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.

寫一個可以將數列分組的語法，透過提供的方法進行分組。

解法如下：

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

思路：
設定結果是一個多陣列(const result = {};)，因為在範例１中是在陣列中再包含陣列；
用this代表Array，並巡歷；
透過fn(item)的回傳值來分組；
所以我們可以判斷result是否有這個回傳值，有就增加，沒有就創造。
那判斷「指定的屬性」是否存在，就可以使用.hasOwnProperty()方法。