{
  $bucket: {
    groupBy: <arbitrary expression>,
    boundaries: [
      // Array of constant values.
    ],
    default: <constant expression>,
    output: {  // Still optional, defaults to {count: {$sum: 1}}
      fieldName1: <accumulator 1>,
      fieldName2: <accumulator 2>
    }
  }
}

> db.example.insert([
  {_id: 0, screenSize: 30},
  {_id: 1, screenSize: 24},
  {_id: 2, screenSize: 42},
  {_id: 3, screenSize: 22},
  {_id: 4, screenSize: 55},
]);
> db.example.aggregate([
  {
    $bucket: {
      groupBy: "$screenSize",
      boundaries: [0, 24, 32, 50, Infinity],  // Makes 4 buckets, the _id is based on the min:
                                              // [0, 24), [24, 32), [32, 50), [50, Infinity)
      default: "other",
      output: {
        count: {$sum: 1},
        matches: {$push: "$screenSize"}
      }
    }
  }
])
{_id: 0, count: 1, matches: [22]}
{_id: 24, count: 2, matches: [24, 30]}
{_id: 32, count: 1, matches: [42]}
{_id: 50, count: 1, matches: [55]}
{_id: "other", count: 2, matches: [null, "not-a-number"]} 

// =====================  Example #2 - Error on unexpected types ==========================
> db.example.insert([
  {_id: 0, screenSize: 30},
  {_id: 1, screenSize: 24},
  {_id: 2, screenSize: 42},
  {_id: 3, screenSize: 22},
  {_id: 4, screenSize: 55},
  {_id: 5, screenSize: "not-a-number"},
  {_id: 6, screenSize: null},
]);
> db.example.aggregate([
  {
    $bucket: {
      groupBy: "$screenSize",
      boundaries: [0, 24, 32, 50, Infinity],
      // No default specified.
    }
  }
])
Error!

// =====================  Example #3 - Group unexpected types together =====================
> db.example.insert([
  {_id: 0, screenSize: 30},
  {_id: 1, screenSize: 24},
  {_id: 2, screenSize: 42},
  {_id: 3, screenSize: 22},
  {_id: 4, screenSize: 55},
  {_id: 5, screenSize: "not-a-number"},
  {_id: 6, screenSize: null},
]);
> db.example.aggregate([
  {
    $bucket: {
      groupBy: "$screenSize",
      boundaries: [0, 24, 32, 50, Infinity],
      default: "other",
      output: {
        count: {$sum: 1},
        matches: {$push: "$screenSize"}
      }
    }
  }
])
{_id: 0, count: 1, matches: [22]}
{_id: 24, count: 2, matches: [24, 30]}
{_id: 32, count: 1, matches: [42]}
{_id: 50, count: 1, matches: [55]}
{_id: "other", count: 2, matches: [null, "not-a-number"]}