why bracket change my out on line 7 in javaScript [closed]

While working with arrays in JavaScript, Here I tried to calculate average using reduce method.
My question is at line 3: without using bracket avg + num/ arr.length gives correct output but using bracket (avg + num) / arr.length gives wrong output.

Can anyone explain what’s the reason behind this wired behavior?

const calcAverage = function (nums) {
  const average = nums.reduce(
    (avg, num, i, arr) => (avg + num) / arr.length,
    0
  );
  return average;
};
const avg = calcAverage([1, 1, 1, 1, 1]);
console.log(avg);