Jest Snapshot Testing for Storybook Components

This challenge focuses on writing robust Jest snapshot tests for your Storybook components using TypeScript. Snapshot testing is a powerful technique for verifying UI consistency by capturing a "snapshot" of a component's rendered output and comparing it against future renders. This helps catch unintended visual regressions effectively.

Problem Description

Your task is to implement Jest snapshot tests for a given set of Storybook components. You'll be provided with sample component code and their corresponding Storybook stories. You need to write Jest tests that render these components using Storybook's testing utilities and then use Jest's snapshot feature to assert their visual integrity.

Key Requirements:

• Write Jest tests for at least three different Storybook stories for the provided components.
• Each test should render a specific story from the component's stories file.
• Utilize Jest's toMatchSnapshot() matcher to capture and assert the rendered output.
• Ensure your tests are written in TypeScript.
• Configure Jest and Storybook for snapshot testing (you can assume a basic setup is provided or create one).

Expected Behavior:

When the Jest tests are run, they should successfully create or update snapshot files for each tested story. On subsequent runs, if the rendered output of a component changes in a way that deviates from the snapshot, the test should fail, indicating a potential regression.

Edge Cases to Consider:

• Components with dynamic content or props that might change between renders.
• Components that rely on external data or browser APIs.

Examples

Example 1: Simple Button Component

Let's say you have a Button.tsx component and a Button.stories.tsx file with a Primary story.

• Button.tsx (simplified):

  import React from 'react';
  
  interface ButtonProps {
    label: string;
    primary?: boolean;
    backgroundColor?: string;
  }
  
  export const Button: React.FC<ButtonProps> = ({
    primary = false,
    backgroundColor,
    label,
    ...props
  }) => {
    const mode = primary ? 'storybook-button--primary' : 'storybook-button--secondary';
    return (
      <button
        type="button"
        className={['storybook-button', `storybook-button--${mode}`].join(' ')}
        style={{ backgroundColor }}
        {...props}
      >
        {label}
      </button>
    );
  };
  

• Button.stories.tsx (simplified):

  import { Story, Meta } from '@storybook/react';
  import { Button, ButtonProps } from './Button';
  
  export default {
    title: 'Example/Button',
    component: Button,
    argTypes: {
      backgroundColor: { control: 'color' },
    },
  } as Meta<typeof Button>;
  
  const Template: Story<ButtonProps> = (args) => <Button {...args} />;
  
  export const Primary = Template.bind({});
  Primary.args = {
    primary: true,
    label: 'Button Label',
  };
  
  export const Secondary = Template.bind({});
  Secondary.args = {
    label: 'Another Button',
  };
  

• Jest Test (Button.test.tsx):

  import React from 'react';
  import { render } from '@testing-library/react';
  import { composeStories } from '@storybook/testing-react';
  import * as ButtonStories from './Button.stories';
  
  const { Primary, Secondary } = composeStories(ButtonStories);
  
  it('renders Primary button correctly', () => {
    const { container } = render(<Primary />);
    expect(container).toMatchSnapshot();
  });
  
  it('renders Secondary button correctly', () => {
    const { container } = render(<Secondary />);
    expect(container).toMatchSnapshot();
  });
  

• Explanation: The test renders the Primary story of the Button component and asserts that its DOM structure matches the saved snapshot. A second test does the same for the Secondary story.

Example 2: Card Component with Slots

Imagine a Card.tsx component that accepts children for its content.

• Card.tsx (simplified):

  import React from 'react';
  
  interface CardProps {
    title: string;
    children: React.ReactNode;
  }
  
  export const Card: React.FC<CardProps> = ({ title, children }) => {
    return (
      <div className="card">
        <h2>{title}</h2>
        <div className="card-content">{children}</div>
      </div>
    );
  };
  

• Card.stories.tsx (simplified):

  import { Story, Meta } from '@storybook/react';
  import { Card, CardProps } from './Card';
  
  export default {
    title: 'Example/Card',
    component: Card,
  } as Meta<typeof Card>;
  
  const Template: Story<CardProps> = (args) => <Card {...args} />;
  
  export const DefaultCard = Template.bind({});
  DefaultCard.args = {
    title: 'Card Title',
    children: <p>This is the content of the card.</p>,
  };
  
  export const CardWithImage = Template.bind({});
  CardWithImage.args = {
    title: 'Card with Image',
    children: (
      <div>
        <img src="https://via.placeholder.com/150" alt="placeholder" />
        <p>Some descriptive text.</p>
      </div>
    ),
  };
  

• Jest Test (Card.test.tsx):

  import React from 'react';
  import { render } from '@testing-library/react';
  import { composeStories } from '@storybook/testing-react';
  import * as CardStories from './Card.stories';
  
  const { DefaultCard, CardWithImage } = composeStories(CardStories);
  
  it('renders DefaultCard correctly', () => {
    const { container } = render(<DefaultCard />);
    expect(container).toMatchSnapshot();
  });
  
  it('renders CardWithImage correctly', () => {
    const { container } = render(<CardWithImage />);
    expect(container).toMatchSnapshot();
  });
  

• Explanation: Tests render stories that include different types of children and verifies their structure using snapshots.

Constraints

• Number of Stories: You must write tests for at least three distinct Storybook stories across one or more components.
• Language: The solution must be implemented in TypeScript.
• Testing Framework: Jest and @storybook/testing-react must be used for testing.
• Snapshotting: The toMatchSnapshot() Jest matcher is mandatory.
• Component Scope: You are expected to test components that are defined within the provided project context (or a simulated equivalent).

Notes

• You will likely need to install and configure Jest, @storybook/testing-react, and their associated type definitions.
• The @storybook/testing-react library provides the composeStories helper, which is crucial for easily accessing and rendering your stories within Jest.
• When a snapshot test fails, carefully examine the diff to understand why the component's rendering has changed. If the change is intentional, you'll need to update the snapshot (usually by running Jest with the --updateSnapshot or -u flag).
• Consider how to handle components that might rely on global styles or context providers. You might need to wrap your rendered stories in these providers within your Jest tests.
• For components with complex styling, the snapshot will capture the HTML structure. For visual regressions, you might later consider integrating visual regression testing tools like Percy or Chromatic.