setup.js

const path = require("path");
const fs = require("fs");
const { MongoMemoryServer } = require("mongodb-memory-server");
const globalConfigPath = path.join(__dirname, "globalConfig.json");

const mongod = new MongoMemoryServer({
  autoStart: false
});

module.exports = async () => {
  if (!mongod.isRunning) {
    await mongod.start();
  }

  const mongoConfig = {
    mongoDBName: "jest",
    mongoUri: await mongod.getConnectionString()
  };

  // Write global config to disk because all tests run  in different contexts.
  fs.writeFileSync(globalConfigPath, JSON.stringify(mongoConfig));

  // Set reference to mongod in order to close the server during teardown.
  global.__MONGOD__ = mongod;
};