Partial Symbol support

Added basic infrastructure to create symbols and convert them
back to strings. In addition, storing and retrieving of symbol
based properties in Objects works.

Change-Id: I185f7aa46e7afa19db5a801102142892e03b7bf1
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

17 files changed, 338 insertions, 35 deletions

diff --git a/src/qml/jsruntime/jsruntime.pri b/src/qml/jsruntime/jsruntime.pri
index 1ec00cc744..c7d80d5a82 100644
--- a/src/qml/jsruntime/jsruntime.pri
+++ b/src/qml/jsruntime/jsruntime.pri
@@ -36,6 +36,7 @@ SOURCES += \
     $$PWD/qv4runtimecodegen.cpp \
     $$PWD/qv4serialize.cpp \
     $$PWD/qv4script.cpp \
+    $$PWD/qv4symbol.cpp \
     $$PWD/qv4include.cpp \
     $$PWD/qv4qobjectwrapper.cpp \
     $$PWD/qv4arraybuffer.cpp \
@@ -86,6 +87,7 @@ HEADERS += \
     $$PWD/qv4regexp_p.h \
     $$PWD/qv4serialize_p.h \
     $$PWD/qv4script_p.h \
+    $$PWD/qv4symbol_p.h \
     $$PWD/qv4scopedvalue_p.h \
     $$PWD/qv4executableallocator_p.h \
     $$PWD/qv4include_p.h \
diff --git a/src/qml/jsruntime/qv4engine.cpp b/src/qml/jsruntime/qv4engine.cpp
index af752cf243..5ede4373f2 100644
--- a/src/qml/jsruntime/qv4engine.cpp
+++ b/src/qml/jsruntime/qv4engine.cpp
@@ -52,6 +52,7 @@
 #include <qv4numberobject_p.h>
 #include <qv4regexpobject_p.h>
 #include <qv4regexp_p.h>
+#include "qv4symbol_p.h"
 #include <qv4variantobject_p.h>
 #include <qv4runtime_p.h>
 #include <private/qv4mm_p.h>
@@ -198,12 +199,12 @@ ExecutionEngine::ExecutionEngine(QJSEngine *jsEngine)
     jsStackLimit = jsStackBase + JSStackLimit/sizeof(Value);
 
     identifierTable = new IdentifierTable(this);
+    symbolTable = new IdentifierTable(this);
 
     memset(classes, 0, sizeof(classes));
     classes[Class_Empty] = memoryManager->allocIC<InternalClass>();
     classes[Class_Empty]->init(this);
 
-    classes[Class_String] = classes[Class_Empty]->changeVTable(QV4::String::staticVTable());
     classes[Class_MemberData] = classes[Class_Empty]->changeVTable(QV4::MemberData::staticVTable());
     classes[Class_SimpleArrayData] = classes[Class_Empty]->changeVTable(QV4::SimpleArrayData::staticVTable());
     classes[Class_SparseArrayData] = classes[Class_Empty]->changeVTable(QV4::SparseArrayData::staticVTable());
@@ -211,6 +212,21 @@ ExecutionEngine::ExecutionEngine(QJSEngine *jsEngine)
     classes[Class_CallContext] = classes[Class_Empty]->changeVTable(QV4::CallContext::staticVTable());
     classes[Class_QmlContext] = classes[Class_Empty]->changeVTable(QV4::QmlContext::staticVTable());
 
+    Scope scope(this);
+    Scoped<InternalClass> ic(scope);
+    ic = classes[Class_Empty]->changeVTable(QV4::Object::staticVTable());
+    jsObjects[ObjectProto] = memoryManager->allocObject<ObjectPrototype>(ic->d());
+    classes[Class_Object] = ic->changePrototype(objectPrototype()->d());
+    classes[Class_QmlContextWrapper] = classes[Class_Object]->changeVTable(QV4::QQmlContextWrapper::staticVTable());
+
+    ic = newInternalClass(QV4::StringObject::staticVTable(), objectPrototype());
+    jsObjects[StringProto] = memoryManager->allocObject<StringPrototype>(ic->d());
+    classes[Class_String] = classes[Class_Empty]->changeVTable(QV4::String::staticVTable())->changePrototype(stringPrototype()->d());
+    Q_ASSERT(stringPrototype()->d() && classes[Class_String]->prototype);
+
+    jsObjects[SymbolProto] = memoryManager->allocate<SymbolPrototype>();
+    classes[Class_Symbol] = classes[EngineBase::Class_Empty]->changeVTable(QV4::Symbol::staticVTable())->changePrototype(symbolPrototype()->d());
+
     jsStrings[String_Empty] = newIdentifier(QString());
     jsStrings[String_undefined] = newIdentifier(QStringLiteral("undefined"));
     jsStrings[String_null] = newIdentifier(QStringLiteral("null"));
@@ -248,13 +264,6 @@ ExecutionEngine::ExecutionEngine(QJSEngine *jsEngine)
     jsStrings[String_buffer] = newIdentifier(QStringLiteral("buffer"));
     jsStrings[String_lastIndex] = newIdentifier(QStringLiteral("lastIndex"));
 
