diff --git a/crates/backend/src/codegen/struct.rs b/crates/backend/src/codegen/struct.rs
index 63bcb09e..bde440fd 100644
--- a/crates/backend/src/codegen/struct.rs
+++ b/crates/backend/src/codegen/struct.rs
@@ -14,6 +14,38 @@ static NAPI_IMPL_ID: AtomicU32 = AtomicU32::new(0);
 // Generate trait implementations for given Struct.
 fn gen_napi_value_map_impl(name: &Ident, to_napi_val_impl: TokenStream) -> TokenStream {
   let name_str = name.to_string();
+  let js_name_str = format!("{}\0", name_str);
+  let validate = quote! {
+    unsafe fn validate(env: napi::sys::napi_env, napi_val: napi::sys::napi_value) -> napi::Result<napi::sys::napi_value> {
+      if let Some(ctor_ref) = napi::bindgen_prelude::get_class_constructor(#js_name_str) {
+        let mut ctor = std::ptr::null_mut();
+        napi::check_status!(
+          napi::sys::napi_get_reference_value(env, ctor_ref, &mut ctor),
+          "Failed to get constructor reference of class `{}`",
+          #name_str
+        )?;
+        let mut is_instance_of = false;
+        napi::check_status!(
+          napi::sys::napi_instanceof(env, napi_val, ctor, &mut is_instance_of),
+          "Failed to get external value of class `{}`",
+          #name_str
+        )?;
+        if is_instance_of {
+          Ok(std::ptr::null_mut())
+        } else {
+          Err(napi::Error::new(
+            napi::Status::InvalidArg,
+            format!("Value is not instanceof class `{}`", #name_str)
+          ))
+        }
+      } else {
+        Err(napi::Error::new(
+          napi::Status::InvalidArg,
+          format!("Failed to get constructor of class `{}`", #name_str)
+        ))
+      }
+    }
+  };
   quote! {
     impl napi::bindgen_prelude::TypeName for #name {
       fn type_name() -> &'static str {
@@ -99,6 +131,14 @@ fn gen_napi_value_map_impl(name: &Ident, to_napi_val_impl: TokenStream) -> Token
       }
     }
 
+    impl napi::bindgen_prelude::ValidateNapiValue for &#name {
+      #validate
+    }
+
+    impl napi::bindgen_prelude::ValidateNapiValue for &mut #name {
+      #validate
+    }
+
     impl napi::NapiRaw for &#name {
       unsafe fn raw(&self) -> napi::sys::napi_value {
         unreachable!()
@@ -523,6 +563,8 @@ impl NapiStruct {
           Ok(val)
         }
       }
+
+      impl napi::bindgen_prelude::ValidateNapiValue for #name {}
     }
   }
 
diff --git a/crates/backend/src/typegen.rs b/crates/backend/src/typegen.rs
index 3f835ef1..81bd9108 100644
--- a/crates/backend/src/typegen.rs
+++ b/crates/backend/src/typegen.rs
@@ -180,6 +180,27 @@ static KNOWN_TYPES: Lazy<HashMap<&'static str, &'static str>> = Lazy::new(|| {
     ("Either3", "{} | {} | {}"),
     ("Either4", "{} | {} | {} | {}"),
     ("Either5", "{} | {} | {} | {} | {}"),
+    ("Either6", "{} | {} | {} | {} | {} | {}"),
+    ("Either7", "{} | {} | {} | {} | {} | {} | {}"),
+    ("Either8", "{} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either9", "{} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either10", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either11", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either12", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either13", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either14", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either15", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either16", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either17", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either18", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either19", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either20", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either21", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either22", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either23", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either24", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either25", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
+    ("Either26", "{} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {} | {}"),
     ("Null", "null"),
     ("JsNull", "null"),
     ("null", "null"),
diff --git a/crates/napi/src/bindgen_runtime/js_values.rs b/crates/napi/src/bindgen_runtime/js_values.rs
index cb1af7f8..50f1e74a 100644
--- a/crates/napi/src/bindgen_runtime/js_values.rs
+++ b/crates/napi/src/bindgen_runtime/js_values.rs
@@ -38,7 +38,6 @@ pub use buffer::*;
 pub use class::*;
 pub use either::*;
 pub use external::*;
-#[cfg(feature = "napi4")]
 pub use function::*;
 pub use nil::*;
 pub use object::*;
@@ -116,16 +115,15 @@ pub trait FromNapiMutRef {
 }
 
 pub trait ValidateNapiValue: FromNapiValue + TypeName {
-  fn type_of() -> Vec<ValueType> {
-    vec![]
-  }
-
   /// # Safety
   ///
   /// this function called to validate whether napi value passed to rust is valid type
+  /// The reason why this function return `napi_value` is that if a `Promise<T>` passed in
+  /// we need to return `Promise.reject(T)`, not the `T`.
+  /// So we need to create `Promise.reject(T)` in this function.
   unsafe fn validate(env: sys::napi_env, napi_val: sys::napi_value) -> Result<sys::napi_value> {
-    let available_types = Self::type_of();
-    if available_types.is_empty() {
+    let value_type = Self::value_type();
+    if value_type == ValueType::Unknown {
       return Ok(ptr::null_mut());
     }
 
