In cases where the empty object is already set as default in the config:

See: stubs, mocks and methods
https://github.com/vuejs/vue-test-utils/blob/dev/packages/test-utils/src/config.js

The ? is not necessary. In this case, we can import the config and we will not be forced to check the existence of attributes:

import { config } from '@vue/test-utils';

if (config.mocks) {
  config.mocks.i18n = { t: jest.fn() };
  config.mocks.$t = jest.fn();
}

and use directly

import { config } from '@vue/test-utils';

config.mocks.i18n = { t: jest.fn() };
config.mocks.$t = jest.fn();

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue-test-utils/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information: