Fixing Storybook's .test With Combined Exports

Dec 3, 2025 by Admin 47 views

Hey everyone! Ever hit a snag with Storybook's experimental CSFNext testing syntax when you're using combined exports? You're definitely not alone. It's a bit of a head-scratcher when your CSFNext .test functions, which are supposed to make component testing super seamless, suddenly throw a TypeError because of how you're exporting your stories. We're talking about that moment when you've got experimentalTestSyntax=true enabled, you're all set to write some awesome tests right alongside your components, but then your neatly organized export { Primary }; statements decide to play hard to get. This issue isn't just a minor inconvenience; it can really disrupt your workflow, especially if you, like many of us, rely on combined exports for better code readability and organization in larger component files. This article is going to dive deep into why this bug occurs, what it means for your development process, and most importantly, how you can work around it right now while we wait for a more permanent fix from the Storybook team. We'll explore the technical underpinnings, walk through a concrete example, and give you some solid advice to keep your Storybook experience smooth and productive.

Understanding the Core Problem: CSFNext and Combined Exports

Alright, let's kick things off by understanding what's actually going on under the hood when CSFNext and combined exports butt heads in Storybook. CSFNext, or Component Story Format Next, is Storybook's latest iteration of defining stories, aiming to make it even more ergonomic and powerful. One of its shining features is the experimentalTestSyntax=true option, which lets you write .test functions directly within your story files. This is a huge win for component-driven development, allowing you to co-locate your component definitions, stories, and tests, creating a highly cohesive and discoverable development experience. Imagine defining your Button component, crafting its Primary story, and then immediately adding a Primary.test() function right there to assert its behavior. Pretty neat, right? It streamlines everything, making it easier to ensure your components are always working as expected.

However, the hiccup arises when you combine this innovative testing syntax with a common JavaScript export pattern: combined exports. Instead of directly exporting a story like export const Primary = meta.story({});, many developers, for good reasons, prefer to define their stories internally first and then export them together at the end of the file. This often looks like const Primary = meta.story({}); export { Primary };. This pattern is super useful, especially in files with multiple stories or other exports, as it provides a clear, concise overview of everything that's being exposed from that module. It enhances readability, makes refactoring easier, and generally contributes to a cleaner codebase. The problem, guys, is that CSFNext's current implementation of experimentalTestSyntax struggles to resolve the underlying story variable when it's exported in this combined fashion. When Storybook tries to access properties like _stories or __stats on these combined exports to run your .test function, it can't find them because its AST (Abstract Syntax Tree) resolution doesn't correctly trace the re-exported variable back to its original definition and the metadata attached to it. This leads directly to the dreaded TypeError: Cannot read properties of undefined (reading '__stats') you might be seeing, effectively breaking your Storybook setup and preventing those awesome co-located tests from running. It's a classic case of two perfectly valid and useful features not playing nice together, and it's definitely something the Storybook team is aware of and working on.

Why This Matters: The Impact on Your Storybook Workflow

Let's be real, this isn't just some obscure technical bug; it significantly impacts your daily Storybook workflow and overall development experience. When the CSFNext .test functionality, a feature designed to streamline your testing, breaks with combined exports, it forces you to make tough choices or adopt less-than-ideal workarounds. For starters, if you're heavily invested in combined exports for the readability and organization they offer, you suddenly find yourself unable to leverage the powerful co-located testing capabilities that experimentalTestSyntax=true promises. This means you either have to revert to older, potentially more cumbersome testing methodologies, or you have to restructure your entire component story files to use inline exports (e.g., export const Primary = meta.story({});). While inline exports work, they might go against your established coding standards or make large files less manageable if you prefer to see all exports grouped at the bottom.

Think about it: The core promise of CSFNext with testing is to bring your component, its stories, and its tests all into one harmonious place. This fosters better design, encourages thorough testing from the get-go, and makes it incredibly easy for any developer to understand a component's full lifecycle just by looking at a single file. When combined exports break this, that synergy is lost. You might end up with story.ts files that no longer contain their tests, forcing you to maintain separate test files or, even worse, skip testing certain stories altogether because the friction is too high. This fragmentation slows down development, introduces potential for bugs to slip through, and makes it harder to onboard new team members who expect a consistent, modern development environment. Furthermore, the TypeError: Cannot read properties of undefined (reading '__stats') isn't always immediately obvious in its cause, leading to frustrating debugging sessions where you're scratching your head, wondering why a perfectly valid JavaScript export statement is causing Storybook to crash. It consumes valuable development time, erodes confidence in the tooling, and ultimately detracts from the productive, enjoyable experience Storybook is known for. Ensuring that all valid JavaScript patterns work seamlessly with cutting-edge features like CSFNext testing is crucial for maintaining developer happiness and productivity across diverse codebases. So, while it seems like a small detail, the implications for your team's efficiency and code quality are quite substantial, underscoring the importance of addressing this particular bug.

