The project's snapshot library

A snapshot library stores the approved output of the code in the repository, compares each new run with it, and shows the difference when they part. Which library the project uses is found in its test dependencies, not assumed.

Finding it

Look in the test dependencies, then for the references themselves: a __snapshots__ directory beside the tests, .snap or .approved. files, or a testdata directory of golden files. The project's test command runs the comparisons.

Using it

• Compare: the comparisons run with the tests, and a mismatch fails its test with a diff of the two outputs.
• Update only on purpose: updating a reference (jest -u, cargo insta review) approves the new output. It is never a way to make a failing test pass.
• No new references in CI: there, a missing reference fails instead of being written (jest --ci), so nothing is approved unseen.
• Review the diff: a changed reference is part of the change and is read in review like the code. What changes on its own (times, random ids, absolute paths) is kept out of it.

Examples: Jest snapshots (JavaScript), insta (Rust), ApprovalTests (several languages), golden files (common in Go).