// Implement a groupBy method that extends the standard array methods. // The method should return a grouped version of the array — an object, // where the keys are the keys of the array from the results of calling the passed function fn(arr[i]), // and the values are arrays containing all the elements of the original array with this key. // code here // Example 1 const array1 = [ { id: 1 }, { id: 1 }, { id: 2 } ]; const fn = (item) => item.id; console.log(array1.groupBy(fn)); // { // 1: [{ id: 1 }, { id: 1 }], // 2: [{ id: 2 }] // } // Example 2 const array2 = [1, 2, 3]; console.log(array2.groupBy(String)); // { // "1": [1], // "2": [2], // "3": [3] // } // Example 3 const array3 = [1.3, 0.5, 1.4]; console.log(array3.groupBy(Math.round)); // { // 1: [0.5, 1.4], // 2: [1.3] // }

To extend the standard array methods in JavaScript, you can add a groupBy method to the Array prototype. This method takes a function fn that computes a key for each element and returns an object where the keys are the results of fn, and the values are arrays of elements with that key.

Example implementation:

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;
  }, {});
};

Testing with examples:

const array1 = [
  { id: 1 },
  { id: 1 },
  { id: 2 }
];
const fn = (item) => item.id;
console.log(array1.groupBy(fn));
// {
//  1: [{ id: 1 }, { id: 1 }],
//  2: [{ id: 2 }]
// }

const array2 = [1, 2, 3];
console.log(array2.groupBy(String));
// {
//  "1": [1],
//  "2": [2],
//  "3": [3]
// }

const array3 = [1.3, 0.5, 1.4];
console.log(array3.groupBy(Math.round));
// {
//  3: [1.3],
//  1: [0.5, 1.4]
// }

This method is useful for grouping data by any criterion.