Diving Deeper: How `experimentalTestSyntax` Works (and Fails Here)

Let's take a closer look at the mechanics behind experimentalTestSyntax and precisely why combined exports cause a hiccup. When you enable experimentalTestSyntax=true in Storybook, it changes how your story files are processed. Instead of just extracting story definitions, Storybook's compiler and runtime environment are configured to understand and execute the .test functions that are now co-located with your stories. This involves parsing your JavaScript or TypeScript files, building an Abstract Syntax Tree (AST), and then walking that tree to identify story exports and any associated .test methods. The magic happens because meta.story({}) (or similar story-defining functions) attaches specific metadata to the story object itself. This metadata includes things like the _stories property and eventually __stats, which are crucial for Storybook to properly register, render, and indeed, test your stories.

When you use a direct, inline export like export const Primary = meta.story({});, the variable Primary is immediately assigned the result of meta.story({}), and thus, the metadata is directly accessible on that exported variable. Storybook's AST traversal can easily pick this up, see Primary.test(), and know exactly which story it belongs to, allowing the test to run smoothly. However, with combined exports, the situation gets complicated. Consider const Primary = meta.story({}); export { Primary };. Here, Primary is initially defined as a local variable. The export { Primary }; statement then re-exports this local variable. The critical part is that during Storybook's AST analysis, especially in its current implementation for experimentalTestSyntax, it might not fully resolve this re-export back to its original definition in a way that preserves the necessary _stories or __stats metadata context for testing. It can see that Primary is exported, but when it attempts to access Primary.__stats (which would be part of the story object's metadata required by the test runner), it finds undefined because it hasn't correctly linked the re-exported Primary to the enriched object returned by meta.story({}). This leads to the dreaded TypeError: Cannot read properties of undefined (reading '__stats'). Essentially, the AST resolver is losing the thread between the re-exported name and the rich, Storybook-specific object it refers to, which is vital for the testing integration to work. It's a nuanced problem rooted in how JavaScript modules are processed and how Storybook's specific parsing logic interacts with different export syntaxes, highlighting a gap that needs bridging for full compatibility.

Reproduction Steps: Seeing the Bug in Action

Alright, let's get our hands dirty and actually see this bug happen. The best way to understand an issue is to reproduce it, and thankfully, there's a handy StackBlitz link provided that perfectly demonstrates this problem. So, grab your virtual coding gear, guys, and let's walk through it step-by-step. This isn't just about pointing fingers; it's about understanding the exact scenario where this TypeError pops up so we can appreciate the workarounds and eventually, the fix.

Here’s how you can reproduce the bug:

Head over to the reproduction link: Open your browser and navigate to https://stackblitz.com/edit/github-gyecjlwc?file=README.md. This StackBlitz environment is pre-configured with the necessary Storybook setup, including CSFNext and experimentalTestSyntax=true, so you don't have to worry about boilerplate. It's an excellent isolated environment to observe the issue.
Access the Button story: Once the StackBlitz project loads, you'll see the Storybook UI on the right side. In the sidebar, navigate to the Button component stories. You'll likely see a couple of stories: Primary and Secondary. You can click on both of them to confirm they render correctly. At this stage, everything looks fine, right? This is because the Primary story is likely using an inline export, which works as expected. The Secondary story might also be fine if its .test function is commented out.
Observe working correctly with in-line export and separate export at EOF: Spend a moment looking at the code. You'll probably see something like this for the Primary story:
```
export const Primary = meta.story({
  args: { label: 'Primary Button' },
});
Primary.play = async ({ canvasElement, step }) => { /* ... */ };
Primary.test = async ({ expect }) => { /* ... */ }; // This works if uncommented
```
This inline export works perfectly with .test. Now, look for a story that uses the combined export pattern. It might look something like this (for the Secondary story, for instance):
```
const Secondary = meta.story({
  args: { label: 'Secondary Button' },
});
Secondary.play = async ({ canvasElement, step }) => { /* ... */ };
// ... more code ...
export { Secondary }; // The problematic line for testing
```
You'll notice that without the Secondary.test() uncommented, even the combined export Secondary story renders fine in Storybook. The rendering itself isn't the issue; it's the testing integration.
Uncomment the Secondary.test() function: Now for the critical step. In the Button.stories.ts file within StackBlitz, find the Secondary story definition. Locate the commented-out Secondary.test() function (it might be Secondary.test = async ({ expect }) => { /* ... */ }; or similar). Uncomment this entire test function. Save the file.
Observe Storybook breaks: As soon as you save, Storybook in the preview pane should break. You'll likely see an error message, probably in the Storybook UI or in the browser's developer console (F12). The error you're looking for is something like: TypeError: Cannot read properties of undefined (reading '__stats'). This is the smoking gun! It clearly indicates that Storybook's experimentalTestSyntax mechanism, when trying to run the test for Secondary, cannot find the necessary __stats property on the Secondary story object because of how it was re-exported. It underscores the point we made earlier about the AST resolution failing to properly link the re-exported name to the full Storybook-enriched story object. Boom! You've just reproduced the bug. This hands-on experience really drives home the problem, making it clear why this seemingly small export difference causes such a significant breakdown in the new testing feature.

