// Implement the groupBy method, extending standard array methods. // The method should return a grouped version of the array – an object, // where each key is the result of the function fn(arr[i]), // and each value is an array containing all original array elements with that key.

It is possible to extend the array prototype by adding a groupBy method, which takes a function fn and returns an object where the keys are the result of calling fn on each element, and the values are arrays of elements with that key.

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

// Example usage:
const arr = [6.1, 4.2, 6.3];
const grouped = arr.groupBy(Math.floor);
console.log(grouped); // { '4': [4.2], '6': [6.1, 6.3] }