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.
[1,2,3,2,4,1,5,1,6].groupBy(val => val % 3);
/*
{
0: [3, 6],
1: [1, 4, 1, 1],
2: [2, 2, 5]
}
*/If no function is passed, the element itself should be taken.
[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]
}
*/