const 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'

}, ];

​

const carsTransformed = cars.reduce((acc, next) => {

  if (acc[next.make]) {

    acc[next.make].push({model: next.model, year: next.year})

  } else {

    acc[next.make] = [{model: next.model, year: next.year}]

  }

  return acc

}, {})

      console.log(carsTransformed)