Repository: TypeScript
Issue: Pre-Compiler Plugin Proposal
Labels: Suggestion,In Discussion

## Search Terms
Extension, Plugin, Vue, Custom extensions

I found some issues but no proposal :/

## Suggestion
Hi! I'm an author of the [`fork-ts-checker-webpack-plugin`](https://github.com/TypeStrong/fork-ts-checker-webpack-plugin). Last few months I was busy with [rewriting this plugin to pay off the technological debt](https://github.com/TypeStrong/fork-ts-checker-webpack-plugin/pull/404).

One of the features that this plugin provides is support for the [_Vue.js Single File Components_](https://vuejs.org/v2/guide/single-file-components.html). I was able to implement it but in order to work with the `Watch` program and `SolutionBuilder` [I had to do some workarounds](https://github.com/TypeStrong/fork-ts-checker-webpack-plugin/blob/feature/refactor-to-version-5/src/typescript-reporter/extension/TypeScriptEmbeddedExtension.ts). It's because there is an assertion in the TypeScript source code that files cannot have a custom extension.

The main issue with this approach is that you have to implement these workarounds in every  TypeScript's API client and they can behave differently (like [`appendTsSuffixTo`](https://github.com/TypeStrong/ts-loader#appendtssuffixto) in the `ts-loader`). I know that there are already Language Service Plugins, but they don't work if you use different API, like `Watch`.

### The proposal
A new type of TypeScript plugins that can pre-compile custom source code to the format that TypeScript understands. The simplified interface would be:
```typescript
////////////////////////////////////////////////
// update existing implementation and typings //

/**
 * Keep it for the backward compatibility
 * @deprecated
 */
interface PluginCreateInfo {
  project: Project;
  languageService: LanguageService;
  languageServiceHost: LanguageServiceHost;
  serverHost: ServerHost;
  config: any;
}
// the new name for this type
type LanguageServicePluginCreateInfo = PluginCreateInfo;

/**
 * Keep it for the backward compatibility
 * @deprecated
 */
interface PluginModule {
  type?: 'language-service-plugin'; // not required due to backward compatibility
  getExternalFiles?(project: Project): string[];
  onConfigurationChanged?(config: any): void;
  create(createInfo: PluginCreateInfo): LanguageService;
}
// the new name for this type
type LanguageServicePluginModule = PluginModule & {
  type: 'language-service-plugin'; // required in the new type
}

/////////////////////////////
// add new type of plugins //

interface PreCompilerPluginModuleCreateInfo {
  moduleResolutionHost: ModuleResolutionHost;
  sourceMapHost: SourceMapHost;
  config: any;
}
interface PreCompilerPluginModule {
  type: 'pre-compiler-plugin'; // to distinguish between LanguageServicePlugin and PreCompilerPlugin
  onConfigurationChanged?(config: any): void;
  create(createInfo: PreCompilerPluginModuleCreateInfo): PreCompilerPlugin;
}
interface PreCompilerPlugin {
  extensions: string[]; // list of supported extensions, for example ['.vue'] or ['.mdx']
  createPreCompiledSourceFile(
    fileName: string,
    sourceText: string,
    languageVersion: ScriptTarget,
    setParentNodes?: boolean
  ): PreCompiledSourceFile;
}
// we need to define an extended version of the SourceFile to support additional sourceMap
interface PreCompiledSourceFile extends SourceFile {
  preSourceMapText: string; // we need to provide source map to calculate diagnostics positions and source maps
}

// we need to define a function to create a PreCompiledSourceFile 
// (as we can't infer ScriptKind so it's not an optional parameter)
function createPreCompiledSourceFile(
  fileName: string,
  sourceText: string,
  languageVersion: ScriptTarget,
  scriptKind: ScriptKind,
  setParentNodes?: boolean,
  sourceMapText?: string
): PreCompiledSourceFile

// this will help with source map generation
interface SourceTextNavigator {
  getLineAndCharacterOfPosition(position: number): LineAndCharacter;
  getPositionOfLineAndCharacter(line: number, character: number): number;
  getLineEndOfPosition(position: number): number;
  getLineStarts(): number[];
}
interface SourceMapHost {
  createSourceMapGenerator(fileName: string): SourceMapGenerator;
  createSourceTextNavigator(text: string): SourceTextNavigator;
}


////////////////////////////////////////////////
// update existing implementation and typings //

type AnyPluginModule = PluginModule | PreCompilerPluginModule;

interface PluginModuleWithName {
  name: string;
  module: AnyPluginModule;
}
type PluginModuleFactory = (mod: {
  typescript: typeof ts;
}) => AnyPluginModule
```

This architecture would allow adding new plugin types in the future (so we could add `ModuleResolutionPlugin` to support Yarn PnP in the future)

The maintenance cost of this feature should be low because the exposed API is pretty simple.

