### Group by

Passing an array of values, a function or a property name thats going to be run against each value in the array,
returns an object where the keys are the mapped results and the values is an array of the original values that generated the same results.

```js
const groupBy = (values, fn) => {
  return (typeof fn === 'function' ? values.map(fn) : values.map((val) => val[fn]))
    .reduce((acc, val, i) => {
      acc[val] = acc[val] === undefined ? [values[i]] : acc[val].concat(values[i]);
      return acc;
    }, {});
}
// groupBy([6.1, 4.2, 6.3], Math.floor) -> {4: [4.2], 6: [6.1, 6.3]}
// groupBy(['one', 'two', 'three'], 'length') -> {3: ['one', 'two'], 5: ['three']}
```