Error handling in environment (#659)

* Map ErrorKind to JS Errors

* Fix fmt

* Modify code with suggestions

* rebase and fix compilation errors

* Remove redundant clone

* Remove duplicate file

* cargo fmt

boa/src/builtins/function/mod.rs

@@ -124,12 +124,14 @@ impl Function {
         // Create binding
         local_env
             .borrow_mut()
-            .create_mutable_binding(param.name().to_owned(), false);
+            .create_mutable_binding(param.name().to_owned(), false)
+            .expect("Failed to create binding for rest param");
 
         // Set Binding to value
         local_env
             .borrow_mut()
-            .initialize_binding(param.name(), array);
+            .initialize_binding(param.name(), array)
+            .expect("Failed to initialize rest param");
     }
 
     // Adds an argument to the environment
@@ -142,12 +144,14 @@ impl Function {
         // Create binding
         local_env
             .borrow_mut()
-            .create_mutable_binding(param.name().to_owned(), false);
+            .create_mutable_binding(param.name().to_owned(), false)
+            .expect("Failed to create binding");
 
         // Set Binding to value
         local_env
             .borrow_mut()
-            .initialize_binding(param.name(), value);
+            .initialize_binding(param.name(), value)
+            .expect("Failed to intialize binding");
     }
 
     /// Returns true if the function object is callable.

boa/src/context.rs

@@ -342,7 +342,7 @@ impl Context {
     #[inline]
     pub fn construct_range_error<M>(&mut self, message: M) -> Value
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         // Runs a `new RangeError(message)`.
         New::from(Call::new(
@@ -357,7 +357,7 @@ impl Context {
     #[inline]
     pub fn throw_range_error<M>(&mut self, message: M) -> Result<Value>
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         Err(self.construct_range_error(message))
     }
@@ -366,7 +366,7 @@ impl Context {
     #[inline]
     pub fn construct_type_error<M>(&mut self, message: M) -> Value
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         // Runs a `new TypeError(message)`.
         New::from(Call::new(
@@ -381,7 +381,7 @@ impl Context {
     #[inline]
     pub fn throw_type_error<M>(&mut self, message: M) -> Result<Value>
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         Err(self.construct_type_error(message))
     }
@@ -390,11 +390,11 @@ impl Context {
     #[inline]
     pub fn construct_reference_error<M>(&mut self, message: M) -> Value
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         New::from(Call::new(
             Identifier::from("ReferenceError"),
-            vec![Const::from(message.into() + " is not defined").into()],
+            vec![Const::from(message.into()).into()],
         ))
         .run(self)
         .expect("Into<String> used as message")
@@ -404,7 +404,7 @@ impl Context {
     #[inline]
     pub fn throw_reference_error<M>(&mut self, message: M) -> Result<Value>
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         Err(self.construct_reference_error(message))
     }
@@ -413,7 +413,7 @@ impl Context {
     #[inline]
     pub fn construct_syntax_error<M>(&mut self, message: M) -> Value
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         New::from(Call::new(
             Identifier::from("SyntaxError"),
@@ -427,7 +427,7 @@ impl Context {
     #[inline]
     pub fn throw_syntax_error<M>(&mut self, message: M) -> Result<Value>
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         Err(self.construct_syntax_error(message))
     }
@@ -435,7 +435,7 @@ impl Context {
     /// Constructs a `EvalError` with the specified message.
     pub fn construct_eval_error<M>(&mut self, message: M) -> Value
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         New::from(Call::new(
             Identifier::from("EvalError"),
@@ -448,7 +448,7 @@ impl Context {
     /// Constructs a `URIError` with the specified message.
     pub fn construct_uri_error<M>(&mut self, message: M) -> Value
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         New::from(Call::new(
             Identifier::from("URIError"),
@@ -461,7 +461,7 @@ impl Context {
     /// Throws a `EvalError` with the specified message.
     pub fn throw_eval_error<M>(&mut self, message: M) -> Result<Value>
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         Err(self.construct_eval_error(message))
     }
@@ -469,7 +469,7 @@ impl Context {
     /// Throws a `URIError` with the specified message.
     pub fn throw_uri_error<M>(&mut self, message: M) -> Result<Value>
     where
-        M: Into<String>,
+        M: Into<Box<str>>,
     {
         Err(self.construct_uri_error(message))
     }
@@ -619,7 +619,8 @@ impl Context {
             Node::Identifier(ref name) => {
                 self.realm
                     .environment
-                    .set_mutable_binding(name.as_ref(), value.clone(), true);
+                    .set_mutable_binding(name.as_ref(), value.clone(), true)
+                    .map_err(|e| e.to_error(self))?;
                 Ok(value)
             }
             Node::GetConstField(ref get_const_field_node) => Ok(get_const_field_node

boa/src/environment/declarative_environment_record.rs

@@ -5,6 +5,7 @@
 //! A declarative Environment Record binds the set of identifiers defined by the declarations contained within its scope.
 //! More info:  [ECMA-262 sec-declarative-environment-records](https://tc39.es/ecma262/#sec-declarative-environment-records)
 
+use super::ErrorKind;
 use crate::{
     environment::{
         environment_record_trait::EnvironmentRecordTrait,
@@ -41,11 +42,12 @@ impl EnvironmentRecordTrait for DeclarativeEnvironmentRecord {
         self.env_rec.contains_key(name)
     }
 
-    fn create_mutable_binding(&mut self, name: String, deletion: bool) {
+    fn create_mutable_binding(&mut self, name: String, deletion: bool) -> Result<(), ErrorKind> {
-        if self.env_rec.contains_key(&name) {
+        assert!(
-            // TODO: change this when error handling comes into play
+            !self.env_rec.contains_key(&name),
-            panic!("Identifier {} has already been declared", name);
+            "Identifier {} has already been declared",
-        }
+            name
+        );
 
         self.env_rec.insert(
             name,
@@ -56,13 +58,15 @@ impl EnvironmentRecordTrait for DeclarativeEnvironmentRecord {
                 strict: false,
             },
         );
+        Ok(())
     }
 
