diff --git a/inputs/failing/operators/in.ts b/inputs/failing/operators/in.ts
new file mode 100644
index 0000000..edc5e31
--- /dev/null
+++ b/inputs/failing/operators/in.ts
@@ -0,0 +1,18 @@
+//! test_output([true,false,false,true,true,true,false,true])
+
+export default function () {
+  return [
+    "foo" in { foo: "bar" },
+    "bar" in { foo: "bar" },
+    "foo" in ["foo"],
+    0 in ["foo"],
+    "0" in ["foo"],
+    "foo" in new C(),
+    "forEach" in [],
+    "map" in [],
+  ];
+}
+
+class C {
+  foo() {}
+}
diff --git a/valuescript_vm/src/builtins/builtin_object.rs b/valuescript_vm/src/builtins/builtin_object.rs
index 27cda56..f4d7c5e 100644
--- a/valuescript_vm/src/builtins/builtin_object.rs
+++ b/valuescript_vm/src/builtins/builtin_object.rs
@@ -70,6 +70,13 @@ where
     Ok(Self::bo_sub(&key.to_string()))
   }
 
+  fn has(&self, key: &Val) -> Option<bool> {
+    match Self::bo_sub(&key.to_string()) {
+      Val::Undefined => Some(false),
+      _ => Some(true),
+    }
+  }
+
   fn submov(&mut self, _key: &Val, _value: Val) -> Result<(), Val> {
     Err(format!("Cannot assign to subscript of {} builtin", Self::bo_name()).to_type_error())
   }
diff --git a/valuescript_vm/src/generator.rs b/valuescript_vm/src/generator.rs
index 70716e7..e1e554f 100644
--- a/valuescript_vm/src/generator.rs
+++ b/valuescript_vm/src/generator.rs
@@ -92,6 +92,18 @@ impl ValTrait for Generator {
     Ok(Val::Undefined)
   }
 
+  fn has(&self, key: &Val) -> Option<bool> {
+    if key.to_string() == "next" {
+      return Some(true);
+    }
+
+    if let Val::Symbol(VsSymbol::ITERATOR) = key {
+      return Some(true);
+    }
+
+    Some(false)
+  }
+
   fn submov(&mut self, _key: &Val, _value: Val) -> Result<(), Val> {
     Err("Cannot assign to subscript of a generator".to_type_error())
   }
diff --git a/valuescript_vm/src/iteration/array_entries_iterator.rs b/valuescript_vm/src/iteration/array_entries_iterator.rs
index afa4951..e0fde88 100644
--- a/valuescript_vm/src/iteration/array_entries_iterator.rs
+++ b/valuescript_vm/src/iteration/array_entries_iterator.rs
@@ -12,7 +12,9 @@ use crate::{
   LoadFunctionResult, ValTrait,
 };
 
-use super::{iteration_result::IterationResult, return_this::RETURN_THIS};
+use super::{
+  iteration_result::IterationResult, iterator_has::iterator_has, return_this::RETURN_THIS,
+};
 
 #[derive(Clone)]
 pub struct ArrayEntriesIterator {
@@ -87,6 +89,10 @@ impl ValTrait for ArrayEntriesIterator {
     Ok(Val::Undefined)
   }
 
+  fn has(&self, key: &Val) -> Option<bool> {
+    iterator_has(key)
+  }
+
   fn submov(&mut self, _key: &Val, _value: Val) -> Result<(), Val> {
     Err("Cannot assign to subscript of array iterator".to_type_error())
   }
diff --git a/valuescript_vm/src/iteration/array_iterator.rs b/valuescript_vm/src/iteration/array_iterator.rs
index edac182..62c5aee 100644
--- a/valuescript_vm/src/iteration/array_iterator.rs
+++ b/valuescript_vm/src/iteration/array_iterator.rs
@@ -12,7 +12,9 @@ use crate::{
   LoadFunctionResult, ValTrait,
 };
 