-    Scope scope(this);
-    Scoped<InternalClass> ic(scope);
-    ic = classes[Class_Empty]->changeVTable(QV4::Object::staticVTable());
-    jsObjects[ObjectProto] = memoryManager->allocObject<ObjectPrototype>(ic->d());
-    classes[Class_Object] = ic->changePrototype(objectPrototype()->d());
-    classes[EngineBase::Class_QmlContextWrapper] = classes[Class_Object]->changeVTable(QV4::QQmlContextWrapper::staticVTable());
-
     ic = newInternalClass(ArrayPrototype::staticVTable(), objectPrototype());
     Q_ASSERT(ic->d()->prototype);
     ic = ic->addMember(id_length()->identifier(), Attr_NotConfigurable|Attr_NotEnumerable);
@@ -277,7 +286,6 @@ ExecutionEngine::ExecutionEngine(QJSEngine *jsEngine)
 
     ic = newInternalClass(QV4::StringObject::staticVTable(), objectPrototype());
     ic = ic->addMember(id_length()->identifier(), Attr_ReadOnly);
-    jsObjects[StringProto] = memoryManager->allocObject<StringPrototype>(ic->d());
     classes[Class_StringObject] = ic->changePrototype(stringPrototype()->d());
     Q_ASSERT(classes[Class_StringObject]->find(id_length()->identifier()) == Heap::StringObject::LengthPropertyIndex);
 
@@ -362,8 +370,10 @@ ExecutionEngine::ExecutionEngine(QJSEngine *jsEngine)
 #endif
 
     ExecutionContext *global = rootContext();
+
     jsObjects[Object_Ctor] = memoryManager->allocate<ObjectCtor>(global);
     jsObjects[String_Ctor] = memoryManager->allocate<StringCtor>(global);
+    jsObjects[Symbol_Ctor] = memoryManager->allocate<SymbolCtor>(global);
     jsObjects[Number_Ctor] = memoryManager->allocate<NumberCtor>(global);
     jsObjects[Boolean_Ctor] = memoryManager->allocate<BooleanCtor>(global);
     jsObjects[Array_Ctor] = memoryManager->allocate<ArrayCtor>(global);
@@ -380,6 +390,7 @@ ExecutionEngine::ExecutionEngine(QJSEngine *jsEngine)
 
     static_cast<ObjectPrototype *>(objectPrototype())->init(this, objectCtor());
     static_cast<StringPrototype *>(stringPrototype())->init(this, stringCtor());
+    static_cast<SymbolPrototype *>(symbolPrototype())->init(this, symbolCtor());
     static_cast<NumberPrototype *>(numberPrototype())->init(this, numberCtor());
     static_cast<BooleanPrototype *>(booleanPrototype())->init(this, booleanCtor());
     static_cast<ArrayPrototype *>(arrayPrototype())->init(this, arrayCtor());
@@ -426,6 +437,7 @@ ExecutionEngine::ExecutionEngine(QJSEngine *jsEngine)
 
     globalObject->defineDefaultProperty(QStringLiteral("Object"), *objectCtor());
     globalObject->defineDefaultProperty(QStringLiteral("String"), *stringCtor());
+    globalObject->defineDefaultProperty(QStringLiteral("Symbol"), *symbolCtor());
     FunctionObject *numberObject = numberCtor();
     globalObject->defineDefaultProperty(QStringLiteral("Number"), *numberObject);
     globalObject->defineDefaultProperty(QStringLiteral("Boolean"), *booleanCtor());
diff --git a/src/qml/jsruntime/qv4engine_p.h b/src/qml/jsruntime/qv4engine_p.h
index 4316967484..ff767a3dde 100644
--- a/src/qml/jsruntime/qv4engine_p.h
+++ b/src/qml/jsruntime/qv4engine_p.h
@@ -161,6 +161,7 @@ public:
         RootContext,
         IntegerNull, // Has to come after the RootContext to make the context stack safe
         ObjectProto,
+        SymbolProto,
         ArrayProto,
         PropertyListProto,
         StringProto,
@@ -187,6 +188,7 @@ public:
 
