LibJS: Include identifier information in nullish property read access

When a GetById / GetByValue bytecode operation results in accessing a
nullish object, we now include the name of the property and the object
being accessed in the exception message (if available). This should make
it easier to debug live websites.

For example, the following errors would all previously produce a generic
error message of "ToObject on null or undefined":

  > foo = null
  > foo.bar
  Uncaught exception:
  [TypeError] Cannot access property "bar" on null object "foo"
      at <unknown>

  > foo = { bar: undefined }
  > foo.bar.baz
  Uncaught exception:
  [TypeError] Cannot access property "baz" on undefined object "foo.bar"
      at <unknown>

Note we certainly don't capture all possible nullish property read
accesses here. This just covers cases I've seen most on live websites;
we can cover more cases as they arise.

Userland/Libraries/LibJS/Bytecode/ASTCodegen.cpp

@@ -1540,7 +1540,8 @@ static Bytecode::CodeGenerationErrorOr<BaseAndValue> get_base_and_value_from_mem
             base,
             generator.intern_identifier(verify_cast<PrivateIdentifier>(member_expression.property()).string()));
     } else {
-        generator.emit_get_by_id(value, base, generator.intern_identifier(verify_cast<Identifier>(member_expression.property()).string()));
+        auto base_identifier = generator.intern_identifier_for_expression(member_expression.object());
+        generator.emit_get_by_id(value, base, generator.intern_identifier(verify_cast<Identifier>(member_expression.property()).string()), move(base_identifier));
     }
 
     return BaseAndValue { base, value };

Userland/Libraries/LibJS/Bytecode/CommonImplementations.h

@@ -58,7 +58,30 @@ inline void fast_typed_array_set_element(TypedArrayBase& typed_array, u32 index,
     *slot = value;
 }
 
-ALWAYS_INLINE ThrowCompletionOr<NonnullGCPtr<Object>> base_object_for_get(VM& vm, Value base_value)
+template<typename BaseType, typename PropertyType>
+ALWAYS_INLINE Completion throw_null_or_undefined_property_access(VM& vm, Value base_value, BaseType const& base_identifier, PropertyType const& property_identifier)
+{
+    VERIFY(base_value.is_nullish());
+
+    bool has_base_identifier = true;
+    bool has_property_identifier = true;
+
+    if constexpr (requires { base_identifier.has_value(); })
+        has_base_identifier = base_identifier.has_value();
+    if constexpr (requires { property_identifier.has_value(); })
+        has_property_identifier = property_identifier.has_value();
+
+    if (has_base_identifier && has_property_identifier)
+        return vm.throw_completion<TypeError>(ErrorType::ToObjectNullOrUndefinedWithPropertyAndName, property_identifier, base_value, base_identifier);
+    if (has_property_identifier)
+        return vm.throw_completion<TypeError>(ErrorType::ToObjectNullOrUndefinedWithProperty, property_identifier, base_value);
+    if (has_base_identifier)
+        return vm.throw_completion<TypeError>(ErrorType::ToObjectNullOrUndefinedWithName, base_identifier, base_value);
+    return vm.throw_completion<TypeError>(ErrorType::ToObjectNullOrUndefined);
+}
+
+template<typename BaseType, typename PropertyType>
+ALWAYS_INLINE ThrowCompletionOr<NonnullGCPtr<Object>> base_object_for_get(VM& vm, Value base_value, BaseType const& base_identifier, PropertyType property_identifier)
 {
     if (base_value.is_object()) [[likely]]
         return base_value.as_object();
@@ -77,10 +100,10 @@ ALWAYS_INLINE ThrowCompletionOr<NonnullGCPtr<Object>> base_object_for_get(VM& vm
         return realm.intrinsics().symbol_prototype();
 
     // NOTE: At this point this is guaranteed to throw (null or undefined).
-    return base_value.to_object(vm);
+    return throw_null_or_undefined_property_access(vm, base_value, base_identifier, property_identifier);
 }
 
-inline ThrowCompletionOr<Value> get_by_id(VM& vm, DeprecatedFlyString const& property, Value base_value, Value this_value, PropertyLookupCache& cache)
+inline ThrowCompletionOr<Value> get_by_id(VM& vm, Optional<DeprecatedFlyString const&> const& base_identifier, DeprecatedFlyString const& property, Value base_value, Value this_value, PropertyLookupCache& cache)
 {
     if (base_value.is_string()) {
         auto string_value = TRY(base_value.as_string().get(vm, property));
@@ -88,7 +111,7 @@ inline ThrowCompletionOr<Value> get_by_id(VM& vm, DeprecatedFlyString const& pro
             return *string_value;
     }
 
