medium

Snapshot mismatch — expected value to match snapshot

The rendered output does not match the stored snapshot. The component output changed since the snapshot was last updated.

Common Causes

  1. 1Component was intentionally updated but snapshot not refreshed
  2. 2Dynamic content (dates, IDs) changed between runs
  3. 3Different environment produces different output
  4. 4Dependency update changed rendered markup

How to Fix

Update the snapshot after verifying the change is correct

# Update all snapshots:
npx jest --updateSnapshot

# Update specific test file:
npx jest path/to/test --updateSnapshot

Review the diff carefully, then update the snapshot if the change is intentional.

Use snapshot serializers for dynamic content

// jest.config.ts
export default {
  snapshotSerializers: ['jest-serializer-html'],
};

// In test:
expect(component).toMatchSnapshot({
  createdAt: expect.any(String),
});

Use property matchers to ignore dynamic values like timestamps or generated IDs.

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Related Errors

SyntaxError: Unexpected token

Jest encountered syntax it cannot parse. This usually means a file needs to be transformed (e.g., TypeScript, JSX, ESM) but the transformer is not configured.

Cannot find module './Component'

Jest could not resolve a module import. The file may not exist, the path may be wrong, or the module resolution config may be misconfigured.

Timeout — Async callback was not invoked within 5000ms

An async test did not complete within the default 5-second timeout. The async operation may be hanging, or done() was never called.

UnhandledPromiseRejectionWarning

A Promise was rejected but no .catch() handler or try/catch block caught the error. This can cause flaky tests or missed failures.

Jest mock not being called / not returning expected value

A jest.mock() or jest.fn() is not behaving as expected — either not being called, not returning the mocked value, or not intercepting the real implementation.