         Object_Ctor,
         String_Ctor,
+        Symbol_Ctor,
         Number_Ctor,
         Boolean_Ctor,
         Array_Ctor,
@@ -214,6 +216,7 @@ public:
     ExecutionContext *rootContext() const { return reinterpret_cast<ExecutionContext *>(jsObjects + RootContext); }
     FunctionObject *objectCtor() const { return reinterpret_cast<FunctionObject *>(jsObjects + Object_Ctor); }
     FunctionObject *stringCtor() const { return reinterpret_cast<FunctionObject *>(jsObjects + String_Ctor); }
+    FunctionObject *symbolCtor() const { return reinterpret_cast<FunctionObject *>(jsObjects + Symbol_Ctor); }
     FunctionObject *numberCtor() const { return reinterpret_cast<FunctionObject *>(jsObjects + Number_Ctor); }
     FunctionObject *booleanCtor() const { return reinterpret_cast<FunctionObject *>(jsObjects + Boolean_Ctor); }
     FunctionObject *arrayCtor() const { return reinterpret_cast<FunctionObject *>(jsObjects + Array_Ctor); }
@@ -232,6 +235,7 @@ public:
     FunctionObject *typedArrayCtors;
 
     Object *objectPrototype() const { return reinterpret_cast<Object *>(jsObjects + ObjectProto); }
+    Object *symbolPrototype() const { return reinterpret_cast<Object *>(jsObjects + SymbolProto); }
     Object *arrayPrototype() const { return reinterpret_cast<Object *>(jsObjects + ArrayProto); }
     Object *propertyListPrototype() const { return reinterpret_cast<Object *>(jsObjects + PropertyListProto); }
     Object *stringPrototype() const { return reinterpret_cast<Object *>(jsObjects + StringProto); }
diff --git a/src/qml/jsruntime/qv4enginebase_p.h b/src/qml/jsruntime/qv4enginebase_p.h
index c01b9b1842..4abf92eea5 100644
--- a/src/qml/jsruntime/qv4enginebase_p.h
+++ b/src/qml/jsruntime/qv4enginebase_p.h
@@ -83,6 +83,7 @@ struct Q_QML_EXPORT EngineBase {
     Value *jsStackBase = nullptr;
 
     IdentifierTable *identifierTable = nullptr;
+    IdentifierTable *symbolTable = nullptr;
     Object *globalObject = nullptr;
 
     // Exception handling
@@ -112,6 +113,7 @@ struct Q_QML_EXPORT EngineBase {
         Class_ErrorObjectWithMessage,
         Class_ErrorProto,
         Class_QmlContextWrapper,
+        Class_Symbol,
         NClasses
     };
     Heap::InternalClass *classes[NClasses];
diff --git a/src/qml/jsruntime/qv4global_p.h b/src/qml/jsruntime/qv4global_p.h
index 6820042406..fce220907e 100644
--- a/src/qml/jsruntime/qv4global_p.h
+++ b/src/qml/jsruntime/qv4global_p.h
@@ -165,7 +165,9 @@ namespace Heap {
     struct MemberData;
     struct ArrayData;
 
+    struct StringOrSymbol;
     struct String;
+    struct Symbol;
     struct Object;
     struct ObjectPrototype;
 
