groupby.js

/**
 * Group an array by value from key
 * @param {Array} arr - Array
 * @param {(String|Function)} key - Key to take value from
 * @returns {Object} Object of groups
 */
function groupBy(arr, key) {
    // if 'key' is a function, set fn to 'key'
    let fn = typeof key === 'function' ? key : null;
    
    return arr.reduce((a, b) => {
        let value = fn ? fn(b) : b[key];
        
        (a[value] = a[value] || []).push(b);
        
        return a;
    }, {});
}