-use super::{iteration_result::IterationResult, return_this::RETURN_THIS};
+use super::{
+  iteration_result::IterationResult, iterator_has::iterator_has, return_this::RETURN_THIS,
+};
 
 #[derive(Clone)]
 pub struct ArrayIterator {
@@ -87,6 +89,10 @@ impl ValTrait for ArrayIterator {
     Ok(Val::Undefined)
   }
 
+  fn has(&self, key: &Val) -> Option<bool> {
+    iterator_has(key)
+  }
+
   fn submov(&mut self, _key: &Val, _value: Val) -> Result<(), Val> {
     Err("Cannot assign to subscript of array iterator".to_type_error())
   }
diff --git a/valuescript_vm/src/iteration/iteration_result.rs b/valuescript_vm/src/iteration/iteration_result.rs
index a4e807c..c97d337 100644
--- a/valuescript_vm/src/iteration/iteration_result.rs
+++ b/valuescript_vm/src/iteration/iteration_result.rs
@@ -69,6 +69,13 @@ impl ValTrait for IterationResult {
     })
   }
 
+  fn has(&self, key: &Val) -> Option<bool> {
+    Some(match key.to_string().as_str() {
+      "value" | "done" => true,
+      _ => false,
+    })
+  }
+
   fn submov(&mut self, _key: &Val, _value: Val) -> Result<(), Val> {
     Err("Cannot assign to subscript of iteration result".to_type_error())
   }
diff --git a/valuescript_vm/src/iteration/iterator_has.rs b/valuescript_vm/src/iteration/iterator_has.rs
new file mode 100644
index 0000000..e2bbc02
--- /dev/null
+++ b/valuescript_vm/src/iteration/iterator_has.rs
@@ -0,0 +1,17 @@
+use crate::{vs_value::Val, VsSymbol};
+
+pub fn iterator_has(key: &Val) -> Option<bool> {
+  if key.to_string() == "next" {
+    return Some(true);
+  }
+
+  if let Val::Symbol(key) = key {
+    match key {
+      VsSymbol::ITERATOR => {
+        return Some(true);
+      }
+    }
+  }
+
+  Some(false)
+}
diff --git a/valuescript_vm/src/iteration/mod.rs b/valuescript_vm/src/iteration/mod.rs
index 4ee9075..ff0fa73 100644
--- a/valuescript_vm/src/iteration/mod.rs
+++ b/valuescript_vm/src/iteration/mod.rs
@@ -1,5 +1,6 @@
 pub mod array_entries_iterator;
 pub mod array_iterator;
 pub mod iteration_result;
+mod iterator_has;
 pub mod return_this;
 pub mod string_iterator;
diff --git a/valuescript_vm/src/iteration/string_iterator.rs b/valuescript_vm/src/iteration/string_iterator.rs
index 7843767..8c347b1 100644
--- a/valuescript_vm/src/iteration/string_iterator.rs
+++ b/valuescript_vm/src/iteration/string_iterator.rs
@@ -12,7 +12,9 @@ use crate::{
   LoadFunctionResult, ValTrait,
 };
 
-use super::{iteration_result::IterationResult, return_this::RETURN_THIS};
+use super::{
+  iteration_result::IterationResult, iterator_has::iterator_has, return_this::RETURN_THIS,
+};
 
 #[derive(Clone)]
 pub struct StringIterator {
@@ -123,6 +125,10 @@ impl ValTrait for StringIterator {
     Ok(Val::Undefined)
   }
 
+  fn has(&self, key: &Val) -> Option<bool> {
+    iterator_has(key)
+  }
+
   fn submov(&mut self, _key: &Val, _value: Val) -> Result<(), Val> {
     Err("Cannot assign to subscript of string iterator".to_type_error())
   }
diff --git a/valuescript_vm/src/native_frame_function.rs b/valuescript_vm/src/native_frame_function.rs
index 9e82df5..e2d180e 100644
--- a/valuescript_vm/src/native_frame_function.rs
+++ b/valuescript_vm/src/native_frame_function.rs
@@ -63,6 +63,10 @@ impl ValTrait for NativeFrameFunction {
     Err("TODO: Subscript native function".to_internal_error())
   }
 
