Categorize data(an array of objects) based on a specified category (a property in the data)

categorizeData.js

/*
*   Categorize data based on a specified category (a property in the data)
*   @data an array of objects
*   @category the criteria for categorizing the @data. @category must be a property in @data
*/

module.exports = (data, category) => {
    // Get the category list with which to categorize the data
    // getCategoryList is assigned to an iife
    let getCategoryList = (() => {
        let categoryList = [];
        data.forEach(value => {
            if (!categoryList.includes(value[category])) {
                categoryList.push(value[category]);
            }
        });
        return categoryList;
    })();

    // Sort the data according to the category list
    let newObject = {};
    getCategoryList
        .forEach(categoryItem => newObject[categoryItem] = data
            .filter(Obj => Obj[category] === categoryItem)
            .map(entry => {
                delete entry[category];
                return entry;
            }));

    // Return categorized data and category list
    return {
        categoryList: getCategoryList,
        data: newObject
    };
};

Example:
// Save to file and import or require it...
import categorizeData from '/path/to/categorizeData.js'

const bills = [
{date: '2018-01-20', amount: '220', category: 'Electricity'},
{date: '2018-01-20', amount: '20', category: 'Gas'},
{date: '2018-02-20', amount: '120', category: 'Electricity'}
];

let categorizedBills = categorizeData(bills, 'category')

// categorizedBills.categoryList returns [ 'Electricity', 'Gas' ]

/*
categorizedBills.data returns:
{
Electricity: [ { date: '2018-01-20', amount: '220' }, { date: '2018-02-20', amount: '120' } ],
Gas: [ { date: '2018-01-20', amount: '20' } ]
}

*/