temp.js

function calsum(arr) {
  const tempArr = [];
  for (let i = 0; i < arr.length; i++) {
    if (i === 0) {
      tempArr.push(arr[0]);
    }
    if (i === 1) {
      const sub = arr[1] - arr[0];
      console.log(sub);
      tempArr.push(sub);
    }
    if (i === 2) {
      tempArr.push(0);
    }
    if (i === 3) {
      const sub = arr[3] - arr[2];
      tempArr.push(sub);
    }
  }

  const sum = tempArr.reduce((acc, data) => {
    return acc + data;
  }, 0);

  console.log(sum);
}

calsum([4, 9, 2, 3]);