-    auto base_obj = TRY(base_object_for_get(vm, base_value));
+    auto base_obj = TRY(base_object_for_get(vm, base_value, base_identifier, property));
 
     // OPTIMIZATION: Fast path for the magical "length" property on Array objects.
     if (base_obj->has_magical_length_property() && property == vm.names.length.as_string()) {
@@ -112,7 +135,7 @@ inline ThrowCompletionOr<Value> get_by_id(VM& vm, DeprecatedFlyString const& pro
     return value;
 }
 
-inline ThrowCompletionOr<Value> get_by_value(VM& vm, Value base_value, Value property_key_value)
+inline ThrowCompletionOr<Value> get_by_value(VM& vm, Optional<DeprecatedFlyString const&> const& base_identifier, Value base_value, Value property_key_value)
 {
     // OPTIMIZATION: Fast path for simple Int32 indexes in array-like objects.
     if (base_value.is_object() && property_key_value.is_int32() && property_key_value.as_i32() >= 0) {
@@ -169,7 +192,7 @@ inline ThrowCompletionOr<Value> get_by_value(VM& vm, Value base_value, Value pro
         }
     }
 
-    auto object = TRY(base_object_for_get(vm, base_value));
+    auto object = TRY(base_object_for_get(vm, base_value, base_identifier, property_key_value));
 
     auto property_key = TRY(property_key_value.to_property_key(vm));
 

Userland/Libraries/LibJS/Bytecode/Executable.h

@@ -74,6 +74,13 @@ public:
     ByteString const& get_string(StringTableIndex index) const { return string_table->get(index); }
     DeprecatedFlyString const& get_identifier(IdentifierTableIndex index) const { return identifier_table->get(index); }
 
+    Optional<DeprecatedFlyString const&> get_identifier(Optional<IdentifierTableIndex> const& index) const
+    {
+        if (!index.has_value())
+            return {};
+        return get_identifier(*index);
+    }
+
     void dump() const;
 
 private:

Userland/Libraries/LibJS/Bytecode/Generator.cpp

@@ -253,13 +253,16 @@ CodeGenerationErrorOr<Generator::ReferenceOperands> Generator::emit_load_from_re
         super_reference.loaded_value = dst;
         return super_reference;
     }
+
     auto base = TRY(expression.object().generate_bytecode(*this)).value();
+    auto base_identifier = intern_identifier_for_expression(expression.object());
+
     if (expression.is_computed()) {
         auto property = TRY(expression.property().generate_bytecode(*this)).value();
         auto saved_property = Operand(allocate_register());
         emit<Bytecode::Op::Mov>(saved_property, property);
         auto dst = preferred_dst.has_value() ? preferred_dst.value() : Operand(allocate_register());
-        emit<Bytecode::Op::GetByValue>(dst, base, property);
+        emit<Bytecode::Op::GetByValue>(dst, base, property, move(base_identifier));
         return ReferenceOperands {
             .base = base,
             .referenced_name = saved_property,
@@ -270,7 +273,7 @@ CodeGenerationErrorOr<Generator::ReferenceOperands> Generator::emit_load_from_re
     if (expression.property().is_identifier()) {
         auto identifier_table_ref = intern_identifier(verify_cast<Identifier>(expression.property()).string());
         auto dst = preferred_dst.has_value() ? preferred_dst.value() : Operand(allocate_register());
-        emit_get_by_id(dst, base, identifier_table_ref);
+        emit_get_by_id(dst, base, identifier_table_ref, move(base_identifier));
         return ReferenceOperands {
             .base = base,
             .referenced_identifier = identifier_table_ref,
@@ -443,6 +446,39 @@ void Generator::emit_set_variable(JS::Identifier const& identifier, Operand valu
     }
 }
 