@@ -197,7 +199,9 @@ namespace Heap {
 
 class MemoryManager;
 class ExecutableAllocator;
+struct StringOrSymbol;
 struct String;
+struct Symbol;
 struct Object;
 struct ObjectPrototype;
 struct ObjectIterator;
@@ -244,6 +248,7 @@ struct Scope;
 struct ScopedValue;
 template<typename T> struct Scoped;
 typedef Scoped<String> ScopedString;
+typedef Scoped<StringOrSymbol> ScopedStringOrSymbol;
 typedef Scoped<Object> ScopedObject;
 typedef Scoped<ArrayObject> ScopedArrayObject;
 typedef Scoped<FunctionObject> ScopedFunctionObject;
diff --git a/src/qml/jsruntime/qv4lookup.cpp b/src/qml/jsruntime/qv4lookup.cpp
index b5a21ca825..e4a7c703fc 100644
--- a/src/qml/jsruntime/qv4lookup.cpp
+++ b/src/qml/jsruntime/qv4lookup.cpp
@@ -109,11 +109,12 @@ ReturnedValue Lookup::resolvePrimitiveGetter(ExecutionEngine *engine, const Valu
         break;
     case Value::Managed_Type: {
         // ### Should move this over to the Object path, as strings also have an internalClass
-        Q_ASSERT(object.isString());
-        primitiveLookup.proto = engine->stringPrototype()->d();
+        Q_ASSERT(object.isStringOrSymbol());
+        primitiveLookup.proto = static_cast<const Managed &>(object).internalClass()->prototype;
+        Q_ASSERT(primitiveLookup.proto);
         Scope scope(engine);
         ScopedString name(scope, engine->currentStackFrame->v4Function->compilationUnit->runtimeStrings[nameIndex]);
-        if (name->equals(engine->id_length())) {
+        if (object.isString() && name->equals(engine->id_length())) {
             // special case, as the property is on the object itself
             getter = stringLengthGetter;
             return stringLengthGetter(this, engine, object);
diff --git a/src/qml/jsruntime/qv4managed.cpp b/src/qml/jsruntime/qv4managed.cpp
index 924d2263c9..6ed2a9d716 100644
--- a/src/qml/jsruntime/qv4managed.cpp
+++ b/src/qml/jsruntime/qv4managed.cpp
@@ -72,8 +72,13 @@ QString Managed::className() const
     const char *s = nullptr;
     switch (Type(vtable()->type)) {
     case Type_Invalid:
-    case Type_String:
         return QString();
+    case Type_String:
+        s = "String";
+        break;
+    case Type_Symbol:
+        s = "Symbol";
+        break;
     case Type_Object:
         s = "Object";
         break;
diff --git a/src/qml/jsruntime/qv4managed_p.h b/src/qml/jsruntime/qv4managed_p.h
index 30adf07ffb..7ef5fd4ab8 100644
--- a/src/qml/jsruntime/qv4managed_p.h
+++ b/src/qml/jsruntime/qv4managed_p.h
@@ -182,6 +182,7 @@ public:
         Type_Invalid,
         Type_String,
         Type_Object,
+        Type_Symbol,
         Type_ArrayObject,
         Type_FunctionObject,
         Type_BooleanObject,
diff --git a/src/qml/jsruntime/qv4objectproto.cpp b/src/qml/jsruntime/qv4objectproto.cpp
index 451566137c..94917a45f6 100644
--- a/src/qml/jsruntime/qv4objectproto.cpp
+++ b/src/qml/jsruntime/qv4objectproto.cpp
@@ -484,10 +484,14 @@ ReturnedValue ObjectPrototype::method_toString(const FunctionObject *b, const Va
         return Encode(v4->newString(QStringLiteral("[object Undefined]")));
     } else if (thisObject->isNull()) {
         return Encode(v4->newString(QStringLiteral("[object Null]")));
+    } else if (thisObject->isBoolean()) {
+        return Encode(v4->newString(QStringLiteral("[object Boolean]")));
+    } else if (thisObject->isNumber()) {
+        return Encode(v4->newString(QStringLiteral("[object Number]")));
     } else {
-        Scope scope(v4);
-        ScopedObject obj(scope, thisObject->toObject(scope.engine));
-        QString className = obj->className();
+        Q_ASSERT(thisObject->isManaged());
+        const Managed *m = static_cast<const Managed *>(thisObject);
+        QString className = m->className();
         return Encode(v4->newString(QStringLiteral("[object %1]").arg(className)));
     }
 }
diff --git a/src/qml/jsruntime/qv4runtime.cpp b/src/qml/jsruntime/qv4runtime.cpp
index 40e08ededb..106ea65679 100644
--- a/src/qml/jsruntime/qv4runtime.cpp
+++ b/src/qml/jsruntime/qv4runtime.cpp
@@ -461,7 +461,10 @@ Heap::Object *RuntimeHelpers::convertToObject(ExecutionEngine *engine, const Val
     case Value::Boolean_Type:
         return engine->newBooleanObject(value.booleanValue());
     case Value::Managed_Type:
-        Q_ASSERT(value.isString());
+        Q_ASSERT(value.isStringOrSymbol());
+        if (!value.isString())
+            // ### this is a symbol, which is an immutable object according to spec
+            return nullptr;
         return engine->newStringObject(value.stringValue());
     case Value::Integer_Type:
     default: // double
@@ -488,6 +491,10 @@ Heap::String *RuntimeHelpers::convertToString(ExecutionEngine *engine, Value val
     case Value::Managed_Type: {
         if (value.isString())
             return static_cast<const String &>(value).d();
+        if (value.isSymbol()) {
+            engine->throwTypeError(QLatin1String("Cannot convert a symbol to a string."));
+            return  nullptr;
+        }
         value = Primitive::fromReturnedValue(RuntimeHelpers::toPrimitive(value, hint));
         Q_ASSERT(value.isPrimitive());
         if (value.isString())
@@ -598,7 +605,7 @@ static Q_NEVER_INLINE ReturnedValue getElementFallback(ExecutionEngine *engine,
         Q_ASSERT(!!o); // can't fail as null/undefined is covered above
     }
 