+  fn has(&self, _key: &Val) -> Option<bool> {
+    Some(false)
+  }
+
   fn submov(&mut self, _key: &Val, _value: Val) -> Result<(), Val> {
     Err("Cannot assign to subscript of native function".to_type_error())
   }
diff --git a/valuescript_vm/src/native_function.rs b/valuescript_vm/src/native_function.rs
index 96b3808..2684804 100644
--- a/valuescript_vm/src/native_function.rs
+++ b/valuescript_vm/src/native_function.rs
@@ -84,6 +84,10 @@ impl ValTrait for NativeFunction {
     Err("TODO: Subscript native function".to_internal_error())
   }
 
+  fn has(&self, _key: &Val) -> Option<bool> {
+    Some(false)
+  }
+
   fn submov(&mut self, _key: &Val, _value: Val) -> Result<(), Val> {
     Err("Cannot assign to subscript of native function".to_type_error())
   }
diff --git a/valuescript_vm/src/operations.rs b/valuescript_vm/src/operations.rs
index c5e3ffb..01af1f6 100644
--- a/valuescript_vm/src/operations.rs
+++ b/valuescript_vm/src/operations.rs
@@ -1,5 +1,4 @@
 use std::collections::BTreeMap;
-use std::mem::take;
 use std::rc::Rc;
 use std::str::FromStr;
 
@@ -523,8 +522,11 @@ pub fn op_instance_of(_left: &Val, _right: &Val) -> Result<Val, Val> {
   Err("TODO: op_instance_of".to_internal_error())
 }
 
-pub fn op_in(_left: &Val, _right: &Val) -> Result<Val, Val> {
-  Err("TODO: op_in".to_internal_error())
+pub fn op_in(left: &Val, right: &Val) -> Result<Val, Val> {
+  match right.has(left) {
+    Some(found) => Ok(found.to_val()),
+    None => Err(format!("Can't use `in` with a {}", right.typeof_()).to_type_error()),
+  }
 }
 
 pub fn op_sub(left: &mut Val, right: &Val) -> Result<Val, Val> {
@@ -541,34 +543,7 @@ pub fn op_sub(left: &mut Val, right: &Val) -> Result<Val, Val> {
     Val::BigInt(bigint) => Ok(op_sub_bigint(bigint, right)),
     Val::Symbol(_) => Ok(Val::Undefined),
     Val::String(string_data) => Ok(op_sub_string(string_data, right)),
-    Val::Array(array_data) => {
-      let right_index = match right.to_index() {
-        None => {
-          // FIXME: Inefficient to_string() that gets duplicated
-          // when subscripting the object
-          if right.to_string() == "length" {
-            return Ok(Val::Number(array_data.elements.len() as f64));
-          }
-
-          return op_sub_array(array_data, right);
-        }
-        Some(i) => i,
-      };
-
-      if right_index >= array_data.elements.len() {
-        return Ok(Val::Undefined);
-      }
-
-      let res = match Rc::get_mut(array_data) {
-        Some(array_data) => take(&mut array_data.elements[right_index]),
-        None => array_data.elements[right_index].clone(),
-      };
-
-      return Ok(match res {
-        Val::Void => Val::Undefined,
-        _ => res,
-      });
-    }
+    Val::Array(array_data) => op_sub_array(array_data, right),
     Val::Object(object_data) => Ok(object_data.sub(right)), // TODO: move on single ref
     Val::Function(_) => Ok(Val::Undefined),
     Val::Class(class) => op_sub(&mut class.static_.clone(), right),
diff --git a/valuescript_vm/src/vs_value.rs b/valuescript_vm/src/vs_value.rs
index 0dfea73..65d9be1 100644
--- a/valuescript_vm/src/vs_value.rs
+++ b/valuescript_vm/src/vs_value.rs
@@ -1,5 +1,6 @@
 use core::fmt;
 use std::any::Any;
+use std::fmt::Display;
 use std::rc::Rc;
 use std::str::FromStr;
 
