Implementing Array.prototype.groupBy method.js

/*
Description:
Add a groupBy method to Array.prototype so that elements in an array could be grouped by the result of evaluating a function on each element.

The method should return an object, in which for each different value returned by the function there is a property whose value is the array of elements that return the same value.

If no function is passed, the element itself should be taken.

Example:

[1,2,3,2,4,1,5,1,6].groupBy()
{
  1: [1, 1, 1],
  2: [2, 2],
  3: [3],
  4: [4],
  5: [5],
  6: [6]
}

[1,2,3,2,4,1,5,1,6].groupBy(function(val) { return val % 3;} )
{
  0: [3, 6],
  1: [1, 4, 1, 1],
  2: [2, 2, 5]
}
For more examples have a look at the example test cases
*/
Array.prototype.groupBy = function(fn) {
  return this.reduce((a, c) => {
    let v = fn ? fn(c) : c;
    (a[v] = a[v] || []).push(c);
    return a;
  }, {});
};