How to sum and recognize and return case '0' , and '-' in

function sum2(num) {
  let sum ;
  let str = String(num)
  for(let i = 0 ; i < str.length; i++){
  if(i === 0) {
    sum = parseInt(str[i]);
    
  } else {
    sum = sum + parseInt(str[i])
  }
  }
    return sum;
}
debugger;
let output = sum2(1148);
console.log(output); // --> 14 = 1 + 1 + 4 + 8

output = sum2(-316);
console.log(output); // --> 4 = -3 + 1 + 6

i need to case num is -316, and num is 0 but it will be return 'NaN'..

what should i do..?