-    fn create_immutable_binding(&mut self, name: String, strict: bool) -> bool {
+    fn create_immutable_binding(&mut self, name: String, strict: bool) -> Result<(), ErrorKind> {
-        if self.env_rec.contains_key(&name) {
+        assert!(
-            // TODO: change this when error handling comes into play
+            !self.env_rec.contains_key(&name),
-            panic!("Identifier {} has already been declared", name);
+            "Identifier {} has already been declared",
-        }
+            name
+        );
 
         self.env_rec.insert(
             name,
@@ -73,32 +77,37 @@ impl EnvironmentRecordTrait for DeclarativeEnvironmentRecord {
                 strict,
             },
         );
-
+        Ok(())
-        true
     }
 
-    fn initialize_binding(&mut self, name: &str, value: Value) {
+    fn initialize_binding(&mut self, name: &str, value: Value) -> Result<(), ErrorKind> {
         if let Some(ref mut record) = self.env_rec.get_mut(name) {
             if record.value.is_none() {
                 record.value = Some(value);
-            } else {
+                return Ok(());
-                // TODO: change this when error handling comes into play
-                panic!("Identifier {} has already been defined", name);
             }
         }
+        panic!("record must have binding for {}", name);
     }
 
     #[allow(clippy::else_if_without_else)]
-    fn set_mutable_binding(&mut self, name: &str, value: Value, mut strict: bool) {
+    fn set_mutable_binding(
+        &mut self,
+        name: &str,
+        value: Value,
+        mut strict: bool,
+    ) -> Result<(), ErrorKind> {
         if self.env_rec.get(name).is_none() {
             if strict {
-                // TODO: change this when error handling comes into play
+                return Err(ErrorKind::new_reference_error(format!(
-                panic!("Reference Error: Cannot set mutable binding for {}", name);
+                    "{} not found",
+                    name
+                )));
             }
 
-            self.create_mutable_binding(name.to_owned(), true);
+            self.create_mutable_binding(name.to_owned(), true)?;
-            self.initialize_binding(name, value);
+            self.initialize_binding(name, value)?;
-            return;
+            return Ok(());
         }
 
         let record: &mut DeclarativeEnvironmentRecordBinding = self.env_rec.get_mut(name).unwrap();
@@ -106,28 +115,35 @@ impl EnvironmentRecordTrait for DeclarativeEnvironmentRecord {
             strict = true
         }
         if record.value.is_none() {
-            // TODO: change this when error handling comes into play
+            return Err(ErrorKind::new_reference_error(format!(
-            panic!("Reference Error: Cannot set mutable binding for {}", name);
+                "{} has not been initialized",
+                name
+            )));
         }
-
         if record.mutable {
             record.value = Some(value);
         } else if strict {
-            // TODO: change this when error handling comes into play
+            return Err(ErrorKind::new_type_error(format!(
-            panic!("TypeError: Cannot mutate an immutable binding {}", name);
+                "Cannot mutate an immutable binding {}",
+                name
+            )));
         }
+
+        Ok(())
     }
 
-    fn get_binding_value(&self, name: &str, _strict: bool) -> Value {
+    fn get_binding_value(&self, name: &str, _strict: bool) -> Result<Value, ErrorKind> {
         if let Some(binding) = self.env_rec.get(name) {
-            binding
+            if let Some(ref val) = binding.value {
-                .value
+                Ok(val.clone())
-                .as_ref()
-                .expect("Could not get record as reference")
-                .clone()
             } else {
-            // TODO: change this when error handling comes into play
+                Err(ErrorKind::new_reference_error(format!(
-            panic!("ReferenceError: Cannot get binding value for {}", name);
+                    "{} is an uninitialized binding",
+                    name
+                )))
+            }
+        } else {
+            panic!("Cannot get binding value for {}", name);
         }
     }
 
@@ -141,7 +157,7 @@ impl EnvironmentRecordTrait for DeclarativeEnvironmentRecord {
                     false
                 }
             }
-            None => false,
+            None => panic!("env_rec has no binding for {}", name),
         }
     }
 
@@ -149,8 +165,8 @@ impl EnvironmentRecordTrait for DeclarativeEnvironmentRecord {
         false
     }
 
-    fn get_this_binding(&self) -> Value {
+    fn get_this_binding(&self) -> Result<Value, ErrorKind> {
-        Value::undefined()
+        Ok(Value::undefined())
     }
 
     fn has_super_binding(&self) -> bool {

boa/src/environment/environment_record_trait.rs

@@ -8,6 +8,7 @@
 //!
 //! There are 5 Environment record kinds. They all have methods in common, these are implemented as a the `EnvironmentRecordTrait`
 //!
+use super::ErrorKind;
 use crate::{
     environment::lexical_environment::{Environment, EnvironmentType},
     gc::{Finalize, Trace},
@@ -25,30 +26,35 @@ pub trait EnvironmentRecordTrait: Debug + Trace + Finalize {
 
     /// Create a new but uninitialized mutable binding in an Environment Record. The String value N is the text of the bound name.
     /// If the Boolean argument deletion is true the binding may be subsequently deleted.
-    fn create_mutable_binding(&mut self, name: String, deletion: bool);
+    fn create_mutable_binding(&mut self, name: String, deletion: bool) -> Result<(), ErrorKind>;
 
     /// Create a new but uninitialized immutable binding in an Environment Record.
     /// The String value N is the text of the bound name.
     /// If strict is true then attempts to set it after it has been initialized will always throw an exception,
     /// regardless of the strict mode setting of operations that reference that binding.
-    fn create_immutable_binding(&mut self, name: String, strict: bool) -> bool;
+    fn create_immutable_binding(&mut self, name: String, strict: bool) -> Result<(), ErrorKind>;
 
     /// Set the value of an already existing but uninitialized binding in an Environment Record.
     /// The String value N is the text of the bound name.
     /// V is the value for the binding and is a value of any ECMAScript language type.
-    fn initialize_binding(&mut self, name: &str, value: Value);
+    fn initialize_binding(&mut self, name: &str, value: Value) -> Result<(), ErrorKind>;
 
