Group your array by multiple keys with comparing customization!

groupify.js

/**
 * Group your items in array by multiple keys!
 *
 * @param collection
 * @param retrieveFunctions
 * @param compareFunctions
 * @returns {Array}
 */
function groupify(
    collection,
    retrieveFunctions = {attribute: value => value},
    compareFunctions = {attribute: (groupValue, value) => groupValue === value}
) {
    let groups = [];
    let keyValues = {};

    collection.forEach(item => {
        for (let attribute in retrieveFunctions) {
            keyValues[attribute] = retrieveFunctions.hasOwnProperty(attribute) ? retrieveFunctions[attribute](item[attribute]) : undefined;
        }
        let group = groups.find(group => {
            for (let key in retrieveFunctions) {
                if (!group || !group.keys[key]) {
                    return false;
                }
                if (compareFunctions[key] instanceof Function) {
                    if (!compareFunctions[key](group.keys[key], keyValues[key])) {
                        return false;
                    }
                } else if (group.keys[key] !== keyValues[key]) {
                    return false;
                }
            }

            return true;
        });

        !group ? groups.push({keys: keyValues, items: [item]}) : group.items.push(item);
        keyValues = {};
    });

    return groups;
}