test-options.js

'use strict';

function createCommandOptions(manager, localOptions) {
  const options = Object.assign(
    {
      host: 'defaultHost',
      port: 27017,
      connectionTimeoutMS: 5000,
      socketTimeoutMS: 0,
      pool: 1,
      reconnect: false,
      emitError: true
    },
    manager.clientOptions,
    localOptions
  );

  return options;
}

const manager = { options: { host: 'managerHost' } };
const localOptions = { host: 'localOptionsHost' };

const result = createCommandOptions(manager, localOptions);
console.log({ result });