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With option `generateExtraImportsInLocalMode` set in local compilation mode, the compiler generates extra side effect imports using this rule: any external module from which an identifier is imported into an NgModule will be added as side effect import to every file in the compilation unit. To illustrate this better assume the compilation unit has source files `a.ts` and `b.ts`, and:
```
// a.ts
import {SomeExternalStuff} from 'path/to/some_where';
import {SomeExternalStuff2} from 'path/to/some_where2';
...
@NgModule({imports: [SomeExternalStuff]})
```
then the extra import `import "path/to/some_where"` will be added to both `a.js` and `b.js`. Note that this is not the case for `import "path/to/some_where2"` though, since the symbol `SomeExternalStuff2` is not imported into any NgModule.
The math behind this mechanism is, in local compilation mode we cannot resolve component external dependencies fully. For example if a component in `a.ts` uses an external component defined in an external file `some_external_comp.ts` then we can generate the import to this file in `a.js`. Instead, we want to generate an import to a file that "gurantees" that `a.js` is placed after `some_external_comp.js` in the bundle. Now since the component in `some_external_comp.ts` is used in `a.ts`, then there must be a chain of imports starting from the NgModule that declares the component in `a.ts` to the component in `some_external_comp.ts`. This chain means some file in the same compilation unit as `a.ts` should import some external NgModule which includes `some_external_comp.ts` in its transitive closure and import it to some NgModule. So by adding this import to `a.js` we ensure that the bundling will have the right order.
PR Close #53543
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| .. | ||
| compliance | ||
| ngtsc | ||
| BUILD.bazel | ||
| downlevel_decorators_transform_spec.ts | ||
| extract_i18n_spec.ts | ||
| mocks.ts | ||
| perform_compile_spec.ts | ||
| perform_watch_spec.ts | ||
| signal_inputs_metadata_transform_spec.ts | ||
| test_support.ts | ||
| typescript_support_spec.ts | ||
| version_helpers_spec.ts | ||