In a React Native app's Jest suite, why does a calendar's untranspiled date library fail on import statements, and how do you fix it?

## The symptom and where it comes from A calendar screen imports a date-formatting package. On the phone the screen works. In Jest, the suite dies before a single assertion with `SyntaxError: Cannot use import statement outside a module`, and the stack trace points into `node_modules/some-date-lib/`. The error means Node's module loader received a file that still contains ES module syntax (`import`/`export`) and was told to treat it as CommonJS. The package was never compiled on its way into the test. ## How Jest decides what to compile Jest's **transform** step runs source files through a compiler (here `babel-jest`) before executing them. Which files get compiled is controlled by **`transformIgnorePatterns`**: an array of regular expressions matched against each file's full path. If a path matches **any** pattern, Jest loads it untouched. The React Native preset sets: ```js transformIgnorePatterns: [ 'node_modules/(?!((jest-)?react-native|@react-native(-community)?)/)', ], ``` Read it as: "ignore anything under `node_modules/` **unless** the next path segment is `react-native`, `jest-react-native`, or the `@react-native` or `@react-native-community` scope." That exception exists because React Native itself ships Flow, JSX and ES modules. Every other package, including your date library, is assumed to be published as ready-to-run CommonJS. Two details of the regex catch people: - The allowlisted name must be followed by `/`, so `react-native-gesture-handler/` is **not** covered by the `react-native` entry; library docs for such packages tell you to add them explicitly. - `@react-native(-community)?/` covers whole scopes, so `@react-native-community/netinfo` is already compiled. ## Why the app runs but the test does not When Metro builds the bundle, it passes every module, dependencies included, through its Babel transformer with `@react-native/babel-preset`. An ES-module dependency is therefore compiled at bundle time and never reaches the engine raw. Jest has no such rule for `node_modules`, so the same package works in the app and fails in the test. ## The fix 1. Identify the untranspiled package from the stack trace (and any untranspiled package **it** imports, which fails next). 2. Add it to the lookahead alternatives of the single pattern. 3. Keep the React Native entries, because your `transformIgnorePatterns` replaces the preset's rather than merging with it. 4. Re-run the suite; if another dependency fails the same way, repeat. | Attempt | Result | |---|---| | `['node_modules/(?!((jest-)?react-native\|@react-native(-community)?\|some-date-lib)/)']` | Works: one lookahead, all exceptions | | `['node_modules/(?!some-date-lib/)']` | Breaks React Native itself: the preset's exceptions are gone | | `['node_modules/(?!react-native/)', 'node_modules/(?!some-date-lib/)']` | Neither is compiled: each file matches the other pattern | **Package managers that nest paths** change the path being matched. With pnpm, files live under `node_modules/.pnpm/<name>@<version>/node_modules/<name>/`, so the first `node_modules/` is followed by `.pnpm`, not the package name; Expo's unit-testing guide shows a pnpm variant of the pattern that allows an optional `.pnpm/` segment for this reason. ## Recognising the same failure in other forms The message depends on which untranspiled syntax Node meets first, but the cause is the same: - `Cannot use import statement outside a module` or an unexpected `export` token: ES module syntax. - An unexpected token at a `<`: JSX left in a published file. - An unexpected token at a type annotation: Flow or TypeScript syntax left in a published file. In every case the stack trace names the file, and the file's path shows which package to add to the allowlist. If the path is inside `react-native` or `@react-native/*` itself, the preset's entries were lost from an override. ## Alternatives and their costs - **Widening the allowlist** is the usual answer. Each extra package compiled costs some transform time on the first run; Jest caches transformed files afterwards. - **Replacing the dependency with a mock** avoids compiling it, but then the test no longer exercises the real date logic, which for a calendar is usually the point. - **Switching Jest to native ESM** is a larger change to the whole runner and is not what React Native's preset is designed around. ## What to say in an interview The strong answer names the mechanism in one breath: Jest does not compile `node_modules`, the preset allowlists only React Native's own packages, Metro compiles everything so the app hides the problem, and the fix is one combined negative lookahead that keeps the preset's entries. Mentioning the "matches any pattern" trap and the pnpm path shows you have fixed this more than once.