Issue #1570 - Implement globalThis

This resolves #1570

js/public/Class.h

@@ -779,7 +779,7 @@ struct JSClass {
 // application.
 #define JSCLASS_GLOBAL_APPLICATION_SLOTS 5
 #define JSCLASS_GLOBAL_SLOT_COUNT                                             \
-    (JSCLASS_GLOBAL_APPLICATION_SLOTS + JSProto_LIMIT * 2 + 45)
+    (JSCLASS_GLOBAL_APPLICATION_SLOTS + JSProto_LIMIT * 2 + 46)
 #define JSCLASS_GLOBAL_FLAGS_WITH_SLOTS(n)                                    \
     (JSCLASS_IS_GLOBAL | JSCLASS_HAS_RESERVED_SLOTS(JSCLASS_GLOBAL_SLOT_COUNT + (n)))
 #define JSCLASS_GLOBAL_FLAGS                                                  \

js/src/jsapi.cpp

@@ -1029,13 +1029,17 @@ JS_ResolveStandardClass(JSContext* cx, HandleObject obj, HandleId id, bool* reso
 
     /* Check whether we're resolving 'undefined', and define it if so. */
     JSAtom* idAtom = JSID_TO_ATOM(id);
-    JSAtom* undefinedAtom = cx->names().undefined;
-    if (idAtom == undefinedAtom) {
+    if (idAtom == cx->names().undefined) {
         *resolved = true;
         return DefineProperty(cx, global, id, UndefinedHandleValue, nullptr, nullptr,
                               JSPROP_PERMANENT | JSPROP_READONLY | JSPROP_RESOLVING);
     }
 
+    // Resolve a "globalThis" self-referential property if necessary.
+    if (idAtom == cx->names().globalThis) {
+        return GlobalObject::maybeResolveGlobalThis(cx, global, resolved);
+    }
+
     /* Try for class constructors/prototypes named by well-known atoms. */
     stdnm = LookupStdName(cx->names(), idAtom, standard_class_names);
 
@@ -1088,6 +1092,7 @@ JS_MayResolveStandardClass(const JSAtomState& names, jsid id, JSObject* maybeObj
     // better, we need a JSContext here; it's fine as it is.)
 
     return atom == names.undefined ||
+           atom == names.globalThis ||
            LookupStdName(names, atom, standard_class_names) ||
            LookupStdName(names, atom, builtin_property_names);
 }

js/src/vm/CommonPropertyNames.h

@@ -160,6 +160,7 @@
     macro(getPropertyDescriptor, getPropertyDescriptor, "getPropertyDescriptor") \
     macro(getPrototypeOf, getPrototypeOf, "getPrototypeOf") \
     macro(global, global, "global") \
+    macro(globalThis, globalThis, "globalThis") \
     macro(Handle, Handle, "Handle") \
     macro(has, has, "has") \
     macro(hasOwn, hasOwn, "hasOwn") \

js/src/vm/GlobalObject.cpp

@@ -296,6 +296,32 @@ GlobalObject::initBuiltinConstructor(JSContext* cx, Handle<GlobalObject*> global
     return true;
 }
 
+// Resolve a "globalThis" self-referential property if necessary,
+// per a stage-3 proposal. https://github.com/tc39/ecma262/pull/702
+//
+// We could also do this in |FinishObjectClassInit| to trim the global
+// resolve hook.  Unfortunately, |ToWindowProxyIfWindow| doesn't work then:
+// the browser's |nsGlobalWindow::SetNewDocument| invokes Object init
+// *before* it sets the global's WindowProxy using |js::SetWindowProxy|.
+//
+// Refactoring global object creation code to support this approach is a
+// challenge for another day.
+/* static */ bool
+GlobalObject::maybeResolveGlobalThis(JSContext* cx, Handle<GlobalObject*> global, bool* resolved)
+{
+    if (global->getSlot(GLOBAL_THIS_RESOLVED).isUndefined()) {
+        RootedValue v(cx, ObjectValue(*ToWindowProxyIfWindow(global)));
+        if (!DefineProperty(cx, global, cx->names().globalThis, v, nullptr, nullptr, JSPROP_RESOLVING)) {
+            return false;
+        }
+
+        *resolved = true;
+        global->setSlot(GLOBAL_THIS_RESOLVED, BooleanValue(true));
+    }
+
+    return true;
+}
+
 GlobalObject*
 GlobalObject::createInternal(JSContext* cx, const Class* clasp)
 {
@@ -419,6 +445,12 @@ GlobalObject::initStandardClasses(JSContext* cx, Handle<GlobalObject*> global)
         return false;
     }
 
+    // Resolve a "globalThis" self-referential property if necessary.
+    bool resolved;
+    if (!GlobalObject::maybeResolveGlobalThis(cx, global, &resolved)) {
+        return false;
+    }
+
     for (size_t k = 0; k < JSProto_LIMIT; ++k) {
         if (!ensureConstructor(cx, global, static_cast<JSProtoKey>(k)))
             return false;

js/src/vm/GlobalObject.h

@@ -121,6 +121,7 @@ class GlobalObject : public NativeObject
         FOR_OF_PIC_CHAIN,
         MODULE_RESOLVE_HOOK,
         WINDOW_PROXY,
+        GLOBAL_THIS_RESOLVED,
 
         /* Total reserved-slot count for global objects. */
         RESERVED_SLOTS
@@ -171,6 +172,8 @@ class GlobalObject : public NativeObject
     static bool initBuiltinConstructor(JSContext* cx, Handle<GlobalObject*> global,
                                        JSProtoKey key, HandleObject ctor, HandleObject proto);
 
+    static bool maybeResolveGlobalThis(JSContext* cx, Handle<GlobalObject*> global, bool* resolved);
+
     void setConstructor(JSProtoKey key, const Value& v) {
         MOZ_ASSERT(key <= JSProto_LIMIT);
         setSlot(APPLICATION_SLOTS + key, v);