2631. Group By

Enhance all arrays in JavaScript so that you can call array.groupBy(fn). It should return an object grouping the items in the array based on the callback's returned key.

📄 Problem Statement

Add a method groupBy(fn) to Array.prototype. The method should:

Call fn(element) for each item in the array.
Group elements that return the same key into the same array.
Return an object where each key maps to an array of grouped items.

❌ Do not use _.groupBy from Lodash or any third-party library.

💡 Example


[1,2,3,4,5].groupBy(n => n % 2 === 0 ? "even" : "odd")
// ➞ { "odd": [1, 3, 5], "even": [2, 4] }

["apple", "banana", "apricot"].groupBy(word => word[0])
// ➞ { "a": ["apple", "apricot"], "b": ["banana"] }

🧠 Explanation

The method loops through the array, applies the callback function to each element to determine its key, then pushes that element into the correct array inside a result object.

If the key hasn't been seen before, initialize it with an empty array first.

✅ Your Solution


/**
 * @param {Function} fn
 * @return {Object}
 */
Array.prototype.groupBy = function(fn) {
    const result = {};

    for (const item of this) {
        const key = fn(item);
        if (!result[key]) {
            result[key] = [];
        }
        result[key].push(item);
    }

    return result;
};