React Native Confusion with Newer Versions

Hi,

This might be a well-known issue in the React Native community, but recent versions have changed the behavior of module resolution. Don't get me wrong—it’s generally a good move.

Metro and the "exports" Field

Previously, Metro (the bundler used in React Native) didn't support the "exports" field in package.json. It was only available with the opt-in flag unstable_enablePackageExports.

With this flag enabled, Metro starts to understand and respect the "exports" field.

Here’s how it looks in Zustand’s package.json:
🔗 View on GitHub

"exports": {
  "./package.json": "./package.json",
  ".": {
    "import": {
      "types": "./esm/index.d.mts",
      "default": "./esm/index.mjs"
    },
    "default": {
      "types": "./index.d.ts",
      "default": "./index.js"
    }
  },
  "./*": {
    "import": {
      "types": "./esm/*.d.mts",
      "default": "./esm/*.mjs"
    },
    "default": {
      "types": "./*.d.ts",
      "default": "./*.js"
    }
  }
}

Roughly speaking:

• When the import condition is met, the bundler picks the ESM build.

• Otherwise, it falls back to the CJS build.

• If a bundler doesn’t understand the "exports" field at all, it defaults to using CJS.

The Hermes and import.meta Problem

The issue arises because the most-used JavaScript runtime in React Native—Hermes—still does not fully support ESM. It throws an error when encountering import.meta.

🔗 Zustand discussion #1967

My hope was that Hermes would support import.meta before unstable_enablePackageExports became stable.

Unfortunately, that didn't happen.

Now it seems that enablePackageExports is enabled by default, but Hermes still errors on import.meta.

The Workaround

From the discussion, I concluded that using the ESM build in React Native is either a bad idea or simply too early. As a workaround, I added a new condition in the exports field specifically for React Native users.

🔗 Zustand PR #3087

Final Thoughts

What’s concerning is that my original plan was to eventually drop CJS and fully move to ESM. But that now seems like a bad idea if the entire React Native ecosystem still relies on CJS.

Happy coding.