Snapshot Testing

Problem

Component refactoring accidentally changes output structure, breaking downstream consumers or causing subtle rendering differences. Developers don’t realize a “harmless” change altered the DOM structure, class names, or data attributes that other code depends on.

Solution

Capture component output and compare against saved snapshots to detect unintended changes. This is useful for catching regressions but requires discipline to keep snapshots meaningful.

Example

This demonstrates capturing component output as a snapshot and comparing against it in future test runs to automatically detect unintended structural changes or regressions.

import renderer from 'react-test-renderer';

test('Button renders correctly', () => {
  // Render component to JSON tree structure
  const tree = renderer.create(<Button label="Click me" />).toJSON();
  // Compare against saved snapshot - fails if output changed
  expect(tree).toMatchSnapshot();
});

Benefits

• Catches unintended changes to component output structure automatically.
• Provides quick regression detection without writing detailed assertions.
• Easy to set up and maintain for stable component interfaces.
• Good for testing presentational components with predictable output.

Tradeoffs

• Creates large diffs that are hard to review when intentional changes are made.
• Can lead to developers blindly accepting snapshot updates without review.
• Becomes brittle for components with dynamic or timestamp-based content.
• Doesn’t test behavior, only output structure.