const numbers = [1, 2, 3, 4, 5];

const reverse = (array) => {
    const reversed = [];
    for (const [index, element] of numbers.entries())
        reversed[array.length - index - 1] = element;
    return reversed;
}

console.log(reverse(numbers)); // [ 5, 4, 3, 2, 1 ] 

We can use a for ... of loop on the entries of the array we want to reverse, and push the element into the desired position of the array.

To calculate the position we want, we can just do array length - index - 1.

This means that if we have an array of 5 items, [1, 2, 3, 4, 5] and we iterate through, we will be doing...

1. 5 - 0 - 1 = 4 for the index of the first item.
2. 5 - 1 - 1 = 3 for the index of the second item.
3. 5 - 2 - 1 = 2 for the index of the third item.
4. 5 - 3 - 1 = 1 for the index of the fourth item.
5. 5 - 4 - 1 = 0 for the index of the fifth item.

function reverse(arr){
// get length
var len = arr.length;
// create new array
var newArr = [];
// loop through array
for(var i = len - 1; i >= 0; i--){
    // push to new array
    newArr.push(arr[i]);
}
// return new array
return newArr;}

reverse([1,2,3,4,5]);

try this.

There are multiple issues :

1. for initialisation happens once, so if you do let a = lemgth - i, it doesn't update a any further, even though i changes.
2. You're reassigning values to your input array itself with reversed array, which is empty itself.
3. You're output-ing the input array (arr).

I tried to fix your snippet, and also adding better approach:

const reverseArr = (...args) => {
      let length = args.length - 1
      let reversed = []
      let i = 0
      // update 'a' as soon as you update 'i'
      for(let a = length - i; i <= length; i++, a = length - i){
        // update the standby array 'reveresed'
        reversed[i] = args[a]
      }
      // print the reversed array instead of 'args' 
      console.log(reversed)
    }


// Alternate approach (in-place reverse)
const reverseInput = (...args) => {
  let left = 0, right = args.length - 1;
  while(left <= right) {
    // swap the left and right elements in  array
    [args[left], args[right]] = [args[right], args[left]] ;
    left++;
    right--;
  }
  console.log(args);
}

    // Approach 1
    console.log(reverseArr(3, 5, 4, 1))
    // Approach 2
    console.log(reverseInput(3,5,4,1));