Group array elements upon a criteria.

groupBy.js

/*
 * @param property string, number or function. This is the crieteria of the grouping function 
 */
function groupBy(arr, property) {
  return arr.reduce( (memo, el, i) => {
  	let grpProperty = (() => {
    	if (typeof property === 'string' || typeof property === 'number') {
      	return el[property];
      } else if (typeof property === 'function') {
      	return property(el, i, arr);
      }
    })();
    if (!grpProperty) return memo;
    if (!memo[grpProperty]) { memo[grpProperty] = []; }
    memo[grpProperty].push(el);
    return memo;
  }, {});
};