fix(napi-derive): async task optional output type (#1796)

crates/backend/src/typegen.rs

@@ -263,6 +263,7 @@ fn is_ts_function_type_notation(ty: &Type) -> bool {
   }
 }
 
+// return (type, is_optional, is_variadic)
 pub fn ty_to_ts_type(
   ty: &Type,
   is_return_ty: bool,

crates/backend/src/typegen/struct.rs

@@ -36,13 +36,17 @@ impl ToTypeDef for NapiImpl {
   fn to_type_def(&self) -> Option<TypeDef> {
     if let Some(output_type) = &self.task_output_type {
       TASK_STRUCTS.with(|t| {
-        let resolved_type = ty_to_ts_type(output_type, false, true, false).0;
+        let (resolved_type, is_optional, _) = ty_to_ts_type(output_type, false, true, false);
         t.borrow_mut().insert(
           self.name.to_string(),
           if resolved_type == "undefined" {
             "void".to_owned()
+          } else {
+            if is_optional {
+              format!("{} | null", resolved_type)
             } else {
               resolved_type
+            }
           },
         );
       });

examples/napi/__tests__/__snapshots__/typegen.spec.ts.md

@@ -246,6 +246,8 @@ Generated by [AVA](https://avajs.dev).
     ␊
     export function asyncReduceBuffer(buf: Buffer): Promise<number>␊
     ␊
+    export function asyncTaskOptionalReturn(): Promise<number | null>␊
+    ␊
     export function asyncTaskVoidReturn(): Promise<void>␊
     ␊
     export interface B {␊

examples/napi/index.d.ts

@@ -236,6 +236,8 @@ export function asyncPlus100(p: Promise<number>): Promise<number>
 
 export function asyncReduceBuffer(buf: Buffer): Promise<number>
 
+export function asyncTaskOptionalReturn(): Promise<number | null>
+
 export function asyncTaskVoidReturn(): Promise<void>
 
 export interface B {

examples/napi/src/task.rs

@@ -49,3 +49,24 @@ impl Task for AsyncTaskVoidReturn {
 fn async_task_void_return() -> AsyncTask<AsyncTaskVoidReturn> {
   AsyncTask::new(AsyncTaskVoidReturn {})
 }
+
+struct AsyncTaskOptionalReturn {}
+
+#[napi]
+impl Task for AsyncTaskOptionalReturn {
+  type JsValue = Option<u32>;
+  type Output = ();
+
+  fn compute(&mut self) -> Result<Self::Output> {
+    Ok(())
+  }
+
+  fn resolve(&mut self, _env: Env, _: Self::Output) -> Result<Self::JsValue> {
+    Ok(None)
+  }
+}
+
+#[napi]
+fn async_task_optional_return() -> AsyncTask<AsyncTaskOptionalReturn> {
+  AsyncTask::new(AsyncTaskOptionalReturn {})
+}