diff --git a/src/playground-code-editor.ts b/src/playground-code-editor.ts
index 74873047..1085ef80 100644
--- a/src/playground-code-editor.ts
+++ b/src/playground-code-editor.ts
@@ -632,7 +632,7 @@ export class PlaygroundCodeEditor extends LitElement {
   private _currentFiletypeSupportsCompletion() {
     // Currently we are only supporting code completion for TS. Change
     // this in a case that we start to support it for other languages too.
-    return this.type === 'ts';
+    return this.type === 'ts' || this.type === 'js';
   }
 
   override focus() {
diff --git a/src/typescript-worker/language-service-context.ts b/src/typescript-worker/language-service-context.ts
index 00d9cdbf..54dec804 100644
--- a/src/typescript-worker/language-service-context.ts
+++ b/src/typescript-worker/language-service-context.ts
@@ -13,6 +13,7 @@ const compilerOptions = {
   skipDefaultLibCheck: true,
   skipLibCheck: true,
   allowJs: true,
+  checkJs: true,
   moduleResolution: ts.ModuleResolutionKind.NodeJs,
   jsx: ts.JsxEmit.React,
   lib: ['dom', 'esnext'],
@@ -66,12 +67,17 @@ class WorkerLanguageServiceHost implements ts.LanguageServiceHost {
    */
   updateFileContentIfNeeded(fileName: string, content: string) {
     const file = this.files.get(fileName);
-    if (file && file.content !== content) {
+    if (file) {
+      if (file.content === content) {
+        // The file hasn't changed, exit early.
+        return;
+      }
       file.content = content;
       file.version += 1;
-    } else {
-      this.files.set(fileName, {content, version: 0});
+      return;
     }
+
+    this.files.set(fileName, {content, version: 0});
   }
 
   /**
diff --git a/src/typescript-worker/typescript-builder.ts b/src/typescript-worker/typescript-builder.ts
index 544d4e44..67a9dc90 100644
--- a/src/typescript-worker/typescript-builder.ts
+++ b/src/typescript-worker/typescript-builder.ts
@@ -10,6 +10,8 @@ import {PackageJson} from './util.js';
 import {makeLspDiagnostic} from './diagnostic.js';
 import {WorkerContext} from './worker-context.js';
 
+const PROCESSED_FILE_ENDINGS = ['.ts', '.jsx', '.tsx'];
+
 export async function* processTypeScriptFiles(
   workerContext: WorkerContext,
   results: AsyncIterable<BuildOutput> | Iterable<BuildOutput>
@@ -20,22 +22,32 @@ export async function* processTypeScriptFiles(
   let packageJson: PackageJson | undefined;
   const compilerInputs = [];
   for await (const result of results) {
+    if (result.kind !== 'file') continue;
+
+    // Collect filetypes that need to be compiled. They will be handled later on in the process.
     if (
-      result.kind === 'file' &&
-      (result.file.name.endsWith('.ts') ||
-        result.file.name.endsWith('.jsx') ||
-        result.file.name.endsWith('.tsx'))
+      PROCESSED_FILE_ENDINGS.some((ending) => result.file.name.endsWith(ending))
     ) {
       compilerInputs.push(result.file);
-    } else {
-      yield result;
-      if (result.kind === 'file' && result.file.name === 'package.json') {
-        try {
-          packageJson = JSON.parse(result.file.content) as PackageJson;
-        } catch (e) {
-          // A bit hacky, but BareModuleTransformer already emits a diagnostic
-          // for this case, so we don't need another one.
-        }
+      continue;
+    }
+    // Everything that reaches this point should be usable out of the box without compiling.
+    // Therefore we can just yield the result to the caller.
+    yield result;
+
+    // Even though we don't need to compile javascript files, we want to append them to our
+    // compilerinputs to get completions and diagnostics for those files.
+    if (result.file.name.endsWith('.js')) {
+      compilerInputs.push(result.file);
+      continue;
+    }
+
+    if (result.file.name === 'package.json') {
+      try {
+        packageJson = JSON.parse(result.file.content) as PackageJson;
+      } catch (e) {
+        // A bit hacky, but BareModuleTransformer already emits a diagnostic
+        // for this case, so we don't need another one.
       }
     }
   }