Snapshot Testing

Snapshot testing is a useful technique in React applications for automatically detecting unintended UI changes. It involves creating and maintaining snapshots of your components and comparing them against the actual rendered output.

When you run snapshot tests, Jest captures a snapshot of the component's rendered HTML or JSON structure. On subsequent test runs, Jest compares the current output with the previously stored snapshot. If there are any changes, Jest alerts you, giving you the opportunity to review and verify the changes.

Creating a Snapshot

To create a snapshot, you can use the toMatchSnapshot matcher provided by Jest. Here's an example:

1import renderer from 'react-test-renderer';
2import MyComponent from './MyComponent';
3
4it('should match the snapshot', () => {
5  const tree = renderer.create(<MyComponent />).toJSON();
6  expect(tree).toMatchSnapshot();
7});

Updating Snapshots

If the component's output changes intentionally, you can update the snapshot by running the test with the --updateSnapshot flag. This flag tells Jest to generate a new snapshot and overwrite the existing one.

Handling Snapshot Failures

When a snapshot test fails, it means there have been unintended changes in the component's output. You can review the diff between the current and expected output and decide whether the changes are expected or not. If the changes are intentional, you can update the snapshot to match the new output.

Snapshot testing is a powerful tool that helps catch UI regressions and provides an efficient way to verify the consistency of component output over time. It is especially useful when dealing with complex components or when refactoring existing code.

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// replace with logic relevant to snapshot testing

// make sure to use a jest snapshot

const player = "Kobe Bryant";

​

expect(player).toMatchSnapshot();