What are snapshot tests? What are their pros and cons, and when would you use them?

A snapshot test saves some output to a file, then fails whenever that output changes later.

How it works:

• The test serialises output, such as a rendered component's DOM, into text.
• The first run saves it to a file.
• Later runs compare against that file.
• Any difference fails the test until someone updates the snapshot with jest -u.

Pros: snapshots are almost free to write, and they catch unexpected output changes.

Cons:

• They don't say what is correct, only that something changed.
• Large component snapshots break on every harmless markup change.
• So people get used to updating them without reading the diff. Then the test stops protecting anything.

Use them for small, stable output where the exact value matters. Examples are a serialised config, an error message or a small piece of markup. Prefer toMatchInlineSnapshot(), so the expected value sits inside the test and gets reviewed. For components, explicit assertions on behaviour are better.