Learn Jest - Migrating & Upgrading Jest

#Introduction

Jest keeps evolving, and so must your suite. Episode 18 covers migrating & upgrading Jest — the process of upgrading versions safely, migrating from other testing frameworks, handling breaking changes and test refactoring, and maintaining long-term suite stability.

A careless migration can stop an entire team for days. With a structured approach, upgrading becomes a calm, predictable routine.

#Upgrading Jest Versions Safely

#Noting Your Current Version

Before upgrading, know where you stand:

npx jest --version
npm outdated jest

npx jest --version shows the installed version, while npm outdated jest shows the latest available version. Also note the major changes between versions — every major version usually has a list of breaking changes in its release notes.

#A Gradual Upgrade Strategy

Never jump several major versions at once without a pause. The safe steps:

1. Read the changelog and note the relevant breaking changes.
2. Upgrade one major version, then run the entire suite.
3. Fix the failures that appear, one by one.
4. Repeat for the next major version.

npm install --save-dev jest@29
npm test

The command npm install --save-dev jest@29 pins a specific major version explicitly. Run npm test afterward; the failures that appear are a real to-do list for fixes.

#Migrating from Another Framework

#Mapping Equivalent APIs

Mocha, Jasmine, and Ava use patterns similar to Jest. Most tests can be mapped directly:

• describe and it → already identical in Jest.
• Chai assertion expect(x).to.equal(y) → expect(x).toBe(y).
• assert.ok → expect(x).toBeTruthy().

Example conversion from Chai to Jest

// Sebelum (Chai):
// expect(nilai).to.be.above(10);
 
// Sesudah (Jest):
test("nilai di atas 10", () => {
  expect(nilai).toBeGreaterThan(10);
});

expect(nilai).toBeGreaterThan(10) is Jest's equivalent of expect(nilai).to.be.above(10). This mapping can be done mechanically for most tests.

#Handling Different Mocking

Sinon and other spy libraries need to be mapped to Jest's mocking engine: sinon.spy becomes jest.spyOn, sinon.stub becomes jest.fn with mockReturnValue, and sinon.mock is replaced by the jest.mock pattern. Focus your attention here — mocking is the part of migration that most often causes surprises.

#Handling Breaking Changes & Refactoring Tests

#Common Types of Breaking Changes

Some categories of breaking changes are common when upgrading Jest:

• Matcher or global API changes: functions removed or behavior changed.
• Default transform changes: transform configuration that must be adjusted.
• Environment changes: default test runner or testEnvironment.
• Output format changes: snapshots that look different because a serializer changed.

For snapshots that change due to a new format, run --updateSnapshot only after verifying the diff — don't accept it blindly.

#Ordered Test Refactoring

Refactoring tests isn't just fixing failures. It's an opportunity to clean up: remove duplicate tests, merge same-pattern ones with test.each, and split helpers that have grown too large. Clean tests are easier to maintain when the next upgrade comes.

#Maintaining Test Stability

#Keeping Long-term Stability

Suite stability is a product of habits, not luck. A few practices that keep a suite stable:

• Run the full suite regularly, not just affected tests.
• Fix flaky tests immediately rather than postponing.
• Keep snapshots small and easy to review.
• Document configuration decisions in the changelog or release notes.

for i in 1 2 3; do npm test || break; done

The command npm test || break inside a loop runs the suite three times to detect flakiness. If the suite fails only on certain iterations, there's a non-deterministic test that needs investigation.

#Wrap Up

Episode 18 covered migration and upgrades: upgrading versions gradually and safely, mapping APIs when migrating from another framework, handling breaking changes with ordered refactoring, and maintaining suite stability through long-term habits.

Key takeaways:

• Upgrade one major version at a time; don't jump ahead.
• Run the full suite after every upgrade step.
• Map assertions and mocking from the old framework to Jest's API.
• Review snapshot diffs before accepting format changes.
• Clean up duplicate tests and use test.each for same patterns.
• Run the suite repeatedly to detect flakiness early.

In the next episode, episode 19, we'll cover operational readiness & runbooks — runbooks for failing suites and flaky tests, managing debugging and test ownership, large-scale maintenance strategies, and recovery from test regressions.