     /// Set the value of an already existing mutable binding in an Environment Record.
     /// The String value `name` is the text of the bound name.
     /// value is the `value` for the binding and may be a value of any ECMAScript language type. S is a Boolean flag.
     /// If `strict` is true and the binding cannot be set throw a TypeError exception.
-    fn set_mutable_binding(&mut self, name: &str, value: Value, strict: bool);
+    fn set_mutable_binding(
+        &mut self,
+        name: &str,
+        value: Value,
+        strict: bool,
+    ) -> Result<(), ErrorKind>;
 
     /// Returns the value of an already existing binding from an Environment Record.
     /// The String value N is the text of the bound name.
     /// S is used to identify references originating in strict mode code or that
     /// otherwise require strict mode reference semantics.
-    fn get_binding_value(&self, name: &str, strict: bool) -> Value;
+    fn get_binding_value(&self, name: &str, strict: bool) -> Result<Value, ErrorKind>;
 
     /// Delete a binding from an Environment Record.
     /// The String value name is the text of the bound name.
@@ -61,7 +67,7 @@ pub trait EnvironmentRecordTrait: Debug + Trace + Finalize {
     fn has_this_binding(&self) -> bool;
 
     /// Return the `this` binding from the environment
-    fn get_this_binding(&self) -> Value;
+    fn get_this_binding(&self) -> Result<Value, ErrorKind>;
 
     /// Determine if an Environment Record establishes a super method binding.
     /// Return true if it does and false if it does not.

boa/src/environment/function_environment_record.rs

@@ -8,6 +8,7 @@
 //! from within the function.
 //! More info: <https://tc39.es/ecma262/#sec-function-environment-records>
 
+use super::ErrorKind;
 use crate::{
     environment::{
         declarative_environment_record::DeclarativeEnvironmentRecordBinding,
@@ -60,39 +61,49 @@ pub struct FunctionEnvironmentRecord {
 }
 
 impl FunctionEnvironmentRecord {
-    pub fn bind_this_value(&mut self, value: Value) {
+    pub fn bind_this_value(&mut self, value: Value) -> Result<Value, ErrorKind> {
         match self.this_binding_status {
             // You can not bind an arrow function, their `this` value comes from the lexical scope above
             BindingStatus::Lexical => {
-                // TODO: change this when error handling comes into play
                 panic!("Cannot bind to an arrow function!");
             }
             // You can not bind a function twice
-            BindingStatus::Initialized => {
+            BindingStatus::Initialized => Err(ErrorKind::new_reference_error(
-                // TODO: change this when error handling comes into play
+                "Cannot bind to an initialised function!",
-                panic!("Reference Error: Cannot bind to an initialised function!");
+            )),
-            }
 
             BindingStatus::Uninitialized => {
-                self.this_value = value;
+                self.this_value = value.clone();
                 self.this_binding_status = BindingStatus::Initialized;
+                Ok(value)
+            }
+        }
     }
+
+    pub fn get_super_base(&self) -> Value {
+        let home = &self.home_object;
+        if home.is_undefined() {
+            Value::Undefined
+        } else {
+            assert!(home.is_object());
+            home.as_object()
+                .expect("home_object must be an Object")
+                .prototype_instance()
         }
     }
 }
 
 impl EnvironmentRecordTrait for FunctionEnvironmentRecord {
-    // TODO: get_super_base can't implement until GetPrototypeof is implemented on object
-
     fn has_binding(&self, name: &str) -> bool {
         self.env_rec.contains_key(name)
     }
 
-    fn create_mutable_binding(&mut self, name: String, deletion: bool) {
+    fn create_mutable_binding(&mut self, name: String, deletion: bool) -> Result<(), ErrorKind> {
-        if self.env_rec.contains_key(&name) {
+        assert!(
-            // TODO: change this when error handling comes into play
+            !self.env_rec.contains_key(&name),
-            panic!("Identifier {} has already been declared", name);
+            "Identifier {} has already been declared",
-        }
+            name
+        );
 
         self.env_rec.insert(
             name,
@@ -103,28 +114,28 @@ impl EnvironmentRecordTrait for FunctionEnvironmentRecord {
                 strict: false,
             },
         );
+        Ok(())
     }
 
-    fn get_this_binding(&self) -> Value {
+    fn get_this_binding(&self) -> Result<Value, ErrorKind> {
         match self.this_binding_status {
             BindingStatus::Lexical => {
-                // TODO: change this when error handling comes into play
                 panic!("There is no this for a lexical function record");
             }
-            BindingStatus::Uninitialized => {
+            BindingStatus::Uninitialized => Err(ErrorKind::new_reference_error(
-                // TODO: change this when error handling comes into play
+                "Uninitialised binding for this function",
-                panic!("Reference Error: Uninitialised binding for this function");
+            )),
-            }
 
-            BindingStatus::Initialized => self.this_value.clone(),
+            BindingStatus::Initialized => Ok(self.this_value.clone()),
         }
     }
 
-    fn create_immutable_binding(&mut self, name: String, strict: bool) -> bool {
+    fn create_immutable_binding(&mut self, name: String, strict: bool) -> Result<(), ErrorKind> {
-        if self.env_rec.contains_key(&name) {
+        assert!(
-            // TODO: change this when error handling comes into play
+            !self.env_rec.contains_key(&name),
-            panic!("Identifier {} has already been declared", name);
+            "Identifier {} has already been declared",
-        }
+            name
+        );
 
         self.env_rec.insert(
             name,
@@ -135,63 +146,73 @@ impl EnvironmentRecordTrait for FunctionEnvironmentRecord {
                 strict,
             },
         );
-
+        Ok(())
-        true
     }
 
-    fn initialize_binding(&mut self, name: &str, value: Value) {
+    fn initialize_binding(&mut self, name: &str, value: Value) -> Result<(), ErrorKind> {
         if let Some(ref mut record) = self.env_rec.get_mut(name) {
-            match record.value {
+            if record.value.is_none() {
-                Some(_) => {
+                record.value = Some(value);
-                    // TODO: change this when error handling comes into play
+                return Ok(());
-                    panic!("Identifier {} has already been defined", name);
-                }
-                None => record.value = Some(value),
             }
         }
+        panic!("record must have binding for {}", name)
     }
 