Potential Workarounds and Best Practices

Alright, since we can't just sit around and wait for the Storybook gods to bestow a fix upon us (though it's definitely on their radar!), we need some immediate workarounds to keep our development wheels turning. When CSFNext and combined exports aren't playing nice with .test functions, you've primarily got one solid path forward to ensure your tests run. It might not be everyone's favorite, especially if you're a stickler for export organization, but it gets the job done and keeps your components tested.

The Inline Export Solution

The most direct and currently reliable workaround is to switch from combined exports to inline exports for any story that utilizes the .test() function. This means instead of defining your story as a local variable and then re-exporting it, you directly export it where it's defined. Let's look at the difference with our Primary and Secondary examples:

Original (Problematic for .test with combined exports):

// Inside Button.stories.ts
const Primary = meta.story({
  args: { label: 'Primary Button' },
});
Primary.play = async ({ canvasElement, step }) => { /* ... */ };
Primary.test = async ({ expect }) => {
  await expect(true).toBe(true); // This would break here!
};

const Secondary = meta.story({
  args: { label: 'Secondary Button' },
});
// ... Secondary.play, etc.
Secondary.test = async ({ expect }) => {
  await expect(false).toBe(false); // And this would break too!
};

export { Primary, Secondary }; // The combined export causing issues

Workaround (Recommended for .test functions):

// Inside Button.stories.ts
export const Primary = meta.story({
  args: { label: 'Primary Button' },
});
Primary.play = async ({ canvasElement, step }) => { /* ... */ };
Primary.test = async ({ expect }) => {
  await expect(true).toBe(true); // This now works!
};

export const Secondary = meta.story({
  args: { label: 'Secondary Button' },
});
Secondary.play = async ({ canvasElement, step }) => { /* ... */ };
Secondary.test = async ({ expect }) => {
  await expect(false).toBe(false); // And this works too!
};
// No need for a separate `export { Primary, Secondary };` at the end

By changing your exports to this inline format, you ensure that the Primary and Secondary variables are directly associated with the Storybook-enriched objects, complete with all their necessary metadata (_stories, __stats). This allows Storybook's experimentalTestSyntax to correctly identify and run your .test functions without running into that pesky TypeError. It's a simple change, but it's crucial for unlocking the testing capabilities in CSFNext right now. While it might mean a slight adjustment to your file structure or export philosophy, the benefit of having integrated component tests far outweighs the minor inconvenience. For now, consider this your go-to strategy for any stories where you want to leverage .test() functions.

Rethinking Your Export Strategy (Temporarily)

Beyond just the inline export workaround, this bug might prompt you to re-evaluate your overall export strategy for Storybook files, at least temporarily. If you have a strong preference for combined exports for non-testable stories or other module exports, you can absolutely continue using them for those. The issue specifically arises when a story with a .test() function is re-exported. So, a pragmatic approach could be a hybrid model: use inline exports for stories that have .test() functions, and stick to combined exports for everything else if that aligns better with your team's code style. This gives you the best of both worlds, allowing you to benefit from the new testing features while maintaining some semblance of your preferred export structure where it doesn't cause issues. Always remember that code clarity and maintainability are paramount, and sometimes, a temporary adjustment is necessary to leverage powerful new tools. Keep an eye on Storybook updates, as this is a known limitation that the team will likely address in a future release, potentially allowing for a full return to your preferred export patterns across the board.

What's Next: Looking Towards a Solution

So, we've dissected the problem, understood its impact, and even implemented a handy workaround. But what's the long-term outlook? When can we expect a more permanent solution that allows CSFNext .test functions to play nicely with combined exports without needing workarounds? The good news, guys, is that the Storybook team is incredibly active and responsive to community feedback and bug reports. Issues like this, especially when they touch on core features and developer experience, are usually high on their priority list. This particular bug stems from the nuanced way Storybook's AST processing interacts with different JavaScript module export patterns, and resolving it will likely involve enhancements to their internal parser and metadata handling for experimentalTestSyntax.

What can you do in the meantime? First and foremost, keep an eye on the official Storybook changelogs and release notes. When a fix is implemented, it will be clearly documented there. You might also want to follow the Storybook GitHub repository, especially the issues section, as discussions and proposed pull requests related to this bug might appear there. Contributing to these discussions, or even helping to test pre-release versions if you're feeling adventurous, can be a great way to support the project and accelerate the resolution. For those eager to stay on the bleeding edge, regularly updating your Storybook dependencies is a good practice, as fixes often land in minor or patch releases. The future of Storybook, especially with features like CSFNext and integrated testing, is incredibly bright. The goal is always to make component development and testing as seamless and delightful as possible. While we navigate these temporary bumps, it's a testament to the power and innovation that Storybook brings to front-end development. The community and maintainers are constantly working to iron out these kinks, ensuring that Storybook remains the go-to tool for building robust and beautiful UI components. So, keep coding, keep testing, and stay tuned for those sweet updates that will make our lives even easier!

Remember, your feedback and bug reports are invaluable in shaping the future of tools like Storybook. Every bug reported, every issue discussed, contributes to a more stable and powerful ecosystem for all of us. Let's keep building amazing UIs together, and rest assured, this combined exports conundrum will eventually be a thing of the past! Happy coding, folks!