There is a point in the [TypeScript Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals)
> 4. Provide an end-to-end build pipeline. Instead, make the system extensible so that external tools can use the compiler for more complex build workflows.

If you think that this feature is incompatible with this point, we could limit it to emitting only `.d.ts` files.

I could try to implement it, but first I need to know if it's something that you would like to add to the TypeScript and if this API is a good direction :)

## Use Cases

I imagine that community would create `PreCompilerPlugin`s for a lot of use cases. For example:
 * `vue-typescript-plugin` - support for the Single File Components
 * `mdx-typescript-plugin` - support for embedded TypeScript in the MDX files
 * `graphql-typescript-pluging` - support for generation of GraphQL types / clients in the runtime (so we don't have to use generators anymore)
 * `css-modules-typescript-plugin` - support for typed css modules (instead of `declare module '*.css';`)

## Examples

#### tsconfig.json
```json
{
  "compilerOptions": {
    "target": "ES6",
    "module": "CommonJS",
    "lib": ["ES6"],
    "moduleResolution": "Node",
    "resolveJsonModule": true,
    "esModuleInterop": true,
    "baseUrl": "./src",
    "outDir": "./lib",
    "plugins": [{ "name": "vue-typescript-plugin" }]
  },
  "include": ["./src"],
  "exclude": ["node_modules"]
}
```