@@ -136,22 +134,15 @@ pub trait ValidateNapiValue: FromNapiValue + TypeName {
     )?;
 
     let received_type = ValueType::from(result);
-    if available_types.contains(&received_type) {
+    if value_type == received_type {
       Ok(ptr::null_mut())
     } else {
       Err(Error::new(
         Status::InvalidArg,
-        if available_types.len() > 1 {
-          format!(
-            "Expect value to be one of {:?}, but received {}",
-            available_types, received_type
-          )
-        } else {
-          format!(
-            "Expect value to be {}, but received {}",
-            available_types[0], received_type
-          )
-        },
+        format!(
+          "Expect value to be {}, but received {}",
+          value_type, received_type
+        ),
       ))
     }
   }
diff --git a/crates/napi/src/bindgen_runtime/js_values/array.rs b/crates/napi/src/bindgen_runtime/js_values/array.rs
index 0f6c6f98..31fdaf63 100644
--- a/crates/napi/src/bindgen_runtime/js_values/array.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/array.rs
@@ -165,11 +165,7 @@ impl Array {
   }
 }
 
-impl ValidateNapiValue for Array {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::Object]
-  }
-}
+impl ValidateNapiValue for Array {}
 
 impl<T> TypeName for Vec<T> {
   fn type_name() -> &'static str {
diff --git a/crates/napi/src/bindgen_runtime/js_values/bigint.rs b/crates/napi/src/bindgen_runtime/js_values/bigint.rs
index cf96a6c9..3dc4a5f5 100644
--- a/crates/napi/src/bindgen_runtime/js_values/bigint.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/bigint.rs
@@ -38,11 +38,7 @@ impl TypeName for BigInt {
   }
 }
 
-impl ValidateNapiValue for BigInt {
-  fn type_of() -> Vec<crate::ValueType> {
-    vec![crate::ValueType::BigInt]
-  }
-}
+impl ValidateNapiValue for BigInt {}
 
 impl FromNapiValue for BigInt {
   unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> crate::Result<Self> {
diff --git a/crates/napi/src/bindgen_runtime/js_values/boolean.rs b/crates/napi/src/bindgen_runtime/js_values/boolean.rs
index cc86d735..5a19b814 100644
--- a/crates/napi/src/bindgen_runtime/js_values/boolean.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/boolean.rs
@@ -10,11 +10,7 @@ impl TypeName for bool {
   }
 }
 
-impl ValidateNapiValue for bool {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::Boolean]
-  }
-}
+impl ValidateNapiValue for bool {}
 
 impl ToNapiValue for bool {
   unsafe fn to_napi_value(env: sys::napi_env, val: bool) -> Result<sys::napi_value> {
diff --git a/crates/napi/src/bindgen_runtime/js_values/either.rs b/crates/napi/src/bindgen_runtime/js_values/either.rs
index aab142b6..e84c25fb 100644
--- a/crates/napi/src/bindgen_runtime/js_values/either.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/either.rs
@@ -1,11 +1,9 @@
-use super::{FromNapiValue, ToNapiValue, TypeName};
+use super::{FromNapiValue, ToNapiValue, TypeName, ValidateNapiValue};
 use crate::{
   bindgen_runtime::{Null, Undefined},
-  sys, type_of, JsUndefined, NapiRaw, Status, ValueType,
+  sys, Error, JsUndefined, NapiRaw, Status, ValueType,
 };
 
-const ERROR_MSG: &str = "The return value of typeof(T) should not be equal in Either";
-
 #[derive(Debug, Clone, Copy)]
 pub enum Either<A, B> {
   A(A),
@@ -61,22 +59,23 @@ impl<T> From<Either<T, Null>> for Option<T> {
   }
 }
 
-impl<A: TypeName + FromNapiValue, B: TypeName + FromNapiValue> FromNapiValue for Either<A, B> {
+impl<
+    A: TypeName + FromNapiValue + ValidateNapiValue,
+    B: TypeName + FromNapiValue + ValidateNapiValue,
+  > FromNapiValue for Either<A, B>
+{
   unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> crate::Result<Self> {
-    debug_assert!(A::value_type() != B::value_type(), "{}", ERROR_MSG);
-    let js_type = type_of!(env, napi_val)?;
-    if js_type == A::value_type() {
-      unsafe { A::from_napi_value(env, napi_val).map(Self::A) }
-    } else if js_type == B::value_type() {
-      unsafe { B::from_napi_value(env, napi_val).map(Self::B) }
+    if unsafe { A::validate(env, napi_val) }.is_ok() {
+      unsafe { A::from_napi_value(env, napi_val) }.map(Either::A)
+    } else if unsafe { B::validate(env, napi_val) }.is_ok() {
+      unsafe { B::from_napi_value(env, napi_val).map(Either::B) }
     } else {
-      Err(crate::Error::new(
+      Err(Error::new(
         Status::InvalidArg,
         format!(
-          "Expect type {} or {}, but got {}",
-          A::value_type(),
-          B::value_type(),
-          js_type
+          "Value is not either {} or {}",
+          A::type_name(),
+          B::type_name()
         ),
       ))
     }
@@ -95,238 +94,82 @@ impl<A: ToNapiValue, B: ToNapiValue> ToNapiValue for Either<A, B> {
   }
 }
 
