Explore essential concepts and practices for snapshot testing using Jest, including snapshot generation, updates, best use cases, and troubleshooting. This quiz helps reinforce your understanding of how snapshot files work within the jest tools ecosystem and highlights common pitfalls to avoid.
What is the primary purpose of using snapshot testing in jest when verifying component output?
Explanation: Snapshot testing is used to ensure that the output of a component remains consistent with a saved reference snapshot. It does not improve rendering performance nor does it enforce naming conventions or automate code refactoring. The other options do not relate to the actual verification process done by snapshot testing, which is about preventing unintended changes.
Which jest command-line flag should you use to update all failing snapshots after an intentional change to the component's output?
Explanation: The --updateSnapshot flag is used to tell jest to update existing snapshots when the expected output changes intentionally. The flags --resetSnapshot, --testUpdate, and --refreshSnapshot do not exist in jest's command-line tool and would not correctly update snapshots; they are distractors intended to resemble the real option.
Where does jest store snapshot files by default when you run snapshot tests on a JavaScript file named 'myComponent.test.js'?
Explanation: Jest stores snapshots inside a special __snapshots__ directory located next to the test file, naming the snapshot after the test file with a .snap extension. The other options list incorrect directory or file naming conventions. For instance, 'snapshots/' and 'myComponent.snap' are not default locations, while '__tests__/' typically stores test files but not snapshots.
For which type of code changes are snapshot tests most effective at preventing unintended alterations?
Explanation: Snapshot testing is ideally suited for verifying the output of UI components since their structure can change inadvertently during development. Business logic functions or scripts like database migrations are less suitable because their outputs are usually not structured for snapshots. Network latency measurements are dynamic and not stable enough for snapshotting.
If a previously passing snapshot test starts failing after you refactor a component, what is the recommended next step before updating the snapshot?
Explanation: Before updating a failing snapshot, it's important to review what changed and ensure that the difference is expected and desired. Deleting the snapshot file could result in lost test coverage, and ignoring the test may allow unintended issues to slip through. Restoring files from a previous commit is only necessary if the change was a mistake, not as a general habit.