> The indexs are numbers as _.some will give number _key for arrays.

In that case `_.findKey` is incorrect for arrays and should always return a string key value too.

If it helps think of `_.findKey` as an "Objects" method and `_.findIndex` as an :Arrays" method, they have distinct behaviors and are not a "Collections" method. So when implementing `_.findKey` you don't want to use `_.some` which is a "Collections" method as this will treat objects with a `length` property as an array (not the desired result).