+static Optional<ByteString> expression_identifier(Expression const& expression)
+{
+    if (expression.is_identifier()) {
+        auto const& identifier = static_cast<Identifier const&>(expression);
+        return identifier.string();
+    }
+
+    if (expression.is_member_expression()) {
+        auto const& member_expression = static_cast<MemberExpression const&>(expression);
+        StringBuilder builder;
+
+        if (auto identifer = expression_identifier(member_expression.object()); identifer.has_value())
+            builder.append(*identifer);
+
+        if (auto identifer = expression_identifier(member_expression.property()); identifer.has_value()) {
+            if (!builder.is_empty())
+                builder.append('.');
+            builder.append(*identifer);
+        }
+
+        return builder.to_byte_string();
+    }
+
+    return {};
+}
+
+Optional<IdentifierTableIndex> Generator::intern_identifier_for_expression(Expression const& expression)
+{
+    if (auto identifer = expression_identifier(expression); identifer.has_value())
+        return intern_identifier(identifer.release_value());
+    return {};
+}
+
 void Generator::generate_scoped_jump(JumpType type)
 {
     TemporaryChange temp { m_current_unwind_context, m_current_unwind_context };
@@ -596,9 +632,9 @@ CodeGenerationErrorOr<Optional<Operand>> Generator::emit_named_evaluation_if_ano
     return expression.generate_bytecode(*this, preferred_dst);
 }
 
-void Generator::emit_get_by_id(Operand dst, Operand base, IdentifierTableIndex id)
+void Generator::emit_get_by_id(Operand dst, Operand base, IdentifierTableIndex property_identifier, Optional<IdentifierTableIndex> base_identifier)
 {
-    emit<Op::GetById>(dst, base, id, m_next_property_lookup_cache++);
+    emit<Op::GetById>(dst, base, property_identifier, move(base_identifier), m_next_property_lookup_cache++);
 }
 
 void Generator::emit_get_by_id_with_this(Operand dst, Operand base, IdentifierTableIndex id, Operand this_value)

Userland/Libraries/LibJS/Bytecode/Generator.h

@@ -183,6 +183,8 @@ public:
         return m_identifier_table->insert(move(string));
     }
 
+    Optional<IdentifierTableIndex> intern_identifier_for_expression(Expression const& expression);
+
     bool is_in_generator_or_async_function() const { return m_enclosing_function_kind == FunctionKind::Async || m_enclosing_function_kind == FunctionKind::Generator || m_enclosing_function_kind == FunctionKind::AsyncGenerator; }
     bool is_in_generator_function() const { return m_enclosing_function_kind == FunctionKind::Generator || m_enclosing_function_kind == FunctionKind::AsyncGenerator; }
     bool is_in_async_function() const { return m_enclosing_function_kind == FunctionKind::Async || m_enclosing_function_kind == FunctionKind::AsyncGenerator; }
@@ -247,7 +249,7 @@ public:
         m_boundaries.take_last();
     }
 
-    void emit_get_by_id(Operand dst, Operand base, IdentifierTableIndex);
+    void emit_get_by_id(Operand dst, Operand base, IdentifierTableIndex property_identifier, Optional<IdentifierTableIndex> base_identifier = {});
 
     void emit_get_by_id_with_this(Operand dst, Operand base, IdentifierTableIndex, Operand this_value);
 

Userland/Libraries/LibJS/Bytecode/Interpreter.cpp

@@ -1083,9 +1083,13 @@ ThrowCompletionOr<void> SetLocal::execute_impl(Bytecode::Interpreter&) const
 
 ThrowCompletionOr<void> GetById::execute_impl(Bytecode::Interpreter& interpreter) const
 {
+    auto base_identifier = interpreter.current_executable().get_identifier(m_base_identifier);
+    auto const& property_identifier = interpreter.current_executable().get_identifier(m_property);
+
     auto base_value = interpreter.get(base());
     auto& cache = interpreter.current_executable().property_lookup_caches[m_cache_index];
-    interpreter.set(dst(), TRY(get_by_id(interpreter.vm(), interpreter.current_executable().get_identifier(m_property), base_value, base_value, cache)));
+
+    interpreter.set(dst(), TRY(get_by_id(interpreter.vm(), base_identifier, property_identifier, base_value, base_value, cache)));
     return {};
 }
 
@@ -1094,7 +1098,7 @@ ThrowCompletionOr<void> GetByIdWithThis::execute_impl(Bytecode::Interpreter& int
     auto base_value = interpreter.get(m_base);
     auto this_value = interpreter.get(m_this_value);
     auto& cache = interpreter.current_executable().property_lookup_caches[m_cache_index];
-    interpreter.set(dst(), TRY(get_by_id(interpreter.vm(), interpreter.current_executable().get_identifier(m_property), base_value, this_value, cache)));
+    interpreter.set(dst(), TRY(get_by_id(interpreter.vm(), {}, interpreter.current_executable().get_identifier(m_property), base_value, this_value, cache)));
     return {};
 }
 
@@ -1499,7 +1503,9 @@ ThrowCompletionOr<void> Await::execute_impl(Bytecode::Interpreter& interpreter)
 