-#[derive(Debug, Clone, Copy)]
-pub enum Either3<A, B, C> {
-  A(A),
-  B(B),
-  C(C),
-}
-
-impl<A: TypeName, B: TypeName, C: TypeName> TypeName for Either3<A, B, C> {
-  fn type_name() -> &'static str {
-    "Either3"
-  }
-
-  fn value_type() -> ValueType {
-    ValueType::Unknown
-  }
-}
-
-impl<A: TypeName + FromNapiValue, B: TypeName + FromNapiValue, C: TypeName + FromNapiValue>
-  FromNapiValue for Either3<A, B, C>
-{
-  unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> crate::Result<Self> {
-    debug_assert!(
-      {
-        let mut types = vec![A::value_type(), B::value_type(), C::value_type()];
-        types.dedup();
-        types.len() == 3
-      },
-      "{}",
-      ERROR_MSG
-    );
-    let js_type = type_of!(env, napi_val)?;
-    if js_type == A::value_type() {
-      unsafe { A::from_napi_value(env, napi_val).map(Self::A) }
-    } else if js_type == B::value_type() {
-      unsafe { B::from_napi_value(env, napi_val).map(Self::B) }
-    } else if js_type == C::value_type() {
-      unsafe { C::from_napi_value(env, napi_val).map(Self::C) }
-    } else {
-      Err(crate::Error::new(
-        Status::InvalidArg,
-        format!(
-          "Expect type {} or {} or {}, but got {}",
-          A::value_type(),
-          B::value_type(),
-          C::value_type(),
-          js_type
-        ),
-      ))
+macro_rules! either_n {
+  ( $either_name:ident, $( $parameter:ident ),+ $( , )* ) => {
+    #[derive(Debug, Clone, Copy)]
+    pub enum $either_name< $( $parameter ),+ > {
+      $( $parameter ( $parameter ) ),+
     }
-  }
-}
 
-impl<A: ToNapiValue, B: ToNapiValue, C: ToNapiValue> ToNapiValue for Either3<A, B, C> {
-  unsafe fn to_napi_value(
-    env: sys::napi_env,
-    value: Self,
-  ) -> crate::Result<crate::sys::napi_value> {
-    match value {
-      Self::A(a) => unsafe { A::to_napi_value(env, a) },
-      Self::B(b) => unsafe { B::to_napi_value(env, b) },
-      Self::C(c) => unsafe { C::to_napi_value(env, c) },
+    impl< $( $parameter ),+ > TypeName for $either_name < $( $parameter ),+ >
+      where $( $parameter: TypeName ),+
+    {
+      fn type_name() -> &'static str {
+        stringify!( $either_name )
+      }
+
+      fn value_type() -> ValueType {
+        ValueType::Unknown
+      }
     }
-  }
-}
 
-#[derive(Debug, Clone, Copy)]
-pub enum Either4<A, B, C, D> {
-  A(A),
-  B(B),
-  C(C),
-  D(D),
-}
-
-impl<A: TypeName, B: TypeName, C: TypeName, D: TypeName> TypeName for Either4<A, B, C, D> {
-  fn type_name() -> &'static str {
-    "Either4"
-  }
-
-  fn value_type() -> ValueType {
-    ValueType::Unknown
-  }
-}
-
-impl<
-    A: TypeName + FromNapiValue,
-    B: TypeName + FromNapiValue,
-    C: TypeName + FromNapiValue,
-    D: TypeName + FromNapiValue,
-  > FromNapiValue for Either4<A, B, C, D>
-{
-  unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> crate::Result<Self> {
-    debug_assert!(
-      {
-        let mut types = vec![
-          A::value_type(),
-          B::value_type(),
-          C::value_type(),
-          D::value_type(),
-        ];
-        types.dedup();
-        types.len() == 4
-      },
-      "{}",
-      ERROR_MSG
-    );
-    let js_type = type_of!(env, napi_val)?;
-    if js_type == A::value_type() {
-      unsafe { A::from_napi_value(env, napi_val).map(Self::A) }
-    } else if js_type == B::value_type() {
-      unsafe { B::from_napi_value(env, napi_val).map(Self::B) }
-    } else if js_type == C::value_type() {
-      unsafe { C::from_napi_value(env, napi_val).map(Self::C) }
-    } else if js_type == D::value_type() {
-      unsafe { D::from_napi_value(env, napi_val).map(Self::D) }
-    } else {
-      Err(crate::Error::new(
-        Status::InvalidArg,
-        format!(
-          "Expect type {} or {} or {} or {}, but got {}",
-          A::value_type(),
-          B::value_type(),
-          C::value_type(),
-          D::value_type(),
-          js_type
-        ),
-      ))
+    impl< $( $parameter ),+ > FromNapiValue for $either_name < $( $parameter ),+ >
+      where $( $parameter: TypeName + FromNapiValue + ValidateNapiValue ),+
+    {
+      unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> crate::Result<Self> {
+        $(
+          if unsafe { $parameter::validate(env, napi_val).is_ok() } {
+            unsafe { $parameter ::from_napi_value(env, napi_val).map(Self:: $parameter ) }
+          } else
+        )+
+        {
+          Err(crate::Error::new(
+            Status::InvalidArg,
+            format!(
+              concat!("Value is non of these types ", $( "`{", stringify!( $parameter ), "}`, " ),+ ),
+              $( $parameter = $parameter::value_type(), )+
+            ),
+          ))
+        }
+      }
     }
-  }
-}
 
