SOURCE

var arr = [2,1,7,4,5,8];
console.log(arr);
console.log(reverseArr(arr));

function reverseArr (arr) {
  var len = arr.length;
  for (var i = 0; i < len / 2; i++) {
  	var temp = arr[i];
  	arr[i] = arr[len-1-i];
  	arr[len-1-i] = temp;
	}
  return arr;
}