     #[allow(clippy::else_if_without_else)]
-    fn set_mutable_binding(&mut self, name: &str, value: Value, mut strict: bool) {
+    fn set_mutable_binding(
+        &mut self,
+        name: &str,
+        value: Value,
+        mut strict: bool,
+    ) -> Result<(), ErrorKind> {
         if self.env_rec.get(name).is_none() {
             if strict {
-                // TODO: change this when error handling comes into play
+                return Err(ErrorKind::new_reference_error(format!(
-                panic!("Reference Error: Cannot set mutable binding for {}", name);
+                    "{} not found",
+                    name
+                )));
             }
 
-            self.create_mutable_binding(name.to_owned(), true);
+            self.create_mutable_binding(name.to_owned(), true)?;
-            self.initialize_binding(name, value);
+            self.initialize_binding(name, value)?;
-            return;
+            return Ok(());
         }
 
         let record: &mut DeclarativeEnvironmentRecordBinding = self.env_rec.get_mut(name).unwrap();
         if record.strict {
             strict = true
         }
-
         if record.value.is_none() {
-            // TODO: change this when error handling comes into play
+            return Err(ErrorKind::new_reference_error(format!(
-            panic!("Reference Error: Cannot set mutable binding for {}", name);
+                "{} has not been initialized",
+                name
+            )));
         }
-
         if record.mutable {
             record.value = Some(value);
         } else if strict {
-            // TODO: change this when error handling comes into play
+            return Err(ErrorKind::new_type_error(format!(
-            panic!("TypeError: Cannot mutate an immutable binding {}", name);
+                "Cannot mutate an immutable binding {}",
+                name
+            )));
         }
+
+        Ok(())
     }
 
-    fn get_binding_value(&self, name: &str, _strict: bool) -> Value {
+    fn get_binding_value(&self, name: &str, _strict: bool) -> Result<Value, ErrorKind> {
         if let Some(binding) = self.env_rec.get(name) {
-            binding
+            if let Some(ref val) = binding.value {
-                .value
+                Ok(val.clone())
-                .as_ref()
+            } else {
-                .expect("Could not get record as reference")
+                Err(ErrorKind::new_reference_error(format!(
-                .clone()
+                    "{} is an uninitialized binding",
+                    name
+                )))
+            }
         } else {
-            // TODO: change this when error handling comes into play
+            panic!("Cannot get binding value for {}", name);
-            panic!("ReferenceError: Cannot get binding value for {}", name);
         }
     }
 
@@ -205,7 +226,7 @@ impl EnvironmentRecordTrait for FunctionEnvironmentRecord {
                     false
                 }
             }
-            None => false,
+            None => panic!("env_rec has no binding for {}", name),
         }
     }
 

boa/src/environment/global_environment_record.rs

@@ -7,6 +7,7 @@
 //! that occur within a Script.
 //! More info:  <https://tc39.es/ecma262/#sec-global-environment-records>
 
+use super::ErrorKind;
 use crate::{
     environment::{
         declarative_environment_record::DeclarativeEnvironmentRecord,
@@ -51,20 +52,49 @@ impl GlobalEnvironmentRecord {
         }
     }
 
-    pub fn create_global_var_binding(&mut self, name: String, deletion: bool) {
+    pub fn can_declare_global_var(&self, name: &str) -> bool {
+        let global_object = &self.object_record.bindings;
+        if global_object.has_field(name) {
+            true
+        } else {
+            global_object.is_extensible()
+        }
+    }
+
+    pub fn can_declare_global_function(&self, name: &str) -> bool {
+        let global_object = &self.object_record.bindings;
+        let existing_prop = global_object.get_property(name);
+        match existing_prop {
+            Some(prop) => {
+                if prop.configurable() {
+                    true
+                } else {
+                    prop.is_data_descriptor() && prop.attributes().writable() && prop.enumerable()
+                }
+            }
+            None => global_object.is_extensible(),
+        }
+    }
+
+    pub fn create_global_var_binding(
+        &mut self,
+        name: String,
+        deletion: bool,
+    ) -> Result<(), ErrorKind> {
         let obj_rec = &mut self.object_record;
         let global_object = &obj_rec.bindings;
         let has_property = global_object.has_field(name.as_str());
         let extensible = global_object.is_extensible();
         if !has_property && extensible {
-            obj_rec.create_mutable_binding(name.clone(), deletion);
+            obj_rec.create_mutable_binding(name.clone(), deletion)?;
-            obj_rec.initialize_binding(&name, Value::undefined());
+            obj_rec.initialize_binding(&name, Value::undefined())?;
         }
 
         let var_declared_names = &mut self.var_names;
         if !var_declared_names.contains(&name) {
             var_declared_names.insert(name);
         }
+        Ok(())
     }
 
     pub fn create_global_function_binding(&mut self, name: &str, value: Value, deletion: bool) {
@@ -90,8 +120,8 @@ impl GlobalEnvironmentRecord {
 }
 
 impl EnvironmentRecordTrait for GlobalEnvironmentRecord {
-    fn get_this_binding(&self) -> Value {
+    fn get_this_binding(&self) -> Result<Value, ErrorKind> {
-        self.global_this_binding.clone()
+        Ok(self.global_this_binding.clone())
     }
 
     fn has_binding(&self, name: &str) -> bool {
@@ -101,36 +131,48 @@ impl EnvironmentRecordTrait for GlobalEnvironmentRecord {
         self.object_record.has_binding(name)
     }
 
-    fn create_mutable_binding(&mut self, name: String, deletion: bool) {
+    fn create_mutable_binding(&mut self, name: String, deletion: bool) -> Result<(), ErrorKind> {
         if self.declarative_record.has_binding(&name) {
-            // TODO: change to exception
+            return Err(ErrorKind::new_type_error(format!(
-            panic!("Binding already exists!");
+                "Binding already exists for {}",
+                name
+            )));
         }
 
         self.declarative_record
             .create_mutable_binding(name, deletion)
     }
 