-impl<A: ToNapiValue, B: ToNapiValue, C: ToNapiValue, D: ToNapiValue> ToNapiValue
-  for Either4<A, B, C, D>
-{
-  unsafe fn to_napi_value(
-    env: sys::napi_env,
-    value: Self,
-  ) -> crate::Result<crate::sys::napi_value> {
-    match value {
-      Self::A(a) => unsafe { A::to_napi_value(env, a) },
-      Self::B(b) => unsafe { B::to_napi_value(env, b) },
-      Self::C(c) => unsafe { C::to_napi_value(env, c) },
-      Self::D(d) => unsafe { D::to_napi_value(env, d) },
+    impl< $( $parameter ),+ > ToNapiValue for $either_name < $( $parameter ),+ >
+      where $( $parameter: ToNapiValue ),+
+    {
+      unsafe fn to_napi_value(
+        env: sys::napi_env,
+        value: Self
+      ) -> crate::Result<crate::sys::napi_value> {
+        match value {
+          $( Self:: $parameter (v) => unsafe { $parameter ::to_napi_value(env, v) } ),+
+        }
+      }
     }
-  }
+  };
 }
 
-#[derive(Debug, Clone, Copy)]
-pub enum Either5<A, B, C, D, E> {
-  A(A),
-  B(B),
-  C(C),
-  D(D),
-  E(E),
-}
-
-impl<A: TypeName, B: TypeName, C: TypeName, D: TypeName, E: TypeName> TypeName
-  for Either5<A, B, C, D, E>
-{
-  fn type_name() -> &'static str {
-    "Either5"
-  }
-
-  fn value_type() -> ValueType {
-    ValueType::Unknown
-  }
-}
-
-impl<
-    A: TypeName + FromNapiValue,
-    B: TypeName + FromNapiValue,
-    C: TypeName + FromNapiValue,
-    D: TypeName + FromNapiValue,
-    E: TypeName + FromNapiValue,
-  > FromNapiValue for Either5<A, B, C, D, E>
-{
-  unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> crate::Result<Self> {
-    debug_assert!(
-      {
-        let mut types = vec![
-          A::value_type(),
-          B::value_type(),
-          C::value_type(),
-          D::value_type(),
-          E::value_type(),
-        ];
-        types.dedup();
-        types.len() == 5
-      },
-      "{}",
-      ERROR_MSG
-    );
-    let js_type = type_of!(env, napi_val)?;
-    if js_type == A::value_type() {
-      unsafe { A::from_napi_value(env, napi_val).map(Self::A) }
-    } else if js_type == B::value_type() {
-      unsafe { B::from_napi_value(env, napi_val).map(Self::B) }
-    } else if js_type == C::value_type() {
-      unsafe { C::from_napi_value(env, napi_val).map(Self::C) }
-    } else if js_type == D::value_type() {
-      unsafe { D::from_napi_value(env, napi_val).map(Self::D) }
-    } else if js_type == E::value_type() {
-      unsafe { E::from_napi_value(env, napi_val).map(Self::E) }
-    } else {
-      Err(crate::Error::new(
-        Status::InvalidArg,
-        format!(
-          "Expect type {} or {} or {} or {} or {}, but got {}",
-          A::value_type(),
-          B::value_type(),
-          C::value_type(),
-          D::value_type(),
-          E::value_type(),
-          js_type
-        ),
-      ))
-    }
-  }
-}
-
-impl<A: ToNapiValue, B: ToNapiValue, C: ToNapiValue, D: ToNapiValue, E: ToNapiValue> ToNapiValue
-  for Either5<A, B, C, D, E>
-{
-  unsafe fn to_napi_value(
-    env: sys::napi_env,
-    value: Self,
-  ) -> crate::Result<crate::sys::napi_value> {
-    match value {
-      Self::A(a) => unsafe { A::to_napi_value(env, a) },
-      Self::B(b) => unsafe { B::to_napi_value(env, b) },
-      Self::C(c) => unsafe { C::to_napi_value(env, c) },
-      Self::D(d) => unsafe { D::to_napi_value(env, d) },
-      Self::E(e) => unsafe { E::to_napi_value(env, e) },
-    }
-  }
-}
+either_n!(Either3, A, B, C);
+either_n!(Either4, A, B, C, D);
+either_n!(Either5, A, B, C, D, E);
+either_n!(Either6, A, B, C, D, E, F);
+either_n!(Either7, A, B, C, D, E, F, G);
+either_n!(Either8, A, B, C, D, E, F, G, H);
+either_n!(Either9, A, B, C, D, E, F, G, H, I);
+either_n!(Either10, A, B, C, D, E, F, G, H, I, J);
+either_n!(Either11, A, B, C, D, E, F, G, H, I, J, K);
+either_n!(Either12, A, B, C, D, E, F, G, H, I, J, K, L);
+either_n!(Either13, A, B, C, D, E, F, G, H, I, J, K, L, M);
+either_n!(Either14, A, B, C, D, E, F, G, H, I, J, K, L, M, N);
+either_n!(Either15, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O);
+either_n!(Either16, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P);
+either_n!(Either17, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q);
+either_n!(Either18, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R);
+either_n!(Either19, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S);
+either_n!(Either20, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T);
+either_n!(Either21, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U);
+either_n!(Either22, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V);
+either_n!(Either23, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W);
+either_n!(Either24, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X);
+either_n!(Either25, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y);
+either_n!(Either26, A, B, C, D, E, F, G, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, Y, Z);
diff --git a/crates/napi/src/bindgen_runtime/js_values/external.rs b/crates/napi/src/bindgen_runtime/js_values/external.rs
index 5ed77a4c..3ce29a13 100644
--- a/crates/napi/src/bindgen_runtime/js_values/external.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/external.rs
@@ -23,11 +23,7 @@ impl<T: 'static> TypeName for External<T> {
   }
 }
 
