Next.js 16 Turbopack: Fixing `exports` In `package.json`

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Next.js 16 Turbopack: Fixing `exports` in `package.json`Oh hey there, fellow developers! Have you recently jumped into the exciting world of Next.js 16 and started playing around with its shiny new default bundler, _Turbopack_? It's super fast, right? But sometimes, with great power comes… well, a few new quirks to learn. Today, we're diving deep into a specific issue that some of you might encounter, especially when integrating libraries like `@merkur/integration-react`: the dreaded _invalid `exports` field_ error in `package.json`. It can be a real head-scratcher, but don't sweat it, guys, we're going to break it down and get you back on track! This isn't just about a quick fix; it's about understanding why this problem crops up and how to navigate modern JavaScript module resolution. We'll explore the *root cause* behind Turbopack's strictness, walk through the exact reproduction steps that lead to this build failure, and then, most importantly, arm you with effective solutions, including temporary workarounds like `pnpm patch` and the *ideal long-term fixes*. Understanding the `package.json` `exports` field is crucial for modern web development, particularly as bundlers like Turbopack become more stringent in their interpretations of module specifications. This article aims to provide a comprehensive guide, ensuring you not only resolve the immediate error but also gain valuable insights into best practices for library integration and `package.json` configuration in the era of Next.js 16 and beyond. By the end of this read, you'll be well-equipped to tackle similar module resolution challenges, making your development journey smoother and your builds more robust. We'll be keeping things super friendly and casual, so grab a coffee, and let's unravel this mystery together!## Understanding the Turbopack `exports` ErrorAlright, let's kick things off by understanding *what* this `exports` error in `package.json` is all about and *why* it's suddenly popping up. If you're using Next.js 16, you're already experiencing the power of _Turbopack_ as the default bundler instead of Webpack. This switch, while bringing incredible performance benefits, also introduces a stricter interpretation of `package.json` fields, especially the `exports` field. The `exports` field is a relatively new addition to `package.json` (introduced in Node.js 12.7) designed to define entry points for a package, offering better control over how modules are resolved and consumed. It allows library authors to specify different entry points for various environments (like CommonJS vs. ES Modules, or client-side vs. server-side builds), and it's super helpful for preventing direct access to internal package files, promoting encapsulation, and improving compatibility. However, with power comes responsibility, and also, sometimes, a bit of syntax strictness!The specific error message we're tackling here is: _"invalid exports field value \"./server/index.js\" for key \"./server/\": folder exports must end with /"_. This little gem tells us exactly what Turbopack is unhappy about. When you define an `exports` key that refers to a folder, like `"./server/"`, Turbopack expects the *value* associated with it to also indicate a folder *by ending with a slash*. So, if your `exports` field looks something like `"./server/": "./server/index.js"`, Turbopack sees `"./server/"` as a key indicating a directory export, but then `"./server/index.js"` as its value, which clearly points to a *file*, not a directory. This mismatch is what causes the build to fail. It's like saying, "Here's a path to a directory!" and then immediately pointing to a specific file within it without clearly defining it as a file export.Turbopack, being the new kid on the block with a focus on speed and correctness, adheres very closely to the Node.js module resolution specification. This strict adherence means it's less forgiving of ambiguities or non-standard practices that older bundlers like Webpack might have silently tolerated. The transition from Webpack to Turbopack in Next.js 16 marks a significant step towards a more robust and compliant module ecosystem. While Webpack often tried its best to *guess* what you meant, Turbopack says, "Nope, tell me precisely!" This stricter approach helps ensure that module resolution is consistent across different environments and tools, ultimately leading to more stable applications. So, when you see this error, it's not Turbopack trying to be difficult; it's Turbopack trying to ensure your `package.json` exports are unambiguous and follow the spec, which is a good thing in the long run for module interoperability and maintainability. Understanding this fundamental principle is the first step towards resolving the issue and writing more robust `package.json` configurations in your projects. It forces us, as developers and library maintainers, to be more intentional about how our packages expose their modules, which is definitely a positive for the wider JavaScript ecosystem.## Reproducing the Turbopack Build Failure with MerkurAlright, guys, let's get our hands dirty and actually *see* this problem in action. It’s always best to understand an issue by reproducing it ourselves, right? This specific problem pops up when you’re building a Next.js 16 project that utilizes _Turbopack_ and tries to integrate `@merkur/integration-react`. We're going to follow a straightforward path to demonstrate how this `package.json` `exports` error rears its head.First things first, you’ll need a fresh Next.js project. We're talking Next.js 16 or later here, because that's where Turbopack becomes the default bundler, triggering our specific problem. So, open up your terminal and let's get this done. The command to create a new Next.js app is super simple:```bashpnpm create next-app@latest merkur-test --yes```This command will scaffold a brand-new Next.js project named `merkur-test` and automatically confirm all the default settings, making it super quick. Once that's done, you'll want to navigate into your shiny new project directory:```bashcd merkur-test```Easy peasy, right? Now, the next crucial step is to introduce the library that triggers our `exports` dilemma: `@merkur/integration-react`. This package, like many others, has its own `package.json` configuration, and that's where the conflict with Turbopack lies. Install it using pnpm:```bashpnpm i @merkur/integration-react```With Merkur installed, we need to actually *use* it in our Next.js application to trigger the build process that will expose the `exports` error. We'll create a simple client component that imports `MerkurWidget`. Why a client component? Because Next.js 13+ introduced the App Router and server components by default, but for client-side interactivity and many existing libraries, you'll still rely on client components. Plus, `MerkurWidget` is likely meant for the client.Create a new file, say `components/MerkurWidgetClient.tsx`, and paste this content:```jsx