How to use snapshots in Jest

Snapshots in JestJest is a JavaScript testing framework that provides us with the APIs for writing tests. are a feature that allows you to capture the rendered output of a component and save it as a baseline for future tests.

When a snapshot test is run, Jest compares the current output with the saved snapshot to determine if there are any differences.

Installation

To install the Jest framework, you need to enter the following command in your Windows/Linux terminal:

// For npm pakage manager

npm install react-test-renderer



// For yarn pakage manager

yarn add react-test-renderer
Enter the relevant command depending upon the pakage manager you're using

Note: This shot assumes that you have already developed your React application. You can learn to creat your react application from here.

Integrating Jest with React

You can follow the following steps to integrate Jest with your React application:

  • Step 1: Navigate to your src folder and locate App.test.js file. You can create one if you do not have it in your folder. The naming convention of your file must be <name>.test.js.

  • Step 2: After navigating your App.test.js file, remove its contents. You do not need to do this if you're creating your own .test.js file.

  • Step 3: In .test.js file, import renderer object from 'react-test-renderer' library.

  • Step 4: Import React object from the 'react' module.

  • Step 5: Import the React application's main file (App.js in our case).

  • Step 6: In the body, add the following snippet:

test("First snapshot test", ()=>{
  const component = renderer.create(
    <App/>
  );
  let tree = component.toJSON();
  expect(tree).toMatchSnapshot();
})

Note: If you're working on the already existing .test.js file, then remove its components and add the above snippet after the imports.

Code explanation

  • Line 1: Creating an arrow function with the label "First snapshot test".

  • Line 2–4: Using the renderer.create method to create an instance of the App component and storing it in a constant component.

  • Line 5: Converting the rendered component into a JSON-like structure by calling the toJSON() method.

  • Line 6: Using the toMatchSnapshot() method from Jest to see if tree matches the stored snapshot or not.

Now your .test.js file will look like the one shown in the sample project below:

Single page application whose snapshot is being saved
Running the test
Open the terminal and enter the following command to run the testcases:
// For npm pakage manager

npm test



// For yarn pakage manager

yarn test
Enter the relevant command depending upon the pakage manager you're using
If you do not make any changes, then your terminal will show the following message after executing the above command:
Output of your .test.js file, if you do not make any changes to your code 
Output of your .test.js file, if you do not make any changes to your code 
Once you enter the command, a folder named __snapshots__ will be created, which will contain the saved snapshot of your React application in App.test.js.snap file (filename might vary).
Now, whenever you make changes to your code and run the same command above, the test cases won't pass because App.test.js.snap file has the last snap stored in it.
The terminal will output the errors and let you know the changes you made as follows:
Output of your .test.js file, if you do not make any changes to your code and the current snapshot doesn't match with the previously saved snapshot
Output of your .test.js file, if you do not make any changes to your code and the current snapshot doesn't match with the previously saved snapshot
Conclusion
Jest snapshots are helpful if we want to experiment with something on our React application that might potentially lead to a crash. We can save our currently working React application somewhere and then do the experiments. This way, we can always revert to our previous state if any crashing event occurs.
Learn about Redux and how to integrate it into React applications.
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