@w-schmitt/jest-mongodb/mongo-aggregate.test.js

Run your tests using Jest & MongoDB in Memory server

const {MongoClient} = require('mongodb');

describe('insert', () => {
  let connection;
  let db;

  beforeAll(async () => {
    connection = await MongoClient.connect(global.__MONGO_URI__, {useNewUrlParser: true});
    db = await connection.db(global.__MONGO_DB_NAME__);
  });

  afterAll(async () => {
    await connection.close();
  });

  it('should aggregate docs from collection', async () => {
    const files = db.collection('files');

    await files.insertMany([
      {type: 'Document'},
      {type: 'Video'},
      {type: 'Image'},
      {type: 'Document'},
      {type: 'Image'},
      {type: 'Document'}
    ]);

    const topFiles = await files
      .aggregate([{$group: {_id: '$type', count: {$sum: 1}}}, {$sort: {count: -1}}])
      .toArray();

    expect(topFiles).toEqual([
      {_id: 'Document', count: 3},
      {_id: 'Image', count: 2},
      {_id: 'Video', count: 1}
    ]);
  });
});