How can I group an array of objects by key?

While it isn't in the interest of answering your question with the most optimal or efficient code, I thought this a fun oportunity to turn this into a Code Golf exercise.

This is a slightly altered version of @metakungfus answer, main difference being that it omits the original key from the resulting objects since it's no longer needed on the object itself in some cases since it's now available in the parent object.

Additionally, it heavily utilizes object, parameter and array expansion along with the reduce JavaScript function.

const groupBy=(_k, a)=>a.reduce((r,{[_k]:k,...p})=>({...r,...{[k]: (r[k]?[...r[k],{...p}]:[{...p}])}}),{});

Considering your original input object:

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'
    },
];

console.log(groupBy('make', cars));

Would result in:

{
  audi: [
    { model: 'r8', year: '2012' },
    { model: 'rs5', year: '2013' }
  ],
  ford: [
    { model: 'mustang', year: '2012' },
    { model: 'fusion', year: '2015' }
  ],
  kia: [
    { model: 'optima', year: '2012' }
  ]
}

Here is another solution to it. As requested.

I want to make a new array of car objects that's grouped by make:

function groupBy() {
  const key = 'make';
  return cars.reduce((acc, x) => ({
    ...acc,
    [x[key]]: (!acc[x[key]]) ? [{
      model: x.model,
      year: x.year
    }] : [...acc[x[key]], {
      model: x.model,
      year: x.year
    }]
  }), {})
}

Output:

console.log('Grouped by make key:',groupBy())