01. Sub Sum

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

    let sum = 0;
    for (let i = startIndex; i <= endIndex; i++) {
        sum += Number(arr[i]);
    }

    return sum
}