var cars = [{ make: 'audi', model: 'r8', year: '2012' }, { make: 'audi', model: 'rs5', year: '2013' }, { make: 'ford', model: 'mustang', year: '2012' }, { make: 'ford', model: 'fusion', year: '2015' }, { make: 'kia', model: 'optima', year: '2012' }];
var result = cars.reduce(function (r, a) {
r[a.make] = r[a.make] || [];
r[a.make].push(a);
return r;

console.log(result);