fix(typescript): fix declaration file generation for single file outp…

…uts (#1201)

Previously, type declaration files were only emitted if outputOptions.dir was
used and not outputOptions.file or, as a side effect, if the tsconfig file was
specified via a qualified path. This commit fixes that, such that types are
also emitted for single file builds.

packages/typescript/README.md

@@ -304,7 +304,7 @@ export default {
   output: {
     file: 'dist/index.mjs'
   },
-  plugins: [typescript({ tsconfig: './tsconfig.json' })]
+  plugins: [typescript()]
 };
 ```
 
@@ -320,7 +320,7 @@ And accompanying `tsconfig.json` file:
 }
 ```
 
-This setup will produce `dist/index.mjs` and `dist/dist/index.d.ts`. To correctly place the declaration file, add an `exclude` setting in `tsconfig` and modify the `declarationDir` setting in `compilerOptions` to resemble:
+This setup will produce `dist/index.mjs` and `dist/src/index.d.ts`. To correctly place the declaration file, add an `exclude` setting in `tsconfig` and modify the `declarationDir` setting in `compilerOptions` to resemble:
 
 ```json
 {

packages/typescript/src/index.ts

@@ -137,7 +137,11 @@ export default function typescript(options: RollupTypescriptOptions = {}): Plugi
         const output = findTypescriptOutput(ts, parsedOptions, fileName, emittedFiles, tsCache);
         output.declarations.forEach((id) => {
           const code = getEmittedFile(id, emittedFiles, tsCache);
-          let baseDir = outputOptions.dir;
+          let baseDir =
+            outputOptions.dir ||
+            (parsedOptions.options.declaration
+              ? parsedOptions.options.declarationDir || parsedOptions.options.outDir
+              : null);
           if (!baseDir && tsconfig) {
             baseDir = tsconfig.substring(0, tsconfig.lastIndexOf('/'));
           }

packages/typescript/test/test.js

@@ -122,6 +122,30 @@ test.serial('ensures multiple outputs can be built', async (t) => {
   ]);
 });
 
+test.serial('supports emitting types also for single file output', async (t) => {
+  // Navigate to folder and use default local tsconfig instead of specifying tsconfig via file path
+  // as that would have the side effect that the tsconfig's path would be used as fallback path for
+  // the here unspecified outputOptions.dir, in which case the original issue wouldn't show.
+  process.chdir('fixtures/basic');
+
+  const warnings = [];
+  const bundle = await rollup({
+    input: 'main.ts',
+    plugins: [typescript({ declaration: true, declarationDir: 'dist' })],
+    onwarn(warning) {
+      warnings.push(warning);
+    }
+  });
+  // generate a single output bundle, in which case, declaration files were not correctly emitted
+  const output = await getCode(bundle, { format: 'esm', file: 'dist/main.js' }, true);
+
+  t.deepEqual(
+    output.map((out) => out.fileName),
+    ['main.js', 'main.d.ts']
+  );
+  t.is(warnings.length, 0);
+});
+
 test.serial('relative paths in tsconfig.json are resolved relative to the file', async (t) => {
   const bundle = await rollup({
     input: 'fixtures/relative-dir/main.ts',