92 lines
3.1 KiB
TypeScript
92 lines
3.1 KiB
TypeScript
/**
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* ## Barrel Optimizations
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*
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* This loader is used to optimize the imports of "barrel" files that have many
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* re-exports. Currently, both Node.js and Webpack have to enter all of these
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* submodules even if we only need a few of them.
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*
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* For example, say a file `foo.js` with the following contents:
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*
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* export { a } from './a'
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* export { b } from './b'
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* export { c } from './c'
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* ...
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*
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* If the user imports `a` only, this loader will accept the `names` option to
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* be `['a']`. Then, it request the "__barrel_transform__" SWC transform to load
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* `foo.js` and receive the following output:
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*
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* export const __next_private_export_map__ = '[["a","./a","a"],["b","./b","b"],["c","./c","c"],...]'
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*
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* format: '["<imported identifier>", "<import path>", "<exported name>"]'
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* e.g.: import { a as b } from './module-a' => '["b", "./module-a", "a"]'
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*
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* The export map, generated by SWC, is a JSON that represents the exports of
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* that module, their original file, and their original name (since you can do
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* `export { a as b }`).
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*
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* Then, this loader can safely remove all the exports that are not needed and
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* re-export the ones from `names`:
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*
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* export { a } from './a'
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*
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* That's the basic situation and also the happy path.
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*
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*
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*
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* ## Wildcard Exports
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*
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* For wildcard exports (e.g. `export * from './a'`), it becomes a bit more complicated.
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* Say `foo.js` with the following contents:
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*
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* export * from './a'
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* export * from './b'
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* export * from './c'
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* ...
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*
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* If the user imports `bar` from it, SWC can never know which files are going to be
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* exporting `bar`. So, we have to keep all the wildcard exports and do the same
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* process recursively. This loader will return the following output:
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*
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* export * from '__barrel_optimize__?names=bar&wildcard!=!./a'
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* export * from '__barrel_optimize__?names=bar&wildcard!=!./b'
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* export * from '__barrel_optimize__?names=bar&wildcard!=!./c'
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* ...
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*
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* The "!=!" tells Webpack to use the same loader to process './a', './b', and './c'.
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* After the recursive process, the "inner loaders" will either return an empty string
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* or:
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*
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* export * from './target'
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*
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* Where `target` is the file that exports `bar`.
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*
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*
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*
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* ## Non-Barrel Files
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*
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* If the file is not a barrel, we can't apply any optimizations. That's because
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* we can't easily remove things from the file. For example, say `foo.js` with:
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*
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* const v = 1
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* export function b () {
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* return v
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* }
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*
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* If the user imports `b` only, we can't remove the `const v = 1` even though
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* the file is side-effect free. In these caes, this loader will simply re-export
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* `foo.js`:
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*
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* export * from './foo'
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*
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* Besides these cases, this loader also carefully handles the module cache so
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* SWC won't analyze the same file twice, and no instance of the same file will
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* be accidentally created as different instances.
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*/
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import type webpack from 'webpack';
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declare const NextBarrelLoader: (this: webpack.LoaderContext<{
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names: string[];
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swcCacheDir: string;
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}>) => Promise<void>;
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export default NextBarrelLoader;
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