-    fn create_immutable_binding(&mut self, name: String, strict: bool) -> bool {
+    fn create_immutable_binding(&mut self, name: String, strict: bool) -> Result<(), ErrorKind> {
         if self.declarative_record.has_binding(&name) {
-            // TODO: change to exception
+            return Err(ErrorKind::new_type_error(format!(
-            panic!("Binding already exists!");
+                "Binding already exists for {}",
+                name
+            )));
         }
 
         self.declarative_record
             .create_immutable_binding(name, strict)
     }
 
-    fn initialize_binding(&mut self, name: &str, value: Value) {
+    fn initialize_binding(&mut self, name: &str, value: Value) -> Result<(), ErrorKind> {
         if self.declarative_record.has_binding(&name) {
-            // TODO: assert binding is in the object environment record
             return self.declarative_record.initialize_binding(name, value);
         }
 
-        panic!("Should not initialized binding without creating first.");
+        assert!(
+            self.object_record.has_binding(name),
+            "Binding must be in object_record"
+        );
+        self.object_record.initialize_binding(name, value)
     }
 
-    fn set_mutable_binding(&mut self, name: &str, value: Value, strict: bool) {
+    fn set_mutable_binding(
+        &mut self,
+        name: &str,
+        value: Value,
+        strict: bool,
+    ) -> Result<(), ErrorKind> {
         if self.declarative_record.has_binding(&name) {
             return self
                 .declarative_record
@@ -139,7 +181,7 @@ impl EnvironmentRecordTrait for GlobalEnvironmentRecord {
         self.object_record.set_mutable_binding(name, value, strict)
     }
 
-    fn get_binding_value(&self, name: &str, strict: bool) -> Value {
+    fn get_binding_value(&self, name: &str, strict: bool) -> Result<Value, ErrorKind> {
         if self.declarative_record.has_binding(&name) {
             return self.declarative_record.get_binding_value(name, strict);
         }

boa/src/environment/lexical_environment.rs

@@ -5,6 +5,7 @@
 //! The following operations are used to operate upon lexical environments
 //! This is the entrypoint to lexical environments.
 
+use super::ErrorKind;
 use crate::{
     environment::{
         declarative_environment_record::DeclarativeEnvironmentRecord,
@@ -113,14 +114,19 @@ impl LexicalEnvironment {
             .get_global_object()
     }
 
-    pub fn get_this_binding(&self) -> Value {
+    pub fn get_this_binding(&self) -> Result<Value, ErrorKind> {
         self.environments()
             .find(|env| env.borrow().has_this_binding())
             .map(|env| env.borrow().get_this_binding())
-            .unwrap_or_else(Value::undefined)
+            .unwrap_or_else(|| Ok(Value::Undefined))
     }
 
-    pub fn create_mutable_binding(&mut self, name: String, deletion: bool, scope: VariableScope) {
+    pub fn create_mutable_binding(
+        &mut self,
+        name: String,
+        deletion: bool,
+        scope: VariableScope,
+    ) -> Result<(), ErrorKind> {
         match scope {
             VariableScope::Block => self
                 .get_current_environment()
@@ -138,7 +144,7 @@ impl LexicalEnvironment {
                     })
                     .expect("No function or global environment");
 
-                env.borrow_mut().create_mutable_binding(name, deletion);
+                env.borrow_mut().create_mutable_binding(name, deletion)
             }
         }
     }
@@ -148,7 +154,7 @@ impl LexicalEnvironment {
         name: String,
         deletion: bool,
         scope: VariableScope,
-    ) -> bool {
+    ) -> Result<(), ErrorKind> {
         match scope {
             VariableScope::Block => self
                 .get_current_environment()
@@ -171,24 +177,43 @@ impl LexicalEnvironment {
         }
     }
 
-    pub fn set_mutable_binding(&mut self, name: &str, value: Value, strict: bool) {
+    pub fn set_mutable_binding(
+        &mut self,
+        name: &str,
+        value: Value,
+        strict: bool,
+    ) -> Result<(), ErrorKind> {
         // Find the first environment which has the given binding
         let env = self
             .environments()
-            .find(|env| env.borrow().has_binding(name))
+            .find(|env| env.borrow().has_binding(name));
-            .expect("Binding does not exists"); // TODO graceful error handling
 
-        env.borrow_mut().set_mutable_binding(name, value, strict);
+        let env = if let Some(env) = env {
+            env
+        } else {
+            // global_env doesn't need has_binding to be satisfied in non strict mode
+            self.environment_stack
+                .get(0)
+                .expect("Environment stack underflow")
+        };
+        env.borrow_mut().set_mutable_binding(name, value, strict)
     }
 
-    pub fn initialize_binding(&mut self, name: &str, value: Value) {
+    pub fn initialize_binding(&mut self, name: &str, value: Value) -> Result<(), ErrorKind> {
         // Find the first environment which has the given binding
         let env = self
             .environments()
-            .find(|env| env.borrow().has_binding(name))
+            .find(|env| env.borrow().has_binding(name));
-            .expect("Binding does not exists"); // TODO graceful error handling
 
-        env.borrow_mut().initialize_binding(name, value);
+        let env = if let Some(env) = env {
+            env
+        } else {
+            // global_env doesn't need has_binding to be satisfied in non strict mode
+            self.environment_stack
+                .get(0)
+                .expect("Environment stack underflow")
+        };
+        env.borrow_mut().initialize_binding(name, value)
     }
 
     /// get_current_environment_ref is used when you only need to borrow the environment
@@ -212,10 +237,16 @@ impl LexicalEnvironment {
             .any(|env| env.borrow().has_binding(name))
     }
 
-    pub fn get_binding_value(&self, name: &str) -> Option<Value> {
+    pub fn get_binding_value(&self, name: &str) -> Result<Value, ErrorKind> {
         self.environments()
             .find(|env| env.borrow().has_binding(name))
             .map(|env| env.borrow().get_binding_value(name, false))
+            .unwrap_or_else(|| {
+                Err(ErrorKind::new_reference_error(format!(
+                    "{} is not defined",
+                    name
+                )))
+            })
     }
 }
 
