diff --git a/crates/backend/src/typegen.rs b/crates/backend/src/typegen.rs
index 350fb9c9..81f9fdf0 100644
--- a/crates/backend/src/typegen.rs
+++ b/crates/backend/src/typegen.rs
@@ -2,6 +2,8 @@ mod r#enum;
 mod r#fn;
 mod r#struct;
 
+use std::collections::HashMap;
+
 use once_cell::sync::Lazy;
 use quote::ToTokens;
 use regex::Regex;
@@ -27,8 +29,16 @@ pub trait ToTypeDef {
   fn to_type_def(&self) -> TypeDef;
 }
 
-pub static VEC_REGEX: Lazy<Regex> = Lazy::new(|| Regex::new(r"^Vec < (.*) >$").unwrap());
-pub static OPTION_REGEX: Lazy<Regex> = Lazy::new(|| Regex::new(r"^Option < (.*) >").unwrap());
+pub static TYPE_REGEXES: Lazy<HashMap<&'static str, Regex>> = Lazy::new(|| {
+  let mut map = HashMap::default();
+  map.extend([
+    ("Vec", Regex::new(r"^Vec < (.*) >$").unwrap()),
+    ("Option", Regex::new(r"^Option < (.*) >").unwrap()),
+    ("Result", Regex::new(r"^Result < (.*) >").unwrap()),
+  ]);
+
+  map
+});
 
 pub fn ty_to_ts_type(ty: &Type) -> String {
   match ty {
@@ -54,6 +64,7 @@ pub fn ty_to_ts_type(ty: &Type) -> String {
 
 pub fn str_to_ts_type(ty: &str) -> String {
   match ty {
+    "()" => "null".to_owned(),
     "i8" | "i16" | "i32" | "i64" | "u8" | "u16" | "u32" => "number".to_owned(),
     "i128" | "isize" | "u64" | "u128" | "usize" => "BigInt".to_owned(),
     "bool" => "boolean".to_owned(),
@@ -61,18 +72,24 @@ pub fn str_to_ts_type(ty: &str) -> String {
     "Object" => "object".to_owned(),
     // nothing but `& 'lifetime str` could ends with ` str`
     s if s.ends_with(" str") => "string".to_owned(),
-    s if s.starts_with("Vec") && VEC_REGEX.is_match(s) => {
-      let captures = VEC_REGEX.captures(s).unwrap();
+    s if s.starts_with("Vec") && TYPE_REGEXES["Vec"].is_match(s) => {
+      let captures = TYPE_REGEXES["Vec"].captures(s).unwrap();
       let inner = captures.get(1).unwrap().as_str();
 
       format!("Array<{}>", str_to_ts_type(inner))
     }
-    s if s.starts_with("Option") && OPTION_REGEX.is_match(s) => {
-      let captures = OPTION_REGEX.captures(s).unwrap();
+    s if s.starts_with("Option") && TYPE_REGEXES["Option"].is_match(s) => {
+      let captures = TYPE_REGEXES["Option"].captures(s).unwrap();
       let inner = captures.get(1).unwrap().as_str();
 
       format!("{} | undefined", str_to_ts_type(inner))
     }
+    s if s.starts_with("Result") && TYPE_REGEXES["Result"].is_match(s) => {
+      let captures = TYPE_REGEXES["Result"].captures(s).unwrap();
+      let inner = captures.get(1).unwrap().as_str();
+
+      format!("Error | {}", str_to_ts_type(inner))
+    }
     s => s.to_owned(),
   }
 }
diff --git a/examples/napi/__test__/typegen.spec.ts.md b/examples/napi/__test__/typegen.spec.ts.md
index 24d4b7e7..b17746ee 100644
--- a/examples/napi/__test__/typegen.spec.ts.md
+++ b/examples/napi/__test__/typegen.spec.ts.md
@@ -12,6 +12,7 @@ Generated by [AVA](https://avajs.dev).
     export function getNums(): Array<number>␊
     export function sumNums(nums: Array<number>): number␊
     export function getCwd(callback: (arg0: string) => void): void␊
+    export function readFile(callback: (arg0: Error | null, arg1: string | undefined) => void): void␊
     export enum Kind { Dog = 0, Cat = 1, Duck = 2 }␊
     export enum CustomNumEnum { One = 1, Two = 2, Three = 3, Four = 4, Six = 6, Eight = 8, Nine = 9, Ten = 10 }␊
     export function enumToI32(e: CustomNumEnum): number␊
diff --git a/examples/napi/__test__/typegen.spec.ts.snap b/examples/napi/__test__/typegen.spec.ts.snap
index 12512286..81dd14dd 100644
Binary files a/examples/napi/__test__/typegen.spec.ts.snap and b/examples/napi/__test__/typegen.spec.ts.snap differ
diff --git a/examples/napi/__test__/values.spec.ts b/examples/napi/__test__/values.spec.ts
index f2aed855..f87f1144 100644
--- a/examples/napi/__test__/values.spec.ts
+++ b/examples/napi/__test__/values.spec.ts
@@ -18,6 +18,7 @@ import {
   listObjKeys,
   createObj,
   mapOption,
+  readFile,
 } from '../'
 
 test('number', (t) => {
@@ -78,6 +79,11 @@ test('callback', (t) => {
     null,
     'Expect value to be Function, but received Undefined',
   )
+
+  readFile((err, content) => {
+    t.is(err, null)
+    t.is(content, 'hello world')
+  })
 })
 
 test('object', (t) => {
diff --git a/examples/napi/index.d.ts b/examples/napi/index.d.ts
index a869439a..7cdb1527 100644
--- a/examples/napi/index.d.ts
+++ b/examples/napi/index.d.ts
@@ -2,6 +2,7 @@ export function getWords(): Array<string>
 export function getNums(): Array<number>
 export function sumNums(nums: Array<number>): number
 export function getCwd(callback: (arg0: string) => void): void
+export function readFile(callback: (arg0: Error | null, arg1: string | undefined) => void): void
 export enum Kind { Dog = 0, Cat = 1, Duck = 2 }
 export enum CustomNumEnum { One = 1, Two = 2, Three = 3, Four = 4, Six = 6, Eight = 8, Nine = 9, Ten = 10 }
 export function enumToI32(e: CustomNumEnum): number
diff --git a/examples/napi/src/callback.rs b/examples/napi/src/callback.rs
index ceff82b4..2fe4c926 100644
--- a/examples/napi/src/callback.rs
+++ b/examples/napi/src/callback.rs
@@ -5,3 +5,18 @@ use std::env;
 fn get_cwd<T: Fn(String) -> Result<()>>(callback: T) {
   callback(env::current_dir().unwrap().to_string_lossy().to_string()).unwrap();
 }
+
+/// napi = { version = 2, features = ["serde-json"] }
+#[napi]
+fn read_file<T: Fn(Result<()>, Option<String>) -> Result<()>>(callback: T) {
+  match read_file_content() {
+    Ok(s) => callback(Ok(()), Some(s)),
+    Err(e) => callback(Err(e), None),
+  }
+  .unwrap();
+}
+
+fn read_file_content() -> Result<String> {
+  // serde_json::from_str(&s)?;
+  Ok("hello world".to_string())
+}