JavaScript オブジェクトを複数のプロパティごとにグループ化し、それらの値を合計するにはどうすればよいでしょうか?

arr = [
    {shape: 'square', color: 'red', used: 1, instances: 1},
    {shape: 'square', color: 'red', used: 2, instances: 1},
    {shape: 'circle', color: 'blue', used: 0, instances: 0},
    {shape: 'square', color: 'blue', used: 4, instances: 4},
    {shape: 'circle', color: 'red', used: 1, instances: 1},
    {shape: 'circle', color: 'red', used: 1, instances: 0},
    {shape: 'square', color: 'blue', used: 4, instances: 5},
    {shape: 'square', color: 'red', used: 2, instances: 1}
];

[{shape: "square", color: "red", used: 5, instances: 3},
{shape: "circle", color: "red", used: 2, instances: 1},
{shape: "square", color: "blue", used: 11, instances: 9},
{shape: "circle", color: "blue", used: 0, instances: 0}]

var helper = {};
var result = arr.reduce(function(r, o) {
  var key = o.shape + '-' + o.color;

  if(!helper[key]) {
    helper[key] = Object.assign({}, o); // create a copy of o
    r.push(helper[key]);
  } else {
    helper[key].used += o.used;
    helper[key].instances += o.instances;
  }

  return r;
}, []);