-impl<T: 'static> ValidateNapiValue for External<T> {
-  fn type_of() -> Vec<crate::ValueType> {
-    vec![crate::ValueType::External]
-  }
-}
+impl<T: 'static> ValidateNapiValue for External<T> {}
 
 impl<T: 'static> External<T> {
   pub fn new(value: T) -> Self {
diff --git a/crates/napi/src/bindgen_runtime/js_values/function.rs b/crates/napi/src/bindgen_runtime/js_values/function.rs
index 208251d5..d7df6aea 100644
--- a/crates/napi/src/bindgen_runtime/js_values/function.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/function.rs
@@ -2,8 +2,4 @@ use super::ValidateNapiValue;
 
 pub use crate::JsFunction;
 
-impl ValidateNapiValue for JsFunction {
-  fn type_of() -> Vec<crate::ValueType> {
-    vec![crate::ValueType::Function]
-  }
-}
+impl ValidateNapiValue for JsFunction {}
diff --git a/crates/napi/src/bindgen_runtime/js_values/map.rs b/crates/napi/src/bindgen_runtime/js_values/map.rs
index 91f6e8ea..6e0c4748 100644
--- a/crates/napi/src/bindgen_runtime/js_values/map.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/map.rs
@@ -13,11 +13,7 @@ impl<K, V, S> TypeName for HashMap<K, V, S> {
   }
 }
 
-impl<K: From<String> + Eq + Hash, V: FromNapiValue> ValidateNapiValue for HashMap<K, V> {
-  fn type_of() -> Vec<crate::ValueType> {
-    vec![crate::ValueType::Object]
-  }
-}
+impl<K: From<String> + Eq + Hash, V: FromNapiValue> ValidateNapiValue for HashMap<K, V> {}
 
 impl<K, V, S> ToNapiValue for HashMap<K, V, S>
 where
diff --git a/crates/napi/src/bindgen_runtime/js_values/nil.rs b/crates/napi/src/bindgen_runtime/js_values/nil.rs
index 24a6d408..91b5a736 100644
--- a/crates/napi/src/bindgen_runtime/js_values/nil.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/nil.rs
@@ -15,11 +15,7 @@ impl TypeName for Null {
   }
 }
 
-impl ValidateNapiValue for Null {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::Null]
-  }
-}
+impl ValidateNapiValue for Null {}
 
 impl FromNapiValue for Null {
   unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> Result<Self> {
@@ -56,11 +52,7 @@ impl TypeName for Undefined {
   }
 }
 
-impl ValidateNapiValue for Undefined {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::Undefined]
-  }
-}
+impl ValidateNapiValue for Undefined {}
 
 impl FromNapiValue for Undefined {
   unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> Result<Self> {
diff --git a/crates/napi/src/bindgen_runtime/js_values/number.rs b/crates/napi/src/bindgen_runtime/js_values/number.rs
index 0c20b619..97296809 100644
--- a/crates/napi/src/bindgen_runtime/js_values/number.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/number.rs
@@ -15,12 +15,7 @@ macro_rules! impl_number_conversions {
         }
       }
 
-      impl $crate::bindgen_prelude::ValidateNapiValue for $t {
-        #[inline(always)]
-        fn type_of() -> Vec<$crate::ValueType> {
-          vec![$crate::ValueType::Number]
-        }
-      }
+      impl $crate::bindgen_prelude::ValidateNapiValue for $t { }
 
       impl $crate::bindgen_prelude::ToNapiValue for $t {
         #[inline(always)]
diff --git a/crates/napi/src/bindgen_runtime/js_values/object.rs b/crates/napi/src/bindgen_runtime/js_values/object.rs
index 512ff5ae..04870e59 100644
--- a/crates/napi/src/bindgen_runtime/js_values/object.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/object.rs
@@ -88,9 +88,3 @@ impl TypeName for Object {
     ValueType::Object
   }
 }
-
-impl ValidateNapiValue for Object {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::Object]
-  }
-}
diff --git a/crates/napi/src/bindgen_runtime/js_values/promise.rs b/crates/napi/src/bindgen_runtime/js_values/promise.rs
index a9fa23c9..b2b710b0 100644
--- a/crates/napi/src/bindgen_runtime/js_values/promise.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/promise.rs
@@ -25,10 +25,6 @@ impl<T: FromNapiValue> TypeName for Promise<T> {
 }
 
 impl<T: FromNapiValue> ValidateNapiValue for Promise<T> {
-  fn type_of() -> Vec<crate::ValueType> {
-    vec![crate::ValueType::Object]
-  }
-
   unsafe fn validate(
     env: crate::sys::napi_env,
     napi_val: crate::sys::napi_value,
diff --git a/crates/napi/src/bindgen_runtime/js_values/string.rs b/crates/napi/src/bindgen_runtime/js_values/string.rs
index 4ab329ac..9fb4635a 100644
--- a/crates/napi/src/bindgen_runtime/js_values/string.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/string.rs
@@ -16,11 +16,7 @@ impl TypeName for String {
   }
 }
 
