr = arr.map(Number).slice(startIndex, endIndex + 1);

  const isNan = inputArr.some((el) => isNaN(el));

  if (!isNan) {
    let sum = 0;

    for (let i = 0; i < inputArr.length; i++) {
      sum += inputArr[i];
    }

    if (sum % 1 === 0) {
      return Number(sum);
    }
    return Number(sum.toFixed(1));
  }
  return NaN;
}