Jest · Unit tests that survive refactors

What are the key options in a Jest configuration?

Mediumconfigurationjest.configprojectsUpdated Oct 2026

Short answer

A Jest config controls roots, test matching, environment, transforms, module mapping and coverage. Keep it typed, put shared defaults in one file, and use projects for different runtimes.

Configuration is where the runner learns how to find, compile and run your tests. A small project needs very little. A monorepo usually needs projects, path mapping and a transform that matches the build setup. Keeping the config in a typed file catches mistakes before a test run.

import type { Config } from "jest";

const config: Config = {
  testEnvironment: "jsdom",
  roots: ["<rootDir>/src"],
  testMatch: ["**/*.test.ts", "**/*.test.tsx"],
  setupFilesAfterEnv: ["<rootDir>/src/test/setup.ts"],
  moduleNameMapper: { "^@/(.*)$": "<rootDir>/src/$1" },
  transform: { "^.+\\.tsx?$": "ts-jest" },
  clearMocks: true,
  restoreMocks: true,
};

export default config;

Options worth knowing

testMatch and testPathIgnorePatterns decide which files are tests
transform and transformIgnorePatterns decide what gets compiled
moduleNameMapper maps aliases and static assets
setupFiles and setupFilesAfterEnv place shared setup at the right time
clearMocks, resetMocks and restoreMocks automate cleanup
projects runs several configurations in one command

Prefer a jest.config.ts or jest.config.js over the jest key in package.json once the config grows. You get comments, imports and type checking. If a setting seems ignored, check whether a project-level config overrides it, because project configs do not automatically inherit every root option.

Changing the transform, module mapping, environment or setup files requires clearing the Jest cache to be safe. npx jest --clearCache resolves many confusing stale results.

Follow-ups you should expect

Interviewers use these to check whether the first answer was understanding or memorisation.

1What is the difference between setupFiles and setupFilesAfterEnv?
2When should you use projects?
3Why does a config change sometimes require clearing the cache?
4What do clearMocks, resetMocks and restoreMocks each do?

Say it in the interview

A Jest config defines how tests are found, which environment they use, how files are transformed and how modules resolve. I keep it in a typed config file and use projects when a repo has both browser and server tests. I enable clearMocks and restoreMocks so cleanup is automatic. When a config change seems ignored, I check project overrides and clear the cache.

Use this as a model, not a script. Saying it in your own words is what makes it survive follow-up pressure.

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Options worth knowing

Follow-ups you should expect

Related questions

Now say it out loud.