-impl ValidateNapiValue for String {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::String]
-  }
-}
+impl ValidateNapiValue for String {}
 
 impl ToNapiValue for String {
   unsafe fn to_napi_value(env: sys::napi_env, val: Self) -> Result<sys::napi_value> {
@@ -81,11 +77,7 @@ impl TypeName for &str {
   }
 }
 
-impl ValidateNapiValue for &str {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::String]
-  }
-}
+impl ValidateNapiValue for &str {}
 
 impl FromNapiValue for &str {
   unsafe fn from_napi_value(env: sys::napi_env, napi_val: sys::napi_value) -> Result<Self> {
@@ -146,11 +138,7 @@ impl ToNapiValue for &str {
 #[derive(Debug)]
 pub struct Utf16String(String);
 
-impl ValidateNapiValue for Utf16String {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::String]
-  }
-}
+impl ValidateNapiValue for Utf16String {}
 
 impl From<String> for Utf16String {
   fn from(s: String) -> Self {
@@ -245,11 +233,7 @@ pub mod latin1_string {
   #[derive(Debug)]
   pub struct Latin1String(String);
 
-  impl ValidateNapiValue for Latin1String {
-    fn type_of() -> Vec<ValueType> {
-      vec![ValueType::String]
-    }
-  }
+  impl ValidateNapiValue for Latin1String {}
 
   impl From<String> for Latin1String {
     fn from(s: String) -> Self {
diff --git a/crates/napi/src/bindgen_runtime/js_values/symbol.rs b/crates/napi/src/bindgen_runtime/js_values/symbol.rs
index 745b884b..fef0edb5 100644
--- a/crates/napi/src/bindgen_runtime/js_values/symbol.rs
+++ b/crates/napi/src/bindgen_runtime/js_values/symbol.rs
@@ -14,15 +14,11 @@ impl TypeName for Symbol {
   }
 
   fn value_type() -> crate::ValueType {
-    crate::ValueType::Object
+    crate::ValueType::Symbol
   }
 }
 
-impl ValidateNapiValue for Symbol {
-  fn type_of() -> Vec<crate::ValueType> {
-    vec![crate::ValueType::Symbol]
-  }
-}
+impl ValidateNapiValue for Symbol {}
 
 impl Symbol {
   pub fn new(desc: String) -> Self {
diff --git a/crates/napi/src/js_values/arraybuffer.rs b/crates/napi/src/js_values/arraybuffer.rs
index d61b4963..deb12b2d 100644
--- a/crates/napi/src/js_values/arraybuffer.rs
+++ b/crates/napi/src/js_values/arraybuffer.rs
@@ -3,8 +3,10 @@ use std::os::raw::c_void;
 use std::ptr;
 use std::slice;
 
-use crate::bindgen_runtime::TypeName;
-use crate::{check_status, sys, JsUnknown, NapiValue, Ref, Result, Value, ValueType};
+use crate::bindgen_runtime::{TypeName, ValidateNapiValue};
+use crate::{
+  check_status, sys, Error, JsUnknown, NapiValue, Ref, Result, Status, Value, ValueType,
+};
 
 pub struct JsArrayBuffer(pub(crate) Value);
 
@@ -18,6 +20,20 @@ impl TypeName for JsArrayBuffer {
   }
 }
 
+impl ValidateNapiValue for JsArrayBuffer {
+  unsafe fn validate(env: sys::napi_env, napi_val: sys::napi_value) -> Result<sys::napi_value> {
+    let mut is_array_buffer = false;
+    check_status!(unsafe { sys::napi_is_arraybuffer(env, napi_val, &mut is_array_buffer) })?;
+    if !is_array_buffer {
+      return Err(Error::new(
+        Status::InvalidArg,
+        "Value is not an array buffer".to_owned(),
+      ));
+    }
+    Ok(ptr::null_mut())
+  }
+}
+
 pub struct JsArrayBufferValue {
   pub(crate) value: JsArrayBuffer,
   len: usize,
diff --git a/crates/napi/src/js_values/bigint.rs b/crates/napi/src/js_values/bigint.rs
index b63fd6e8..245cddb1 100644
--- a/crates/napi/src/js_values/bigint.rs
+++ b/crates/napi/src/js_values/bigint.rs
@@ -20,6 +20,8 @@ impl TypeName for JsBigInt {
   }
 }
 
