fix: don't overwrite target from tsconfig.json (#408)

src/transformers/typescript.ts

@@ -249,24 +249,20 @@ function getCompilerOptions({
   options: Options.Typescript;
   basePath: string;
 }): CompilerOptions {
-  // default options
-  const compilerOptionsJSON = {
-    moduleResolution: 'node',
-    target: 'es6',
-  };
-
-  Object.assign(compilerOptionsJSON, options.compilerOptions);
+  const inputOptions = options.compilerOptions ?? {};
 
   const { errors, options: convertedCompilerOptions } =
     options.tsconfigFile !== false || options.tsconfigDirectory
-      ? loadTsconfig(compilerOptionsJSON, filename, options)
-      : ts.convertCompilerOptionsFromJson(compilerOptionsJSON, basePath);
+      ? loadTsconfig(inputOptions, filename, options)
+      : ts.convertCompilerOptionsFromJson(inputOptions, basePath);
 
   if (errors.length) {
     throw new Error(formatDiagnostics(errors, basePath));
   }
 
   const compilerOptions: CompilerOptions = {
+    target: ts.ScriptTarget.ES2015,
+    moduleResolution: ts.ModuleResolutionKind.NodeJs,
     ...(convertedCompilerOptions as CompilerOptions),
     importsNotUsedAsValues: ts.ImportsNotUsedAsValues.Error,
     allowNonTsExtensions: true,

test/fixtures/TypeScriptES2021.svelte

@@ -0,0 +1,9 @@
+<script lang="ts">
+  let x = true;
+  x &&= false;
+  x ||= true;
+
+  async function doIt() {
+    await (() => true)();
+  }
+</script>

test/fixtures/tsconfig.es2021target.json

@@ -0,0 +1,6 @@
+{
+  "compilerOptions": {
+    "target": "es2021",
+    "skipLibCheck": true
+  }
+}

test/transformers/typescript.test.ts

@@ -1,5 +1,6 @@
 import { resolve } from 'path';
 
+import { compile } from 'svelte/compiler';
 import type { Diagnostic } from 'typescript';
 
 import sveltePreprocess from '../../src';
@@ -202,5 +203,42 @@ describe('transformer - typescript', () => {
         esModuleInterop: true,
       });
     });
+
+    it('should not override the target setting if one is present', async () => {
+      const tpl = getFixtureContent('TypeScriptES2021.svelte');
+
+      const opts = sveltePreprocess({
+        typescript: {
+          tsconfigFile: './test/fixtures/tsconfig.es2021target.json',
+        },
+      });
+
+      const { code } = await preprocess(tpl, opts);
+
+      expect(code).toContain('await ');
+      expect(code).toContain('async function doIt');
+      expect(code).toContain('&&=');
+      expect(code).toContain('||=');
+
+      // Svelte should be able to compile ES2021.
+      const { warnings } = compile(code, { name: 'Test' });
+
+      expect(warnings).toHaveLength(0);
+    });
+
+    it('should target ES6 if the configuration has no target setting', async () => {
+      const tpl = getFixtureContent('TypeScriptES2021.svelte');
+
+      const opts = sveltePreprocess({
+        typescript: { tsconfigFile: false },
+      });
+
+      const { code } = await preprocess(tpl, opts);
+
+      expect(code).not.toContain('await ');
+      expect(code).not.toContain('async function doIt');
+      expect(code).not.toContain('&&=');
+      expect(code).not.toContain('||=');
+    });
   });
 });