-    ScopedString name(scope, index.toString(engine));
+    ScopedStringOrSymbol  name(scope, index.toStringOrSymbol(engine));
     if (scope.hasException())
         return Encode::undefined();
     return o->get(name);
@@ -652,7 +659,7 @@ static Q_NEVER_INLINE bool setElementFallback(ExecutionEngine *engine, const Val
         return o->putIndexed(idx, value);
     }
 
-    ScopedString name(scope, index.toString(engine));
+    ScopedStringOrSymbol name(scope, index.toStringOrSymbol(engine));
     return o->put(name, value);
 }
 
@@ -1033,24 +1040,30 @@ ReturnedValue Runtime::method_callName(ExecutionEngine *engine, int nameIndex, V
 ReturnedValue Runtime::method_callProperty(ExecutionEngine *engine, Value *base, int nameIndex, Value *argv, int argc)
 {
     Scope scope(engine);
+    ScopedString name(scope, engine->currentStackFrame->v4Function->compilationUnit->runtimeStrings[nameIndex]);
+    ScopedObject lookupObject(scope, base);
 
-    if (!base->isObject()) {
+    if (!lookupObject) {
         Q_ASSERT(!base->isEmpty());
         if (base->isNullOrUndefined()) {
             QString message = QStringLiteral("Cannot call method '%1' of %2")
-                    .arg(engine->currentStackFrame->v4Function->compilationUnit->runtimeStrings[nameIndex]->toQString(),
-                         base->toQStringNoThrow());
+                    .arg(name->toQString(), base->toQStringNoThrow());
             return engine->throwTypeError(message);
         }
 
-        ScopedValue thisObject(scope, RuntimeHelpers::convertToObject(engine, *base));
-        if (engine->hasException) // type error
-            return Encode::undefined();
-        base = thisObject;
+        if (base->isManaged()) {
+            Managed *m = static_cast<Managed *>(base);
+            lookupObject = m->internalClass()->prototype;
+            Q_ASSERT(m->internalClass()->prototype);
+        } else {
+            lookupObject = RuntimeHelpers::convertToObject(engine, *base);
+            if (engine->hasException) // type error
+                return Encode::undefined();
+            base = lookupObject;
+        }
     }
 
-    ScopedString name(scope, engine->currentStackFrame->v4Function->compilationUnit->runtimeStrings[nameIndex]);
-    ScopedFunctionObject f(scope, static_cast<Object *>(base)->get(name));
+    ScopedFunctionObject f(scope, static_cast<Object *>(lookupObject)->get(name));
 
     if (!f) {
         QString error = QStringLiteral("Property '%1' of object %2 is not a function")
@@ -1080,7 +1093,7 @@ ReturnedValue Runtime::method_callElement(ExecutionEngine *engine, Value *base,
     ScopedValue thisObject(scope, base->toObject(engine));
     base = thisObject;
 
-    ScopedString str(scope, index.toString(engine));
+    ScopedStringOrSymbol str(scope, index.toStringOrSymbol(engine));
     if (engine->hasException)
         return Encode::undefined();
 
diff --git a/src/qml/jsruntime/qv4string.cpp b/src/qml/jsruntime/qv4string.cpp
index 81044103eb..78377286de 100644
--- a/src/qml/jsruntime/qv4string.cpp
+++ b/src/qml/jsruntime/qv4string.cpp
@@ -138,7 +138,8 @@ void Heap::ComplexString::init(Heap::String *ref, int from, int len)
     this->len = len;
 }
 