@@ -57,6 +58,24 @@ pub enum VsType {
   Class,
 }
 
+impl Display for VsType {
+  fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+    match self {
+      VsType::Undefined => f.write_str("undefined"),
+      VsType::Null => f.write_str("null"),
+      VsType::Bool => f.write_str("bool"),
+      VsType::Number => f.write_str("number"),
+      VsType::BigInt => f.write_str("bigint"),
+      VsType::Symbol => f.write_str("symbol"),
+      VsType::String => f.write_str("string"),
+      VsType::Array => f.write_str("array"),
+      VsType::Object => f.write_str("object"),
+      VsType::Function => f.write_str("function"),
+      VsType::Class => f.write_str("class"),
+    }
+  }
+}
+
 pub enum LoadFunctionResult {
   NotAFunction,
   StackFrame(StackFrame),
@@ -80,6 +99,7 @@ pub trait ValTrait: fmt::Display {
   fn load_function(&self) -> LoadFunctionResult;
 
   fn sub(&self, key: &Val) -> Result<Val, Val>;
+  fn has(&self, key: &Val) -> Option<bool>;
   fn submov(&mut self, key: &Val, value: Val) -> Result<(), Val>;
 
   fn pretty_fmt(&self, f: &mut std::fmt::Formatter<'_>) -> std::fmt::Result;
@@ -355,6 +375,97 @@ impl ValTrait for Val {
     op_sub(&mut self.clone(), key)
   }
 
+  fn has(&self, key: &Val) -> Option<bool> {
+    match self {
+      Val::Void
+      | Val::Undefined
+      | Val::Null
+      | Val::Bool(_)
+      | Val::Number(_)
+      | Val::BigInt(_)
+      | Val::Symbol(_)
+      | Val::String(_) => None,
+
+      Val::Array(array) => {
+        let index = match key.to_index() {
+          None => {
+            return Some(match key.to_string().as_str() {
+              "at" => true,
+              "concat" => true,
+              "copyWithin" => true,
+              "entries" => true,
+              "every" => true,
+              "fill" => true,
+              "filter" => true,
+              "find" => true,
+              "findIndex" => true,
+              "flat" => true,
+              "flatMap" => true,
+              "includes" => true,
+              "indexOf" => true,
+              "join" => true,
+              "keys" => true,
+              "lastIndexOf" => true,
+              "length" => true,
+              "map" => true,
+              "pop" => true,
+              "push" => true,
+              "reduce" => true,
+              "reduceRight" => true,
+              "reverse" => true,
+              "shift" => true,
+              "slice" => true,
+              "some" => true,
+              "sort" => true,
+              "splice" => true,
+              "toLocaleString" => true,
+              "toString" => true,
+              "unshift" => true,
+              "values" => true,
+
+              _ => false,
+            });
+          }
+          Some(i) => i,
+        };
+
+        return Some(index < array.elements.len());
+      }
+      Val::Object(object) => match key {
+        Val::Symbol(symbol) => {
+          if object.symbol_map.contains_key(symbol) {
+            return Some(true);
+          }
+
+          if let Some(proto) = &object.prototype {
+            return proto.has(key);
+          }
+
+          return Some(false);
+        }
+        _ => {
+          if object.string_map.contains_key(&key.to_string()) {
+            return Some(true);
+          }
+
+          if let Some(proto) = &object.prototype {
+            return proto.has(key);
+          }
+
+          return Some(false);
+        }
+      },
+      Val::Function(_) => Some(false),
+      Val::Class(class) => class.static_.has(key),
+      Val::Static(static_) => static_.has(key),
+      Val::Dynamic(dynamic) => dynamic.has(key),
+      Val::CopyCounter(_) => Some(match key.to_string().as_str() {
+        "tag" | "count" => true,
+        _ => false,
+      }),
+    }
+  }
+
   fn submov(&mut self, key: &Val, value: Val) -> Result<(), Val> {
     op_submov(self, key, value)
   }