Proposal: Pass SourceFile being emitted to Program.emit's callback

[chuckjaz]

I propose including the SourceFile being emitted in the emit callback.

TypeScript Version:

1.7.5 / 1.8.0-beta / nightly (1.9.0-dev.20160217)

Code

import * as ts from 'typescript';
import * as fs from 'fs';
import * as fse from 'fse';

const fileName = 'index.ts';
const fileNames = [fileName];

class LocalHost implements ts.LanguageServiceHost {
  getCompilationSettings(): ts.CompilerOptions {
    return {
      experimentalDecorators: true,
      modules:  ts.ModuleKind.CommonJS,
      target: ts.ScriptTarget.ES5
    };
  }

  getProjectVersion(): string { return "1";}
  getScriptFileNames(): string[] { return fileNames; }
  getScriptVersion(fileName: string): string { return "1"; }
  getCurrentDirectory(): string { return dir; }

  getScriptSnapshot(fileName: string): ts.IScriptSnapshot {
    const filteredName = nameFilter(fileName);
    if (fs.existsSync(filteredName))
      return ts.ScriptSnapshot.fromString(fs.readFileSync(filteredName, 'utf8'));
  }

  getDefaultLibFileName(options: ts.CompilerOptions): string {
    return './node_modules/typescript/lib/lib.es7.d.ts';  
  }
}

const host = new LocalHost();
const ls = ts.createLanguageService(host);
const program = ls.getProgram();

const emitResult = program.emit(undefined, (absoluteFilePath, fileContent, sourceFile) => {
    console.log('emitting', absoluteFilePath, 'for', sourceFile.fileName);
    fse.mkdirsSync(path.dirname(absoluteFilePath));
    fs.writeFileSync(absoluteFilePath, this.fixSourceMapSources(fileContent), { encoding: 'utf-8' });
});

Expected behavior:
If this proposal is taken then the sourceFile parameter above will contain the SourceFile instance emit currently emitting the file absoluteFilePath on behalf of.

Justification:
.d.ts files do not contain information about the decorators used in the .ts files, nor can it. However, these decorators often contain valuable information that other processors might need, such as, in our case, the Angular 2 offline compiler and the Angular 2 language service. To preserve this information we emit a .metadata.json file for each .d.ts file emitted if the corresponding module contains classes with decorators.

We use program.emit() in our build process and use the callback to trigger the emit of the .metadata.json file but we must make an educated guess as to which SourceFile the .d.ts file is being emitted for. It would be more straight forward for the SourceFile to passed into the callback to emit.

[mhegazy]

if you have --out, there is not a single SourceFile to pass. If you are not using --out/--outFile consider calling emit on each file at a time*, i.e.:

program.getSourceFiles().forEach(f => {
   program.emit(f, ...);
});

* if you end up following this approach, i would not --noEmitOnError, as that will add some unnecessary overhead.

[chuckjaz]

I tried that first but am seeing a huge difference in performance between calling program.emit(undefined, ...) and program.getSourceFiles().forEach(f => program.emit(f, ...)). In my case the first takes consistently around 6 seconds and the second takes 40 seconds.

As for --out/--outFile it seems reasonable to pass null or undefined in this case.

[mhegazy]

I tried that first but am seeing a huge difference in performance between calling

i think this is because you have --noEmitOnError along with --declaration. The declaration diagnostics are not cached. We could cache them.

[chuckjaz]

Caching would be useful but it seems innocuous and useful to pass the SourceFile into the callback. This means it would work and perform well without having to rely on caching. Is there a technical reason I am missing that prevents this?

[mhegazy]

i do not think there is a specific reason. my main concern was consistency with the --out case.

[chuckjaz]

Do you me to create a pull request?

[mhegazy]

sure.

[alexeagle]

When the declaration file is emitted, the sourceFiles are not present. This breaks our primary purpose in adding this feature, which is to write an extra file next to the .d.ts outputs.
cc @chuckjaz