Front-end and back-end developers can avoid many issues by writing tests, greatly enhancing the software development process. Different types of testing, such as Unit, Functional, Integration, and even manual testing for React Apps, play a vital role in ensuring a product works ideally and is free of bugs. This article focuses on another crucial form of testing called snapshot testing and why it prevents an app's UI from changing unexpectedly. We also explore how snapshot tests work, their pros and cons, some practical tools for snapshot testing, and how it works by providing a short tutorial and other reading materials.
Testing is a significant part of software development for front-end and back-end developers. Product testing is an effective way to improve your code's quality and build trust in it. In addition, it makes it easier to detect errors quickly and catch bugs before they cause any major havoc.
There are various forms of testing, like Unit, Integration, or functional testing, for React components, but in this article, the focus is on snapshot testing. This testing benefits React components, ensuring the app's UI doesn't change unexpectedly.
How Do Snapshot Tests Work?
A typical snapshot test works by rendering a UI component, taking a snapshot, and finally comparing its properties to the characteristics of another reference snapshot file saved alongside the just concluded test.
It compares the "saved output" with the current output of a component, which makes it quite different from other forms of testing. Unit and functional testing, for instance, focuses on creating assertions about the expected behavior or output of a component, whereas snapshot testing is mainly concerned with detecting unexpected UI changes. A snapshot test won't give you any information regarding the component's behavior; it only tells you if the current output is the same as the previous one.
It's important to note that a snapshot test will be tagged as unsuccessful if both snapshots don't match. An unsuccessful snapshot test can happen for two reasons: either the change was unexpected, or the reference snapshot requires an update to the latest version of the UI component.
Why is it essential to carry out snapshot tests?
Snapshot tests are written as part of front-end test automation, and using Jest snapshots will help to ensure UI changes are self-consistent and that the creator is aware of the changes made. The information can help determine whether the changes were intended or not. Snapshot testing was also created due to the need for an easier way to write tests for React components. According to most React developers, they spend more time writing tests than the actual component. Hence, snapshot testing allows React developers to write tests quickly because they're shorter than traditional tests.
Snapshot tests are also relatively easy to update because developers generally need to run a single command to make the testing system record new snapshots. This method is much easier than going through the stress of editing multiple test files for tests to be back in sync with the most recent version.
Constantly changing text assertions can be painful, so snapshot testing is a powerful solution that helps fix that issue.
Cons of Snapshot Testing
While snapshot testing has many benefits, there are also some drawbacks to this form of testing;
- Since snapshot test results don't tell you precisely what has changed in a component, failures become hard to understand. Seeing what has changed in a snapshot test failure is usually challenging.
- Snapshots are connected to an application's output, which makes them very fragile. They can fail when any change is made to even the most irrelevant part of the output.
- Developers usually have to manually verify everything works properly before updating the snapshots.
- Snapshot tests don't provide meaningful output expectations compared to other forms of testing that highlight the crucial aspects of test output.
- They are not built for dynamic content. Frequently, an app will fail snapshot tests because its random quote during output may not match the stored app's data.
- Snapshots require larger storage space. As a big implementation tool, it saves extensive data like component names, prop names, and many others that might break on every refactor.
- The existence of false negatives in snapshot tests makes developers not trust the tool's ability to find bugs. Having to double-check tests manually makes the entire process a chore on a checklist, and most developers will update the snapshot to get it over with.
Essential Tools for Snapshot testing
Here are the primary tools used for snapshot testing front-end code:
Most of the tools mentioned above are used for local testing, but many cloud-based services are used for visual snapshot testing. These cloud-based tools manage the browsers that tests may run against and save the test snapshots afterward. Two good examples are Chromatic
Snapshot testing in Jest
Snapshot testing is a built-in feature of Jest that generates a JSON value for the React tree and compares it to a reference snapshot to look for discrepancies.
- Install Node.js locally
- Have some familiarity with React and Jest
- Any code editor and terminal of your choice.
A typical Snapshot test using Jest goes through the following steps:
- Create a React Component to Test
- Write Snapshot tests
- Upgrade Snapshot tests.
To get a detailed understanding of snapshot testing, here's our team's typical example of a full-on snapshot test
for a React app.
Also, check out Jest's GitHub repository
, which showcases written code samples for snapshot testing.
Jest snapshot testing is one of the most highly recommended tools for React developers to detect unexpected UI changes, as it is easy to create and maintain.
Writing test cases for all possible flows in a React component is crucial, but note that you can't replace unit or functional tests with snapshot tests. Snapshot tests act as a simple tool to improve code quality, but consider that unit and functional tests are necessary to monitor and verify an application's behavior.
Though we have explored the basics of snapshot tests, there is still much to learn about better writing. Check out the best practices
documentation by Jest to learn more.
*This is not the official Fetchly opinion but the opinion of the writer who is employed by Fetchly*