{$project: {
        fieldName: {
            $switch: {
                branches: [
                    {
                        case: <expression>,
                        then: <expression>
                    },
                    …
                ],
                default: <optional, expression>
            }
        }
    }}

> db.coll.insert([
  {_id: 0, x: 0}
  {_id: 1, x: -1}
  {_id: 2, x: 2}
]);
> db.coll.aggregate([{
  $project: {
    z: {
      $switch: {
        branches: [
          {
            case: {$eq: ["$x", 0]},
            then: "Equal to zero."
          },
          {
            case: {$eq: [{$abs: "$x"}, 1]},
            then: "Equal to plus or minus one."
          },
        ],
        default: "Something else."
      }
    }
  }
}])
{_id: 0, z: "Equal to zero."}
{_id: 1, z: "Equal to plus or minus one."}
{_id: 2, z: "Something else."}