diff --git a/src/generators/jsx-ast/utils/__tests__/buildSignature.test.mjs b/src/generators/jsx-ast/utils/__tests__/buildSignature.test.mjs
new file mode 100644
index 00000000..f65d7e6f
--- /dev/null
+++ b/src/generators/jsx-ast/utils/__tests__/buildSignature.test.mjs
@@ -0,0 +1,45 @@
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+
+import { generateSignature } from '../buildSignature.mjs';
+
+describe('generateSignature', () => {
+  it('formats union return types without spaces as spaced ("|" surrounded)', () => {
+    const sig = generateSignature(
+      'foo',
+      {
+        params: [],
+        return: { type: 'string|number' },
+      },
+      ''
+    );
+
+    assert.equal(sig, 'foo(): string | number');
+  });
+
+  it('preserves already spaced union return types', () => {
+    const sig = generateSignature(
+      'bar',
+      {
+        params: [],
+        return: { type: 'Promise<string> | undefined' },
+      },
+      ''
+    );
+
+    assert.equal(sig, 'bar(): Promise<string> | undefined');
+  });
+
+  it('omits return type when undefined', () => {
+    const sig = generateSignature(
+      'baz',
+      {
+        params: [],
+        return: undefined,
+      },
+      ''
+    );
+
+    assert.equal(sig, 'baz()');
+  });
+});
diff --git a/src/generators/jsx-ast/utils/buildSignature.mjs b/src/generators/jsx-ast/utils/buildSignature.mjs
index b6ba238a..e649b66e 100644
--- a/src/generators/jsx-ast/utils/buildSignature.mjs
+++ b/src/generators/jsx-ast/utils/buildSignature.mjs
@@ -23,7 +23,11 @@ export const generateSignature = (
   }
 
   // Function or method
-  const returnStr = returnType ? `: ${returnType.type}` : '';
+  const returnStr = (returnType ? `: ${returnType.type}` : '')
+    .split('|')
+    .map(part => part.trim())
+    .filter(Boolean)
+    .join(' | ');
 
   const paramsStr = params
     .map(param => {