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