suggestion: for group by

1. make it acceptable of iterable objects
2. use `Object.create(null)` to avoid having to check if xyz is an array using Array.isArray
3. allow using a 3th argument as a destination object that defaults to an object. (incase you need to use the same object for more stuff)
4. use `??=` assignment to clamp it down even more

```js
function groupBy(iterable, cb, dest = Object.create(null)) {
if (typeof cb !== 'function') throw new Error('expected a function for second argument')
if (typeof dest !== 'object') throw new Error('expected a object for third argument')

for (var item of iterable) (dest[cb(item)] ??= []).push(item)

return dest
}
```