Edit in JSFiddle

const reverseArr = (arr) => {
  let temp = 0;
  for (let i = 0; i < arr.length/2; i++) {
    temp = arr[i];
    arr[i] = arr[arr.length - 1 -i];
    arr[arr.length - 1 - i] = temp; 
  }
  return arr;
  
}

const arr = [3, 5, 6, 2, 4, 7];
//7,4,2,6,5,3
console.log(reverseArr(arr));