Javascript - Counting array elements by reduce method until specific value occurs doesn't give a correct output

Question

const arr = [5,6,0,7,8];
const sum = (arr,num) => arr.reduce((total)=>(num==0 ? total : total+num), 0) 
console.log(sum(arr, 0))

Please check how

Answer 1

Since you need to stop summing values part way through the array, this might be most simply implemented using a for loop:

const arr = [5, 6, 0, 7, 8];
const num = 0;

let sum = 0;
for (let i = 0; i < arr.length; i++) {
  if (arr[i] == num) break;
  sum += arr[i];
}
console.log(sum);

If you want to use reduce, you need to keep a flag that says whether you have seen the num value so you can stop adding values from the array:

const arr = [5, 6, 0, 7, 8];

const sum = (arr, num) => {
  let seen = false;
  return arr.reduce((c, v) => {
    if (seen || v == num) {
      seen = true;
      return c;
    }
    return c + v;
  }, 0);
}

console.log(sum(arr, 0));
console.log(sum(arr, 8));