@@ -246,7 +277,7 @@ pub fn new_function_environment(
     };
     // If a `this` value has been passed, bind it to the environment
     if let Some(v) = this {
-        func_env.bind_this_value(v);
+        func_env.bind_this_value(v).unwrap();
     }
     Gc::new(GcCell::new(Box::new(func_env)))
 }

boa/src/environment/mod.rs

@@ -6,3 +6,35 @@ pub mod function_environment_record;
 pub mod global_environment_record;
 pub mod lexical_environment;
 pub mod object_environment_record;
+
+#[derive(Debug)]
+pub enum ErrorKind {
+    ReferenceError(Box<str>),
+    TypeError(Box<str>),
+}
+
+use crate::value::Value;
+use crate::Context;
+
+impl ErrorKind {
+    pub fn to_error(&self, ctx: &mut Context) -> Value {
+        match self {
+            ErrorKind::ReferenceError(msg) => ctx.construct_reference_error(msg.clone()),
+            ErrorKind::TypeError(msg) => ctx.construct_type_error(msg.clone()),
+        }
+    }
+
+    pub fn new_reference_error<M>(msg: M) -> Self
+    where
+        M: Into<Box<str>>,
+    {
+        Self::ReferenceError(msg.into())
+    }
+
+    pub fn new_type_error<M>(msg: M) -> Self
+    where
+        M: Into<Box<str>>,
+    {
+        Self::TypeError(msg.into())
+    }
+}

boa/src/environment/object_environment_record.rs

@@ -6,6 +6,7 @@
 //! Property keys that are not strings in the form of an `IdentifierName` are not included in the set of bound identifiers.
 //! More info:  [Object Records](https://tc39.es/ecma262/#sec-object-environment-records)
 
+use super::*;
 use crate::{
     environment::{
         environment_record_trait::EnvironmentRecordTrait,
@@ -35,7 +36,7 @@ impl EnvironmentRecordTrait for ObjectEnvironmentRecord {
         }
     }
 
-    fn create_mutable_binding(&mut self, name: String, deletion: bool) {
+    fn create_mutable_binding(&mut self, name: String, deletion: bool) -> Result<(), ErrorKind> {
         // TODO: could save time here and not bother generating a new undefined object,
         // only for it to be replace with the real value later. We could just add the name to a Vector instead
         let bindings = &mut self.bindings;
@@ -46,13 +47,14 @@ impl EnvironmentRecordTrait for ObjectEnvironmentRecord {
         prop.set_configurable(deletion);
 
         bindings.set_property(name, prop);
+        Ok(())
     }
 
-    fn create_immutable_binding(&mut self, _name: String, _strict: bool) -> bool {
+    fn create_immutable_binding(&mut self, _name: String, _strict: bool) -> Result<(), ErrorKind> {
-        true
+        Ok(())
     }
 
-    fn initialize_binding(&mut self, name: &str, value: Value) {
+    fn initialize_binding(&mut self, name: &str, value: Value) -> Result<(), ErrorKind> {
         // We should never need to check if a binding has been created,
         // As all calls to create_mutable_binding are followed by initialized binding
         // The below is just a check.
@@ -60,26 +62,32 @@ impl EnvironmentRecordTrait for ObjectEnvironmentRecord {
         self.set_mutable_binding(name, value, false)
     }
 
-    fn set_mutable_binding(&mut self, name: &str, value: Value, strict: bool) {
+    fn set_mutable_binding(
+        &mut self,
+        name: &str,
+        value: Value,
+        strict: bool,
+    ) -> Result<(), ErrorKind> {
         debug_assert!(value.is_object() || value.is_function());
-
         let mut property = DataDescriptor::new(value, Attribute::ENUMERABLE);
         property.set_configurable(strict);
         self.bindings
             .as_object()
             .expect("binding object")
             .insert(name, property);
+        Ok(())
     }
 
-    fn get_binding_value(&self, name: &str, strict: bool) -> Value {
+    fn get_binding_value(&self, name: &str, strict: bool) -> Result<Value, ErrorKind> {
         if self.bindings.has_field(name) {
-            self.bindings.get_field(name)
+            Ok(self.bindings.get_field(name))
+        } else if strict {
+            Err(ErrorKind::new_reference_error(format!(
+                "{} has no binding",
+                name
+            )))
         } else {
-            if strict {
+            Ok(Value::undefined())
-                // TODO: throw error here
-                // Error handling not implemented yet
-            }
-            Value::undefined()
         }
     }
 
@@ -92,8 +100,8 @@ impl EnvironmentRecordTrait for ObjectEnvironmentRecord {
         false
     }
 
-    fn get_this_binding(&self) -> Value {
+    fn get_this_binding(&self) -> Result<Value, ErrorKind> {
-        Value::undefined()
+        Ok(Value::undefined())
     }
 
     fn has_super_binding(&self) -> bool {

boa/src/object/gcobject.rs

@@ -162,10 +162,12 @@ impl GcObject {
                         let arguments_obj = create_unmapped_arguments_object(args);
                         local_env
                             .borrow_mut()
-                            .create_mutable_binding("arguments".to_string(), false);
+                            .create_mutable_binding("arguments".to_string(), false)
+                            .map_err(|e| e.to_error(context))?;
                         local_env
                             .borrow_mut()
-                            .initialize_binding("arguments", arguments_obj);
+                            .initialize_binding("arguments", arguments_obj)
+                            .map_err(|e| e.to_error(context))?;
 
                         context.realm_mut().environment.push(local_env);
 
@@ -257,10 +259,12 @@ impl GcObject {
                         let arguments_obj = create_unmapped_arguments_object(args);
                         local_env
                             .borrow_mut()
-                            .create_mutable_binding("arguments".to_string(), false);
+                            .create_mutable_binding("arguments".to_string(), false)
+                            .map_err(|e| e.to_error(context))?;
                         local_env
                             .borrow_mut()
-                            .initialize_binding("arguments", arguments_obj);
+                            .initialize_binding("arguments", arguments_obj)
+                            .map_err(|e| e.to_error(context))?;
 
