Group By.js

// 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 which generate 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.

// Please solve it without lodash's _.groupBy function.

 

// Example 1:

// Input: 
// array = [
//   {"id":"1"},
//   {"id":"1"},
//   {"id":"2"}
// ], 
// fn = function (item) { 
//   return item.id; 
// }
// Output: 
// { 
//   "1": [{"id": "1"}, {"id": "1"}],   
//   "2": [{"id": "2"}] 
// }
// Explanation:
// Output is from array.groupBy(fn).
// The selector function gets the "id" out of each item in the array.
// There are two objects with an "id" of 1. Both of those objects are put in the first array.
// There is one object with an "id" of 2. That object is put in the second array.


// Code

/**
 * @param {Function} fn
 * @return {Object}
 */
Array.prototype.groupBy = function(fn) {
    return this.reduce((grouped, item) => {
        const key = fn(item); // Compute the key using the provided function
        if (!grouped[key]) {
            grouped[key] = []; // Initialize an array for the key if it doesn't exist
        }
        grouped[key].push(item); // Add the current item to the corresponding group
        return grouped;
    }, {}); // Start with an empty object
};