It also seems wrong to mutate the accumulator in the reduce example, it would just be simpler to use `forEach` and use a variable defined outside if you're going to write code this way?
1 : http://www.nicoespeon.com/en/2015/01/pure-functions-javascri...
let acc = {...}
footballPlayers.reduce(...,acc)
I'd do it like this (yay for one-ish-liners!):
const playersByCountry = footballPlayers.reduce((acc, player) => {
return {...acc, [player.country]: [...(acc[player.country] || []), player]};
}, {});