Method takes a key and groups objects of an array (if running `groupCondition` on the object yields true) by the key passed.

groupBy.js

/**
 * Method takes a key and groups objects of an array
 * (if running `groupCondition` on the object yields true) 
 * by the key passed.
 *
 * @param {string} key  The key to group the object of the array by
 * @param {function} groupCondition   Function that is run on every
 *  object in the array and if it returns true, the object is then grouped
 * 
 * @returns {array} Array of objects grouped by key
 */
Array.prototype.groupBy = function(key, groupCondition) {
    if (key === undefined || typeof key !== 'string' || this[0][key] === undefined ) {
        throw new Error(`[key ${typeof key}]: Grouping key must be a string and a key in all the objects of the array to be grouped.`);
    }
    
    if (groupCondition && typeof groupCondition !== 'function') {
        throw new Error(`[groupCondition ${typeof groupCondition}]: Grouping condition must be a function.`);
    }
    
    if (!groupCondition) {
        groupCondition = () => true;
    }
    
    return this.reduce((objectsByKeyValue = [], obj) => {
        if (groupCondition(obj)) {
            const value = obj[key];
            delete obj[key];
        
            objectsByKeyValue[value] = (objectsByKeyValue[value] || []).concat(obj);
        
            return objectsByKeyValue;
        }
    }, {});
}


// USAGE
const groupingCondition = person => person.age >= 30 && person.age <= 40);

const grouped = [
    {name: 'Kator', age: 21, gender: 'male' },
    {name: 'Sandra', age: 31, gender: 'female' },
    { name: 'John', age: 30, gender: 'male' },
    { name: 'Juliet', age: 39, gender: 'female' }
].groupBy('gender', groupingCondition);

console.log(grouped);