                         context.realm_mut().environment.push(local_env);
 
@@ -285,7 +289,7 @@ impl GcObject {
 
                 // local_env gets dropped here, its no longer needed
                 let binding = context.realm_mut().environment.get_this_binding();
-                Ok(binding)
+                binding.map_err(|e| e.to_error(context))
             }
         }
     }

boa/src/syntax/ast/node/declaration/const_decl_list/mod.rs

@@ -43,17 +43,17 @@ impl Executable for ConstDeclList {
             } else {
                 return context.throw_syntax_error("missing = in const declaration");
             };
-
+            context
-            context.realm_mut().environment.create_immutable_binding(
+                .realm_mut()
-                decl.name().to_owned(),
+                .environment
-                false,
+                .create_immutable_binding(decl.name().to_owned(), false, VariableScope::Block)
-                VariableScope::Block,
+                .map_err(|e| e.to_error(context))?;
-            );
 
             context
                 .realm_mut()
                 .environment
-                .initialize_binding(decl.name(), val);
+                .initialize_binding(decl.name(), val)
+                .map_err(|e| e.to_error(context))?;
         }
         Ok(Value::undefined())
     }

boa/src/syntax/ast/node/declaration/function_decl/mod.rs

@@ -94,16 +94,17 @@ impl Executable for FunctionDecl {
 
         // Set the name and assign it in the current environment
         val.set_field("name", self.name());
-        context.realm_mut().environment.create_mutable_binding(
+        context
-            self.name().to_owned(),
+            .realm_mut()
-            false,
+            .environment
-            VariableScope::Function,
+            .create_mutable_binding(self.name().to_owned(), false, VariableScope::Function)
-        );
+            .map_err(|e| e.to_error(context))?;
 
         context
             .realm_mut()
             .environment
-            .initialize_binding(self.name(), val);
+            .initialize_binding(self.name(), val)
+            .map_err(|e| e.to_error(context))?;
 
         Ok(Value::undefined())
     }

boa/src/syntax/ast/node/declaration/let_decl_list/mod.rs

@@ -41,15 +41,16 @@ impl Executable for LetDeclList {
                 Some(v) => v.run(context)?,
                 None => Value::undefined(),
             };
-            context.realm_mut().environment.create_mutable_binding(
-                var.name().to_owned(),
-                false,
-                VariableScope::Block,
-            );
             context
                 .realm_mut()
                 .environment
-                .initialize_binding(var.name(), val);
+                .create_mutable_binding(var.name().to_owned(), false, VariableScope::Block)
+                .map_err(|e| e.to_error(context))?;
+            context
+                .realm_mut()
+                .environment
+                .initialize_binding(var.name(), val)
+                .map_err(|e| e.to_error(context))?;
         }
         Ok(Value::undefined())
     }

boa/src/syntax/ast/node/declaration/var_decl_list/mod.rs

@@ -46,15 +46,18 @@ impl Executable for VarDeclList {
 
             if environment.has_binding(var.name()) {
                 if var.init().is_some() {
-                    environment.set_mutable_binding(var.name(), val, true);
+                    environment
+                        .set_mutable_binding(var.name(), val, true)
+                        .map_err(|e| e.to_error(context))?;
                 }
             } else {
-                environment.create_mutable_binding(
+                environment
-                    var.name().to_owned(),
+                    .create_mutable_binding(var.name().to_owned(), false, VariableScope::Function)
-                    false,
+                    .map_err(|e| e.to_error(context))?;
-                    VariableScope::Function,
+                let environment = &mut context.realm_mut().environment;
-                );
+                environment
-                environment.initialize_binding(var.name(), val);
+                    .initialize_binding(var.name(), val)
+                    .map_err(|e| e.to_error(context))?;
             }
         }
         Ok(Value::undefined())

boa/src/syntax/ast/node/identifier/mod.rs

@@ -40,7 +40,7 @@ impl Executable for Identifier {
             .realm()
             .environment
             .get_binding_value(self.as_ref())
-            .ok_or_else(|| context.construct_reference_error(self.as_ref()))
+            .map_err(|e| e.to_error(context))
     }
 }
 

boa/src/syntax/ast/node/iteration/for_of_loop/mod.rs

@@ -90,14 +90,21 @@ impl Executable for ForOfLoop {
 
                     if environment.has_binding(name.as_ref()) {
                         // Binding already exists
-                        environment.set_mutable_binding(name.as_ref(), next_result.clone(), true);
+                        environment
+                            .set_mutable_binding(name.as_ref(), next_result.clone(), true)
+                            .map_err(|e| e.to_error(context))?;
                     } else {
-                        environment.create_mutable_binding(
+                        environment
+                            .create_mutable_binding(
                                 name.as_ref().to_owned(),
                                 true,
                                 VariableScope::Function,
-                        );
+                            )
-                        environment.initialize_binding(name.as_ref(), next_result.clone());
+                            .map_err(|e| e.to_error(context))?;
+                        let environment = &mut context.realm_mut().environment;
+                        environment
+                            .initialize_binding(name.as_ref(), next_result.clone())
+                            .map_err(|e| e.to_error(context))?;
                     }
                 }
                 Node::VarDeclList(ref list) => match list.as_ref() {
@@ -109,14 +116,21 @@ impl Executable for ForOfLoop {
                         }
 
                         if environment.has_binding(var.name()) {
-                            environment.set_mutable_binding(var.name(), next_result, true);
+                            environment
+                                .set_mutable_binding(var.name(), next_result, true)
+                                .map_err(|e| e.to_error(context))?;
                         } else {
-                            environment.create_mutable_binding(
+                            environment
+                                .create_mutable_binding(
                                     var.name().to_owned(),
                                     false,
                                     VariableScope::Function,
-                            );
+                                )
-                            environment.initialize_binding(var.name(), next_result);
+                                .map_err(|e| e.to_error(context))?;
+                            let environment = &mut context.realm_mut().environment;
+                            environment
+                                .initialize_binding(var.name(), next_result)
+                                .map_err(|e| e.to_error(context))?;
                         }
                     }
                     _ => {
@@ -133,12 +147,18 @@ impl Executable for ForOfLoop {
                             return context.throw_syntax_error("a declaration in the head of a for-of loop can't have an initializer");
                         }
 