-void Heap::String::destroy() {
+void Heap::StringOrSymbol::destroy()
+{
     if (text) {
         internalClass->engine->memoryManager->changeUnmanagedHeapSizeUsage(qptrdiff(-text->size) * (int)sizeof(QChar));
         if (!text->ref.deref())
diff --git a/src/qml/jsruntime/qv4string_p.h b/src/qml/jsruntime/qv4string_p.h
index 4e35853f69..c48f46fe6e 100644
--- a/src/qml/jsruntime/qv4string_p.h
+++ b/src/qml/jsruntime/qv4string_p.h
@@ -68,7 +68,9 @@ struct Q_QML_PRIVATE_EXPORT StringOrSymbol : Base
 {
     mutable QStringData *text;
     mutable Identifier identifier;
+
     static void markObjects(Heap::Base *that, MarkStack *markStack);
+    void destroy();
 
     inline QString toQString() const {
         if (!text)
@@ -96,7 +98,6 @@ struct Q_QML_PRIVATE_EXPORT String : StringOrSymbol {
     }
 
     void init(const QString &text);
-    void destroy();
     void simplifyString() const;
     int length() const;
     std::size_t retainedTextSize() const {
@@ -175,6 +176,9 @@ struct Q_QML_PRIVATE_EXPORT StringOrSymbol : public Managed {
 
     uint asArrayIndex() const;
 
+    inline QString toQString() const {
+        return d()->toQString();
+    }
 #endif
 };
 
diff --git a/src/qml/jsruntime/qv4stringobject.cpp b/src/qml/jsruntime/qv4stringobject.cpp
index 61176b3706..db873d32e2 100644
--- a/src/qml/jsruntime/qv4stringobject.cpp
+++ b/src/qml/jsruntime/qv4stringobject.cpp
@@ -44,6 +44,7 @@
 #include "qv4objectproto_p.h"
 #include <private/qv4mm_p.h>
 #include "qv4scopedvalue_p.h"
+#include "qv4symbol_p.h"
 #include "qv4alloca_p.h"
 #include "qv4jscall_p.h"
 #include <QtCore/QDateTime>
@@ -152,16 +153,18 @@ ReturnedValue StringCtor::callAsConstructor(const FunctionObject *f, const Value
         value = argv[0].toString(v4);
     else
         value = v4->newString();
+    CHECK_EXCEPTION();
     return Encode(v4->newStringObject(value));
 }
 
 ReturnedValue StringCtor::call(const FunctionObject *m, const Value *, const Value *argv, int argc)
 {
     ExecutionEngine *v4 = m->engine();
-    if (argc)
-        return argv[0].toString(v4)->asReturnedValue();
-    else
+    if (!argc)
         return v4->newString()->asReturnedValue();
+    if (argv[0].isSymbol())
+        return v4->newString(argv[0].symbolValue()->descriptiveString())->asReturnedValue();
+    return argv[0].toString(v4)->asReturnedValue();
 }
 
 void StringPrototype::init(ExecutionEngine *engine, Object *ctor)
@@ -169,6 +172,11 @@ void StringPrototype::init(ExecutionEngine *engine, Object *ctor)
     Scope scope(engine);
     ScopedObject o(scope);
 