#### node_modules/vue-typescript-plugin/index.ts
It's a pseudo-code based on the `fork-ts-checker-webpack-plugin` implementation. It adds support for the `.vue` files. 
```typescript
import compiler from 'vue-template-compiler';

function init(modules: { typescript: typeof import("typescript/lib/typescript") }) {
  const ts = modules.typescript;

  function create({ sourceMapHost }: ts.PreCompilerPluginModuleCreateInfo): ts.PreCompilerPlugin {
    function getScriptKindByLang(lang: string | undefined) {
      switch (lang) {
        case 'ts':
          return ts.ScriptKind.TS;
        case 'tsx':
          return ts.ScriptKind.TSX;
        case 'jsx':
          return ts.ScriptKind.JSX;
        case 'json':
          return ts.ScriptKind.JSON;
        case 'js':
        default:
          return ts.ScriptKind.JS;
      }
    }

    function createNoScriptSourceFile(
      fileName: string,
      sourceText: string,
      languageVersion: ts.ScriptTarget,
      setParentNodes: boolean | undefined
    ): ts.PreCompilerSourceFile {
      const compiledText = 'export default {};';

      // generate source map
      const sourceMap = sourceMapHost.createSourceMapGenerator(fileName);
      const sourceIndex = sourceMap.addSource(fileName);
      sourceMap.setSourceContent(sourceIndex, sourceText);

      const sourceTextNavigator = sourceMapHost.createSourceTextNavigator(sourceText);

      const sourceStart = sourceTextNavigator.getLineAndCharacterOfPosition(0);
      const sourceEnd = sourceTextNavigator.getLineAndCharacterOfPosition(sourceText.length);

      sourceMap.addMapping(0, 0, sourceIndex, sourceStart.line, sourceStart.character);
      sourceMap.addMapping(0, compiledText.length, sourceIndex, sourceEnd.line, sourceEnd.character);

      // create source file
      return ts.createPreCompiledSourceFile(
        fileName,
        'export default {};',
        languageVersion,
        ts.ScriptKind.JS,
        setParentNodes
      );
    }

    function createSrcScriptSourceFile(
      fileName: string,
      sourceText: string,
      languageVersion: ts.ScriptTarget,
      setParentNodes: boolean | undefined,
      sourceStartPosition: number,
      sourceEndPosition: number,
      scriptTagSrc: string,
      scriptTagLang: string | undefined,
    ): ts.PreCompilerSourceFile {
      // import path cannot be end with '.ts[x]'
      const compiledText = `export * from "${scriptTagSrc.replace(/\.tsx?$/i, '')}";`;

      // generate source map
      const sourceMap = sourceMapHost.createSourceMapGenerator(fileName);
      const sourceIndex = sourceMap.addSource(fileName);
      sourceMap.setSourceContent(sourceIndex, sourceText);

      const sourceTextNavigator = sourceMapHost.createSourceTextNavigator(sourceText);
      const sourceStart = sourceTextNavigator.getLineAndCharacterOfPosition(sourceStartPosition);
      const sourceEnd = sourceTextNavigator.getLineAndCharacterOfPosition(sourceEndPosition);

      sourceMap.addMapping(0, 0, sourceIndex, sourceStart.line, sourceStart.character);
      sourceMap.addMapping(0, compiledText.length, sourceIndex, sourceEnd.line, sourceEnd.character);

      // create source file
      return ts.createPreCompiledSourceFile(
        fileName,
        compiledText,
        languageVersion,
        getScriptKindByLang(scriptTagLang),
        setParentNodes,
        sourceMap.toString()
      );
    }

    function createInlineScriptSourceFile(
      fileName: string,
      sourceText: string,
      languageVersion: ts.ScriptTarget,
      setParentNodes: boolean | undefined,
      sourceStartPosition: number,
      scriptTagContent: string,
      scriptTagLang: string | undefined
    ): ts.PreCompilerSourceFile {
      const compiledText = sourceTagContent;

      // generate source map
      const sourceMap = sourceMapHost.createSourceMapGenerator(fileName);
      const sourceIndex = sourceMap.addSource(fileName);
      sourceMap.setSourceContent(sourceIndex, sourceText);

      const sourceTextNavigator = sourceMapHost.createSourceTextNavigator(sourceText);
      const compiledTextNavigator = sourceMapHost.createSourceTextNavigator(compiledText);

      const compiledLineStarts = compiledTextNavigator.getLineStarts();
      compiledLineStarts.forEach((compiledLineStart) => {
        // map line by line
        const sourceStart = sourceTextNavigator.getLineAndCharacterOfPosition(sourceStartPosition + compiledLineStart);
        const sourceEnd = sourceTextNavigator.getLineAndCharacterOfPosition(sourceTextNavigator.getLineEndOfPosition(sourceStartPosition + compiledLineStart));
        const compiledStart = compiledTextNavigator.getLineAndCharacterOfPosition(compiledLineStart);
        const compiledEnd = compiledTextNavigator.getLineAndCharacterOfPosition(compiledTextNavigator.getLineEndOfPosition(compiledLineStart));

        sourceMap.addMapping(compiledStart.line, compiledStart.character, sourceIndex, sourceStart.line, sourceStart.character);
        sourceMap.addMapping(compiledEnd.line, compiledEnd.character, sourceIndex, sourceEnd.line, sourceEnd.character);
      });

      // create source file
      return ts.createPreCompiledSourceFile(
        fileName,
        compiledText,
        languageVersion,
        getScriptKindByLang(scriptTagLang),
        setParentNodes,
        sourceMap.toString()
      );
    }


    return {
      extensions: ['.vue'],
      createPreCompiledSourceFile(fileName, sourceText, languageVersion, setParentNodes) {
        const { script } = compiler.parseComponent(sourceText, { pad: 'space' });
    
        if (!script) {
          // No <script> block
          return createNoScriptSourceFile(fileName, sourceText, languageVersion, setParentNodes);
        } else if (script.attrs.src) {
          // <script src="file.ts" /> block
          return createSrcScriptSourceFile(
            fileName, 
            sourceText, 
            languageVersion, 
            setParentNodes, 
            script.start, 
            script.end, 
            script.attrs.src, 
            script.attrs.lang
          );
        } else {
          // <script lang="ts"></script> block
          return createInlineScriptSourceFile(
            fileName, 
            sourceText, 
            languageVersion, 
            setParentNodes, 
            script.start,
            sourceText.slice(script.start, script.end), 
            script.attrs.lang
          );
        }
      }
    }
  }

  return {
    type: 'pre-compiler-plugin',
    create
  };
}

export = init;
```

## Checklist

My suggestion meets these guidelines:

* [x] This wouldn't be a breaking change in existing TypeScript/JavaScript code
* [x] This wouldn't change the runtime behavior of existing JavaScript code
* [x] This could be implemented without emitting different JS based on the types of the expressions
* [x] This isn't a runtime feature (e.g. library functionality, non-ECMAScript syntax with JavaScript output, etc.)
* [x] This feature would agree with the rest of [TypeScript's Design Goals](https://github.com/Microsoft/TypeScript/wiki/TypeScript-Design-Goals).