+impl ValidateNapiValue for JsBigInt {}
+
 impl JsBigInt {
   pub(crate) fn from_raw_unchecked(
     env: sys::napi_env,
diff --git a/crates/napi/src/js_values/boolean.rs b/crates/napi/src/js_values/boolean.rs
index 277d3dd5..c5d5d69f 100644
--- a/crates/napi/src/js_values/boolean.rs
+++ b/crates/napi/src/js_values/boolean.rs
@@ -1,7 +1,7 @@
 use std::convert::TryFrom;
 
 use super::Value;
-use crate::bindgen_runtime::TypeName;
+use crate::bindgen_runtime::{TypeName, ValidateNapiValue};
 use crate::{check_status, ValueType};
 use crate::{sys, Error, Result};
 
@@ -18,6 +18,8 @@ impl TypeName for JsBoolean {
   }
 }
 
+impl ValidateNapiValue for JsBoolean {}
+
 impl JsBoolean {
   pub fn get_value(&self) -> Result<bool> {
     let mut result = false;
diff --git a/crates/napi/src/js_values/buffer.rs b/crates/napi/src/js_values/buffer.rs
index 1ce009de..ab21b83d 100644
--- a/crates/napi/src/js_values/buffer.rs
+++ b/crates/napi/src/js_values/buffer.rs
@@ -3,7 +3,10 @@ use std::ops::{Deref, DerefMut};
 use std::ptr;
 
 use super::{Value, ValueType};
-use crate::{bindgen_runtime::TypeName, check_status, sys, JsUnknown, NapiValue, Ref, Result};
+use crate::bindgen_runtime::ValidateNapiValue;
+use crate::{
+  bindgen_runtime::TypeName, check_status, sys, Error, JsUnknown, NapiValue, Ref, Result, Status,
+};
 
 pub struct JsBuffer(pub(crate) Value);
 
@@ -17,6 +20,20 @@ impl TypeName for JsBuffer {
   }
 }
 
+impl ValidateNapiValue for JsBuffer {
+  unsafe fn validate(env: sys::napi_env, napi_val: sys::napi_value) -> Result<sys::napi_value> {
+    let mut is_buffer = false;
+    check_status!(unsafe { sys::napi_is_buffer(env, napi_val, &mut is_buffer) })?;
+    if !is_buffer {
+      return Err(Error::new(
+        Status::InvalidArg,
+        "Value is not a buffer".to_owned(),
+      ));
+    }
+    Ok(ptr::null_mut())
+  }
+}
+
 pub struct JsBufferValue {
   pub(crate) value: JsBuffer,
   data: mem::ManuallyDrop<Vec<u8>>,
diff --git a/crates/napi/src/js_values/mod.rs b/crates/napi/src/js_values/mod.rs
index 0787e541..0c56a7b2 100644
--- a/crates/napi/src/js_values/mod.rs
+++ b/crates/napi/src/js_values/mod.rs
@@ -73,6 +73,8 @@ impl TypeName for JsNull {
   }
 }
 
+impl ValidateNapiValue for JsNull {}
+
 #[derive(Clone, Copy)]
 pub struct JsSymbol(pub(crate) Value);
 
@@ -86,11 +88,7 @@ impl TypeName for JsSymbol {
   }
 }
 
-impl ValidateNapiValue for JsSymbol {
-  fn type_of() -> Vec<ValueType> {
-    vec![ValueType::Symbol]
-  }
-}
+impl ValidateNapiValue for JsSymbol {}
 
 pub struct JsExternal(pub(crate) Value);
 
@@ -104,6 +102,8 @@ impl TypeName for JsExternal {
   }
 }
 
+impl ValidateNapiValue for JsExternal {}
+
 macro_rules! impl_napi_value_trait {
   ($js_value:ident, $value_type:ident) => {
     impl NapiValue for $js_value {
diff --git a/crates/napi/src/js_values/number.rs b/crates/napi/src/js_values/number.rs
index e85fab73..9dc0965d 100644
--- a/crates/napi/src/js_values/number.rs
+++ b/crates/napi/src/js_values/number.rs
@@ -1,7 +1,7 @@
 use std::convert::TryFrom;
 
 use super::Value;
-use crate::bindgen_runtime::TypeName;
+use crate::bindgen_runtime::{TypeName, ValidateNapiValue};
 use crate::{check_status, ValueType};
 use crate::{sys, Error, Result};
 
@@ -18,6 +18,8 @@ impl TypeName for JsNumber {
   }
 }
 
+impl ValidateNapiValue for JsNumber {}
+
 impl JsNumber {
   pub fn get_uint32(&self) -> Result<u32> {
     let mut result = 0;
diff --git a/crates/napi/src/js_values/string/mod.rs b/crates/napi/src/js_values/string/mod.rs
index 8aa26538..0e5eeff8 100644
--- a/crates/napi/src/js_values/string/mod.rs
+++ b/crates/napi/src/js_values/string/mod.rs
@@ -2,6 +2,7 @@ use std::mem;
 use std::ptr;
 
 use crate::bindgen_runtime::TypeName;
+use crate::bindgen_runtime::ValidateNapiValue;
 use crate::ValueType;
 use crate::{check_status, sys, Result, Value};
 
