Group By

Write code that enhances all arrays such that you can call the array.groupBy(fn) method on any array and it will return a grouped version of the array.

A grouped array is an object where each key is the output of fn(arr[i]) and each value is an array containing all items in the original array with that key.

The provided callback fn will accept an item in the array and return a string key.

The order of each value list should be the order the items appear in the array. Any order of keys is acceptable.

Solve it without lodash's _.groupBy function.

Array.prototype.groupBy = function (fn) {
  const grouped = {};
  
  this.forEach((item) => {
    const key = fn(item);
    
    if (!grouped[key]) {
      grouped[key] = [];
      grouped[key].push(item);
    }
    
    grouped[key].push(item);
  });
  
  return grouped;
};

// Example usage:
const arr = [{ id: 1 }, { id: 1 }, { id: 2 }];
const grouped = arr.groupBy(function (item) { 
  return item.id; 
});

console.log(grouped);