docs: add return Promise callback function

Fix https://github.com/napi-rs/napi-rs/issues/1128

examples/napi/__test__/typegen.spec.ts.md

@@ -40,6 +40,7 @@ Generated by [AVA](https://avajs.dev).
     /** napi = { version = 2, features = ["serde-json"] } */␊
     export function readFile(callback: (arg0: Error | undefined, arg1?: string | undefined | null) => void): void␊
     export function returnJsFunction(): (...args: any[]) => any␊
+    export function callbackReturnPromise<T>(functionInput: () => T | Promise<T>, callback: (err: Error | null, result: T) => void): T | Promise<T>␊
     export function dateToNumber(input: Date): number␊
     export function chronoDateToMillis(input: Date): number␊
     export function chronoDateAdd1Minute(input: Date): Date␊

examples/napi/__test__/values.spec.ts

@@ -2,6 +2,7 @@ import { exec } from 'child_process'
 import { join } from 'path'
 
 import test from 'ava'
+import { spy } from 'sinon'
 
 import {
   DEFAULT_COST,
@@ -89,6 +90,7 @@ import {
   bufferPassThrough,
   JsRepo,
   asyncReduceBuffer,
+  callbackReturnPromise,
 } from '../'
 
 test('export const', (t) => {
@@ -224,6 +226,21 @@ test('return function', (t) => {
   })
 })
 
+test('function return Promise', async (t) => {
+  const cbSpy = spy()
+  await callbackReturnPromise<string>(() => '1', spy)
+  t.is(cbSpy.callCount, 0)
+  await callbackReturnPromise(
+    () => Promise.resolve('42'),
+    (err, res) => {
+      t.is(err, null)
+      cbSpy(res)
+    },
+  )
+  t.is(cbSpy.callCount, 1)
+  t.deepEqual(cbSpy.args, [['42']])
+})
+
 test('object', (t) => {
   t.deepEqual(listObjKeys({ name: 'John Doe', age: 20 }), ['name', 'age'])
   t.deepEqual(createObj(), { test: 1 })

examples/napi/index.d.ts

@@ -30,6 +30,7 @@ export function optionOnly(callback: (arg0?: string | undefined | null) => void)
 /** napi = { version = 2, features = ["serde-json"] } */
 export function readFile(callback: (arg0: Error | undefined, arg1?: string | undefined | null) => void): void
 export function returnJsFunction(): (...args: any[]) => any
+export function callbackReturnPromise<T>(functionInput: () => T | Promise<T>, callback: (err: Error | null, result: T) => void): T | Promise<T>
 export function dateToNumber(input: Date): number
 export function chronoDateToMillis(input: Date): number
 export function chronoDateAdd1Minute(input: Date): Date

examples/napi/src/callback.rs

@@ -53,7 +53,8 @@ fn return_js_function(env: Env) -> Result<JsFunction> {
 
 #[napi(
   ts_generic_types = "T",
-  ts_args_type = "functionInput: () => T | Promise<T>, callback: (err: Error | null, result: T) => void"
+  ts_args_type = "functionInput: () => T | Promise<T>, callback: (err: Error | null, result: T) => void",
+  ts_return_type = "T | Promise<T>"
 )]
 fn callback_return_promise<T: Fn() -> Result<JsUnknown>>(
   env: Env,