+    // need to set this once again, as these were not fully defined when creating the string proto
+    Heap::InternalClass *ic = scope.engine->classes[ExecutionEngine::Class_StringObject]->changePrototype(scope.engine->objectPrototype()->d());
+    d()->internalClass.set(scope.engine, ic);
+    d()->string.set(scope.engine, scope.engine->id_empty()->d());
+
     ctor->defineReadonlyProperty(engine->id_prototype(), (o = this));
     ctor->defineReadonlyProperty(engine->id_length(), Primitive::fromInt32(1));
     ctor->defineDefaultProperty(QStringLiteral("fromCharCode"), method_fromCharCode, 1);
diff --git a/src/qml/jsruntime/qv4symbol.cpp b/src/qml/jsruntime/qv4symbol.cpp
new file mode 100644
index 0000000000..82602c7cf2
--- /dev/null
+++ b/src/qml/jsruntime/qv4symbol.cpp
@@ -0,0 +1,116 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtQml module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+
+#include <qv4symbol_p.h>
+#include <qv4functionobject_p.h>
+
+using namespace QV4;
+
+DEFINE_OBJECT_VTABLE(SymbolCtor);
+DEFINE_MANAGED_VTABLE(Symbol);
+
+void Heap::Symbol::init(Heap::String *description)
+{
+    identifier = Identifier::fromHeapObject(this);
+    QString desc = description->toQString();
+    text = desc.data_ptr();
+    text->ref.ref();
+}
+
+void Heap::SymbolCtor::init(QV4::ExecutionContext *scope)
+{
+    Heap::FunctionObject::init(scope, QStringLiteral("Symbol"));
+}
+
+ReturnedValue QV4::SymbolCtor::call(const QV4::FunctionObject *f, const QV4::Value *, const QV4::Value *argv, int argc)
+{
+    Scope scope(f);
+    ScopedString s(scope);
+    if (argc)
+        s = argv[0].toString(scope.engine);
+    if (scope.hasException())
+        return Encode::undefined();
+    return f->engine()->memoryManager->alloc<Symbol>(s->d())->asReturnedValue();
+}
+
+ReturnedValue SymbolCtor::method_for(const FunctionObject *f, const Value *thisObject, const Value *argv, int argc)
+{
+//    Scope scope(f);
+//    ScopedValue k(s, argc ? argv[0]: Encode::undefined());
+//    ScopedString key(scope, k->toString(scope.engine));
+//    CHECK_EXCEPTION();
+}
+
+ReturnedValue SymbolCtor::method_keyFor(const FunctionObject *f, const Value *thisObject, const Value *argv, int argc)
+{
+}
+
+void SymbolPrototype::init(ExecutionEngine *engine, Object *ctor)
+{
+    Scope scope(engine);
+    ScopedValue v(scope);
+    ctor->defineReadonlyProperty(engine->id_prototype(), (v = this));
+
+    defineDefaultProperty(QStringLiteral("toString"), method_toString);
+    defineDefaultProperty(QStringLiteral("valueOf"), method_valueOf);
+}
+
+ReturnedValue SymbolPrototype::method_toString(const FunctionObject *f, const Value *thisObject, const Value *, int)
+{
+    ExecutionEngine *e = f->engine();
+    const Symbol *s = thisObject->as<Symbol>();
+    if (!s)
+        return e->throwTypeError();
+    return e->newString(s->descriptiveString())->asReturnedValue();
+}
+
+ReturnedValue SymbolPrototype::method_valueOf(const FunctionObject *f, const Value *thisObject, const Value *argv, int argc)
+{
+    const Symbol *s = thisObject->as<Symbol>();
+    if (!s) {
+        ExecutionEngine *e = f->engine();
+        return e->throwTypeError();
+    }
+    return s->asReturnedValue();
+}
+
+QString Symbol::descriptiveString() const
+{
+    return QLatin1String("Symbol(") + toQString() + QLatin1String(")");
+}
diff --git a/src/qml/jsruntime/qv4symbol_p.h b/src/qml/jsruntime/qv4symbol_p.h
new file mode 100644
index 0000000000..c33833a971
--- /dev/null
+++ b/src/qml/jsruntime/qv4symbol_p.h
@@ -0,0 +1,105 @@
+/****************************************************************************
+**
+** Copyright (C) 2018 The Qt Company Ltd.
+** Contact: https://www.qt.io/licensing/
+**
+** This file is part of the QtQml module of the Qt Toolkit.
+**
+** $QT_BEGIN_LICENSE:LGPL$
+** Commercial License Usage
+** Licensees holding valid commercial Qt licenses may use this file in
+** accordance with the commercial license agreement provided with the
+** Software or, alternatively, in accordance with the terms contained in
+** a written agreement between you and The Qt Company. For licensing terms
+** and conditions see https://www.qt.io/terms-conditions. For further
+** information use the contact form at https://www.qt.io/contact-us.
+**
+** GNU Lesser General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU Lesser
+** General Public License version 3 as published by the Free Software
+** Foundation and appearing in the file LICENSE.LGPL3 included in the
+** packaging of this file. Please review the following information to
+** ensure the GNU Lesser General Public License version 3 requirements
+** will be met: https://www.gnu.org/licenses/lgpl-3.0.html.
+**
+** GNU General Public License Usage
+** Alternatively, this file may be used under the terms of the GNU