 ThrowCompletionOr<void> GetByValue::execute_impl(Bytecode::Interpreter& interpreter) const
 {
-    interpreter.set(dst(), TRY(get_by_value(interpreter.vm(), interpreter.get(m_base), interpreter.get(m_property))));
+    auto base_identifier = interpreter.current_executable().get_identifier(m_base_identifier);
+
+    interpreter.set(dst(), TRY(get_by_value(interpreter.vm(), base_identifier, interpreter.get(m_base), interpreter.get(m_property))));
     return {};
 }
 

Userland/Libraries/LibJS/Bytecode/Op.h

@@ -629,11 +629,12 @@ private:
 
 class GetById final : public Instruction {
 public:
-    GetById(Operand dst, Operand base, IdentifierTableIndex property, u32 cache_index)
+    GetById(Operand dst, Operand base, IdentifierTableIndex property, Optional<IdentifierTableIndex> base_identifier, u32 cache_index)
         : Instruction(Type::GetById, sizeof(*this))
         , m_dst(dst)
         , m_base(base)
         , m_property(property)
+        , m_base_identifier(move(base_identifier))
         , m_cache_index(cache_index)
     {
     }
@@ -650,6 +651,7 @@ private:
     Operand m_dst;
     Operand m_base;
     IdentifierTableIndex m_property;
+    Optional<IdentifierTableIndex> m_base_identifier;
     u32 m_cache_index { 0 };
 };
 
@@ -874,11 +876,12 @@ private:
 
 class GetByValue final : public Instruction {
 public:
-    explicit GetByValue(Operand dst, Operand base, Operand property)
+    GetByValue(Operand dst, Operand base, Operand property, Optional<IdentifierTableIndex> base_identifier = {})
         : Instruction(Type::GetByValue, sizeof(*this))
         , m_dst(dst)
         , m_base(base)
         , m_property(property)
+        , m_base_identifier(move(base_identifier))
     {
     }
 
@@ -889,10 +892,13 @@ public:
     Operand base() const { return m_base; }
     Operand property() const { return m_property; }
 
+    Optional<DeprecatedFlyString const&> base_identifier(Bytecode::Interpreter const&) const;
+
 private:
     Operand m_dst;
     Operand m_base;
     Operand m_property;
+    Optional<IdentifierTableIndex> m_base_identifier;
 };
 
 class GetByValueWithThis final : public Instruction {

Userland/Libraries/LibJS/Runtime/ErrorTypes.h

@@ -302,6 +302,9 @@
     M(ThisHasNotBeenInitialized, "|this| has not been initialized")                                                                     \
     M(ThisIsAlreadyInitialized, "|this| is already initialized")                                                                        \
     M(ToObjectNullOrUndefined, "ToObject on null or undefined")                                                                         \
+    M(ToObjectNullOrUndefinedWithName, "\"{}\" is {}")                                                                                  \
+    M(ToObjectNullOrUndefinedWithProperty, "Cannot access property \"{}\" on {} object")                                                \
+    M(ToObjectNullOrUndefinedWithPropertyAndName, "Cannot access property \"{}\" on {} object \"{}\"")                                  \
     M(TopLevelVariableAlreadyDeclared, "Redeclaration of top level variable '{}'")                                                      \
     M(ToPrimitiveReturnedObject, "Can't convert {} to primitive with hint \"{}\", its @@toPrimitive method returned an object")         \
     M(TypedArrayContentTypeMismatch, "Can't create {} from {}")                                                                         \

Userland/Libraries/LibJS/Tests/null-or-undefined-access.js

@@ -0,0 +1,36 @@
+test("null/undefined object", () => {
+    [null, undefined].forEach(value => {
+        let foo = value;
+
+        expect(() => {
+            foo.bar;
+        }).toThrowWithMessage(TypeError, `Cannot access property "bar" on ${value} object "foo"`);
+
+        expect(() => {
+            foo[0];
+        }).toThrowWithMessage(TypeError, `Cannot access property "0" on ${value} object "foo"`);
+    });
+});
+
+test("null/undefined object key", () => {
+    [null, undefined].forEach(value => {
+        let foo = { bar: value };
+
+        expect(() => {
+            foo.bar.baz;
+        }).toThrowWithMessage(
+            TypeError,
+            `Cannot access property "baz" on ${value} object "foo.bar"`
+        );
+    });
+});
+
+test("null/undefined array index", () => {
+    [null, undefined].forEach(value => {
+        let foo = [value];
+
+        expect(() => {
+            foo[0].bar;
+        }).toThrowWithMessage(TypeError, `Cannot access property "bar" on ${value} object`);
+    });
+});