@@ -26,6 +27,8 @@ impl TypeName for JsString {
   }
 }
 
+impl ValidateNapiValue for JsString {}
+
 impl JsString {
   pub fn utf8_len(&self) -> Result<usize> {
     let mut length = 0;
diff --git a/crates/napi/src/js_values/undefined.rs b/crates/napi/src/js_values/undefined.rs
index ede22046..bd5f5e0b 100644
--- a/crates/napi/src/js_values/undefined.rs
+++ b/crates/napi/src/js_values/undefined.rs
@@ -1,4 +1,7 @@
-use crate::{bindgen_runtime::TypeName, ValueType};
+use crate::{
+  bindgen_runtime::{TypeName, ValidateNapiValue},
+  ValueType,
+};
 
 use super::Value;
 
@@ -14,3 +17,5 @@ impl TypeName for JsUndefined {
     ValueType::Undefined
   }
 }
+
+impl ValidateNapiValue for JsUndefined {}
diff --git a/examples/napi/__test__/typegen.spec.ts.md b/examples/napi/__test__/typegen.spec.ts.md
index cfc7453c..4271220a 100644
--- a/examples/napi/__test__/typegen.spec.ts.md
+++ b/examples/napi/__test__/typegen.spec.ts.md
@@ -62,6 +62,7 @@ Generated by [AVA](https://avajs.dev).
     export function either4(input: string | number | boolean | Obj): number␊
     export function receiveClassOrNumber(either: number | JsClassForEither): number␊
     export function receiveMutClassOrNumber(either: number | JsClassForEither): number␊
+    export function receiveDifferentClass(either: JsClassForEither | AnotherClassForEither): number␊
     export function returnEitherClass(input: number): number | JsClassForEither␊
     export function eitherFromOption(): JsClassForEither | undefined␊
     /** default enum values are continuos i32s start from 0 */␊
@@ -276,6 +277,9 @@ Generated by [AVA](https://avajs.dev).
     export class JsClassForEither {␊
       constructor()␊
     }␊
+    export class AnotherClassForEither {␊
+      constructor()␊
+    }␊
     export class Fib {␊
       [Symbol.iterator](): Iterator<number, void, number>␊
       constructor()␊
diff --git a/examples/napi/__test__/typegen.spec.ts.snap b/examples/napi/__test__/typegen.spec.ts.snap
index a7eaeb36..a5f80b44 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 412e053e..e04a1d76 100644
--- a/examples/napi/__test__/values.spec.ts
+++ b/examples/napi/__test__/values.spec.ts
@@ -99,6 +99,8 @@ import {
   overrideIndividualArgOnFunctionWithCbArg,
   createObjectWithClassField,
   receiveObjectWithClassField,
+  AnotherClassForEither,
+  receiveDifferentClass,
 } from '../'
 
 test('export const', (t) => {
@@ -500,6 +502,13 @@ test('receive class reference in either', (t) => {
   t.is(receiveMutClassOrNumber(c), 100)
 })
 
+test('receive different class', (t) => {
+  const a = new JsClassForEither()
+  const b = new AnotherClassForEither()
+  t.is(receiveDifferentClass(a), 42)
+  t.is(receiveDifferentClass(b), 100)
+})
+
 test('return either class', (t) => {
   t.is(returnEitherClass(1), 1)
   t.true(returnEitherClass(-1) instanceof JsClassForEither)
diff --git a/examples/napi/index.d.ts b/examples/napi/index.d.ts
index 0531adfe..5d537f7f 100644
--- a/examples/napi/index.d.ts
+++ b/examples/napi/index.d.ts
@@ -52,6 +52,7 @@ export interface Obj {
 export function either4(input: string | number | boolean | Obj): number
 export function receiveClassOrNumber(either: number | JsClassForEither): number
 export function receiveMutClassOrNumber(either: number | JsClassForEither): number
+export function receiveDifferentClass(either: JsClassForEither | AnotherClassForEither): number
 export function returnEitherClass(input: number): number | JsClassForEither
 export function eitherFromOption(): JsClassForEither | undefined
 /** default enum values are continuos i32s start from 0 */
@@ -266,6 +267,9 @@ export class ClassWithFactory {
 export class JsClassForEither {
   constructor()
 }
+export class AnotherClassForEither {
+  constructor()
+}
 export class Fib {
   [Symbol.iterator](): Iterator<number, void, number>
   constructor()
diff --git a/examples/napi/src/either.rs b/examples/napi/src/either.rs
index 0f5fa8cd..d5c3498d 100644
--- a/examples/napi/src/either.rs
+++ b/examples/napi/src/either.rs
@@ -67,6 +67,17 @@ impl JsClassForEither {
   }
 }
 
+#[napi]
+struct AnotherClassForEither {}
+
+#[napi]
+impl AnotherClassForEither {
+  #[napi(constructor)]
+  pub fn new() -> Self {
+    Self {}
+  }
+}
+
 #[napi]
 fn receive_class_or_number(either: Either<u32, &JsClassForEither>) -> u32 {
   match either {
@@ -83,6 +94,14 @@ fn receive_mut_class_or_number(either: Either<u32, &mut JsClassForEither>) -> u3
   }
 }
 
+#[napi]
+fn receive_different_class(either: Either<&JsClassForEither, &AnotherClassForEither>) -> u32 {
+  match either {
+    Either::A(_) => 42,
+    Either::B(_) => 100,
+  }
+}
+
 #[napi]
 fn return_either_class(input: i32) -> Either<u32, JsClassForEither> {
   if input > 0 {