Creates an object composed of keys generated from the results of running each element of collection through iteratee. The corresponding value of each key is an array of the elements responsible for generating the key. The iteratee is invoked with one argument: (value).

_.groupBy([6.1, 4.2, 6.3], Math.floor);
// => { '4': [4.2], '6': [6.1, 6.3] }

// using the `_.property` callback shorthand
_.groupBy(['one', 'two', 'three'], 'length');
// => { '3': ['one', 'two'], '5': ['three'] }