+** General Public License version 2.0 or (at your option) the GNU General
+** Public license version 3 or any later version approved by the KDE Free
+** Qt Foundation. The licenses are as published by the Free Software
+** Foundation and appearing in the file LICENSE.GPL2 and LICENSE.GPL3
+** included in the packaging of this file. Please review the following
+** information to ensure the GNU General Public License requirements will
+** be met: https://www.gnu.org/licenses/gpl-2.0.html and
+** https://www.gnu.org/licenses/gpl-3.0.html.
+**
+** $QT_END_LICENSE$
+**
+****************************************************************************/
+#ifndef QV4_SYMBOL_H
+#define QV4_SYMBOL_H
+
+//
+//  W A R N I N G
+//  -------------
+//
+// This file is not part of the Qt API.  It exists purely as an
+// implementation detail.  This header file may change from version to
+// version without notice, or even be removed.
+//
+// We mean it.
+//
+
+#include "qv4string_p.h"
+#include "qv4functionobject_p.h"
+
+QT_BEGIN_NAMESPACE
+
+
+namespace QV4 {
+
+namespace Heap {
+
+struct SymbolCtor : FunctionObject {
+    void init(QV4::ExecutionContext *scope);
+};
+
+struct Symbol : StringOrSymbol {
+    void init(Heap::String *description);
+};
+
+}
+
+struct SymbolCtor : FunctionObject
+{
+    V4_OBJECT2(SymbolCtor, FunctionObject)
+
+    static ReturnedValue call(const FunctionObject *, const Value *thisObject, const Value *argv, int argc);
+    static ReturnedValue method_for(const FunctionObject *, const Value *thisObject, const Value *argv, int argc);
+    static ReturnedValue method_keyFor(const FunctionObject *, const Value *thisObject, const Value *argv, int argc);
+};
+
+struct SymbolPrototype : Object
+{
+    V4_PROTOTYPE(objectPrototype)
+    void init(ExecutionEngine *engine, Object *ctor);
+
+    static ReturnedValue method_toString(const FunctionObject *, const Value *thisObject, const Value *argv, int argc);
+    static ReturnedValue method_valueOf(const FunctionObject *, const Value *thisObject, const Value *argv, int argc);
+};
+
+struct Symbol : StringOrSymbol
+{
+    V4_MANAGED(Symbol, StringOrSymbol)
+    Q_MANAGED_TYPE(Symbol)
+    V4_INTERNALCLASS(Symbol)
+    V4_NEEDS_DESTROY
+
+    QString descriptiveString() const;
+};
+
+}
+
+QT_END_NAMESPACE
+
+#endif
diff --git a/src/qml/jsruntime/qv4value.cpp b/src/qml/jsruntime/qv4value.cpp
index c558c27521..e095a2b720 100644
--- a/src/qml/jsruntime/qv4value.cpp
+++ b/src/qml/jsruntime/qv4value.cpp
@@ -40,6 +40,7 @@
 #include <qv4runtime_p.h>
 #include <qv4string_p.h>
 #ifndef V4_BOOTSTRAP
+#include <qv4symbol_p.h>
 #include <qv4object_p.h>
 #include <qv4objectproto_p.h>
 #include <private/qv4mm_p.h>
@@ -145,6 +146,8 @@ QString Value::toQStringNoThrow() const
     case Value::Managed_Type:
         if (String *s = stringValue())
             return s->toQString();
+        if (Symbol *s = symbolValue())
+            return s->descriptiveString();
         {
             Q_ASSERT(isObject());
             Scope scope(objectValue()->engine());
diff --git a/src/qml/jsruntime/qv4value_p.h b/src/qml/jsruntime/qv4value_p.h
index 85c345f645..b89011a9a0 100644
--- a/src/qml/jsruntime/qv4value_p.h
+++ b/src/qml/jsruntime/qv4value_p.h
@@ -365,7 +365,17 @@ public:
     QML_NEARLY_ALWAYS_INLINE String *stringValue() const {
         if (!isString())
             return nullptr;
-        return reinterpret_cast<String*>(const_cast<Value *>(this));
+        return reinterpret_cast<String *>(const_cast<Value *>(this));
+    }
+    QML_NEARLY_ALWAYS_INLINE StringOrSymbol *stringOrSymbolValue() const {
+        if (!isStringOrSymbol())
+            return nullptr;
+        return reinterpret_cast<StringOrSymbol *>(const_cast<Value *>(this));
+    }
+    QML_NEARLY_ALWAYS_INLINE Symbol *symbolValue() const {
+        if (!isSymbol())
+            return nullptr;
+        return reinterpret_cast<Symbol *>(const_cast<Value *>(this));
     }
     QML_NEARLY_ALWAYS_INLINE Object *objectValue() const {
         if (!isObject())
@@ -411,6 +421,11 @@ public:
             return reinterpret_cast<Heap::String *>(m());
         return toString(e, *this);
     }
+    Heap::StringOrSymbol *toStringOrSymbol(ExecutionEngine *e) const {
+        if (isStringOrSymbol())
+            return reinterpret_cast<Heap::StringOrSymbol *>(m());
+        return reinterpret_cast<Heap::StringOrSymbol *>(toString(e, *this));
+    }
     static Heap::String *toString(ExecutionEngine *e, Value val);
     Heap::Object *toObject(ExecutionEngine *e) const {
         if (isObject())
@@ -518,7 +533,9 @@ bool Value::isSymbol() const
     Heap::Base *b = heapObject();
     return b && b->internalClass->vtable->isStringOrSymbol && !b->internalClass->vtable->isString;
 }
+
 inline bool Value::isObject() const
+
 {
     Heap::Base *b = heapObject();
     return b && b->internalClass->vtable->isObject;