rsnext/errors/import-esm-externals.md
Tobias Koppers 7a8da9741d
add support for esm externals (#27069)
add `experimental.esmExternals: boolean | 'loose'` config option

remove `output.environment` configuration in favor of `target`

|                          | `esmExternals: false` (default) | `esmExternals: 'loose'` | `esmExternals: true` |
| ------------------------ | ------------------------------- | ----------------------- | -------------------- |
| import cjs package       | `require()`                     | `require()`             | `require()`          |
| require cjs package      | `require()`                     | `require()`             | `require()`          |
| import mixed package     | `require()` ***                 | `import()`              | `import()`           |
| require mixed package    | `require()`                     | `require()`             | `require()`          |
| import pure esm package  | `import()`                      | `import()`              | `import()`           |
| require pure esm package | Error **                        | `import()` *            | Error **             |
| import pure cjs package  | `require()`                     | `require()`             | Resolving error      |
| require pure cjs package | `require()`                     | `require()`             | `require()`          |

cjs package: Offers only CJS implementation (may not even have an `exports` field)
mixed package: Offers CJS and ESM implementation via `exports` field
pure esm package: Only offers an ESM implementation (may not even have an `exports` field)
pure cjs package: CommonJs package that prevents importing via `exports` field when `import` is used.

`*` This case will behave a bit unexpected for now, since `require` will return a Promise. So that need to be awaited. This will be fixed once the whole next.js bundle is ESM. It didn't work at all before this PR.
`**` This is a new Error when trying to require an esm package.
`***` For mixed packages we prefer the CommonJS variant to avoid a breaking change.

## Feature

- [ ] Implements an existing feature request or RFC. Make sure the feature request has been accepted for implementation before opening a PR.
- [ ] Related issues linked using `fixes #number`
- [x] Integration tests added
- [ ] Documentation added
- [ ] Telemetry added. In case of a feature if it's used or not.
- [x] Errors have helpful link attached, see `contributing.md`

## Documentation / Examples

- [ ] Make sure the linting passes
2021-07-10 16:49:02 +00:00

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# ESM packages need to be imported
#### Why This Error Occurred
Packages in node_modules that are published as EcmaScript Module, need to be `import`ed via `import ... from 'package'` or `import('package')`.
You get this error when using a different way to reference the package, e. g. `require()`.
#### Possible Ways to Fix It
1. Use `import` or `import()` to reference the package instead. (Recommended)
2. If you are already using `import`, make sure that this is not changed by a transpiler, e. g. TypeScript or Babel.
3. Switch to loose mode (`experimental.esmExternals: 'loose'`), which tries to automatically correct this error.
### Useful Links
- [Node.js ESM require docs](https://nodejs.org/dist/latest-v16.x/docs/api/esm.html#esm_require)