-                        environment.create_mutable_binding(
+                        environment
+                            .create_mutable_binding(
                                 var.name().to_owned(),
                                 false,
                                 VariableScope::Block,
-                        );
+                            )
-                        environment.initialize_binding(var.name(), next_result);
+                            .map_err(|e| e.to_error(context))?;
+
+                        let environment = &mut context.realm_mut().environment;
+                        environment
+                            .initialize_binding(var.name(), next_result)
+                            .map_err(|e| e.to_error(context))?;
                     }
                     _ => {
                         return context.throw_syntax_error(
@@ -154,12 +174,17 @@ impl Executable for ForOfLoop {
                             return context.throw_syntax_error("a declaration in the head of a for-of loop can't have an initializer");
                         }
 
-                        environment.create_immutable_binding(
+                        environment
+                            .create_immutable_binding(
                                 var.name().to_owned(),
                                 false,
                                 VariableScope::Block,
-                        );
+                            )
-                        environment.initialize_binding(var.name(), next_result);
+                            .map_err(|e| e.to_error(context))?;
+                        let environment = &mut context.realm_mut().environment;
+                        environment
+                            .initialize_binding(var.name(), next_result)
+                            .map_err(|e| e.to_error(context))?;
                     }
                     _ => {
                         return context.throw_syntax_error(

boa/src/syntax/ast/node/mod.rs

@@ -312,7 +312,11 @@ impl Executable for Node {
             Node::Spread(ref spread) => spread.run(context),
             Node::This => {
                 // Will either return `this` binding or undefined
-                Ok(context.realm().environment.get_this_binding())
+                context
+                    .realm()
+                    .environment
+                    .get_this_binding()
+                    .map_err(|e| e.to_error(context))
             }
             Node::Try(ref try_node) => try_node.run(context),
             Node::Break(ref break_node) => break_node.run(context),

boa/src/syntax/ast/node/operator/assign/mod.rs

@@ -62,14 +62,21 @@ impl Executable for Assign {
 
                 if environment.has_binding(name.as_ref()) {
                     // Binding already exists
-                    environment.set_mutable_binding(name.as_ref(), val.clone(), true);
+                    environment
+                        .set_mutable_binding(name.as_ref(), val.clone(), true)
+                        .map_err(|e| e.to_error(context))?;
                 } else {
-                    environment.create_mutable_binding(
+                    environment
+                        .create_mutable_binding(
                             name.as_ref().to_owned(),
                             true,
                             VariableScope::Function,
-                    );
+                        )
-                    environment.initialize_binding(name.as_ref(), val.clone());
+                        .map_err(|e| e.to_error(context))?;
+                    let environment = &mut context.realm_mut().environment;
+                    environment
+                        .initialize_binding(name.as_ref(), val.clone())
+                        .map_err(|e| e.to_error(context))?;
                 }
             }
             Node::GetConstField(ref get_const_field) => {

boa/src/syntax/ast/node/operator/bin_op/mod.rs

@@ -174,14 +174,15 @@ impl Executable for BinOp {
                         .realm()
                         .environment
                         .get_binding_value(name.as_ref())
-                        .ok_or_else(|| context.construct_reference_error(name.as_ref()))?;
+                        .map_err(|e| e.to_error(context))?;
+
                     let v_b = self.rhs().run(context)?;
                     let value = Self::run_assign(op, v_a, v_b, context)?;
-                    context.realm_mut().environment.set_mutable_binding(
+                    context
-                        name.as_ref(),
+                        .realm_mut()
-                        value.clone(),
+                        .environment
-                        true,
+                        .set_mutable_binding(name.as_ref(), value.clone(), true)
-                    );
+                        .map_err(|e| e.to_error(context))?;
                     Ok(value)
                 }
                 Node::GetConstField(ref get_const_field) => {

boa/src/syntax/ast/node/try_node/mod.rs

@@ -108,9 +108,11 @@ impl Executable for Try {
                                 param.to_owned(),
                                 false,
                                 VariableScope::Block,
-                            );
+                            )
-
+                            .map_err(|e| e.to_error(context))?;
-                            env.initialize_binding(param, err);
+                            let env = &mut context.realm_mut().environment;
+                            env.initialize_binding(param, err)
+                                .map_err(|e| e.to_error(context))?;
                         }
                     }
 

boa/src/value/operations.rs

@@ -320,7 +320,7 @@ impl Value {
                 a.as_inner()
                     .clone()
                     .shift_left(b.as_inner().clone())
-                    .map_err(|msg| context.construct_range_error(&msg))?,
+                    .map_err(|msg| context.construct_range_error(msg))?,
             ),
 
             // Slow path:
@@ -332,7 +332,7 @@ impl Value {
                     x.as_inner()
                         .clone()
                         .shift_left(y.as_inner().clone())
-                        .map_err(|msg| context.construct_range_error(&msg))?,
+                        .map_err(|msg| context.construct_range_error(msg))?,
                 ),
                 (_, _) => {
                     return context.throw_type_error(
@@ -362,7 +362,7 @@ impl Value {
                 a.as_inner()
                     .clone()
                     .shift_right(b.as_inner().clone())
-                    .map_err(|msg| context.construct_range_error(&msg))?,
+                    .map_err(|msg| context.construct_range_error(msg))?,
             ),
 
             // Slow path:
@@ -374,7 +374,7 @@ impl Value {
                     x.as_inner()
                         .clone()
                         .shift_right(y.as_inner().clone())
-                        .map_err(|msg| context.construct_range_error(&msg))?,
+                        .map_err(|msg| context.construct_range_error(msg))?,
                 ),
                 (_, _) => {
                     return context.throw_type_error(