Bug 1331092 - Part 2: Implement Async Generator except yield*.

Tag #1287

22 files changed, 1013 insertions, 54 deletions

diff --git a/js/src/builtin/AsyncIteration.js b/js/src/builtin/AsyncIteration.js
new file mode 100644
index 000000000..029c7c0f8
--- /dev/null
+++ b/js/src/builtin/AsyncIteration.js
@@ -0,0 +1,7 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+function AsyncIteratorIdentity() {
+    return this;
+}
diff --git a/js/src/builtin/Promise.h b/js/src/builtin/Promise.h
index 8f716dfbc..680ab2abd 100644
--- a/js/src/builtin/Promise.h
+++ b/js/src/builtin/Promise.h
@@ -151,6 +151,23 @@ AsyncFunctionThrown(JSContext* cx, Handle<PromiseObject*> resultPromise);
 MOZ_MUST_USE bool
 AsyncFunctionAwait(JSContext* cx, Handle<PromiseObject*> resultPromise, HandleValue value);
 
+class AsyncGeneratorObject;
+
+MOZ_MUST_USE bool
+AsyncGeneratorAwait(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj, HandleValue value);
+
+MOZ_MUST_USE bool
+AsyncGeneratorResolve(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                      HandleValue value, bool done);
+
+MOZ_MUST_USE bool
+AsyncGeneratorReject(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                     HandleValue exception);
+
+MOZ_MUST_USE bool
+AsyncGeneratorEnqueue(JSContext* cx, HandleValue asyncGenVal, CompletionKind completionKind,
+                      HandleValue completionValue, MutableHandleValue result);
+
 /**
  * A PromiseTask represents a task that can be dispatched to a helper thread
  * (via StartPromiseTask), executed (by implementing PromiseTask::execute()),
diff --git a/js/src/frontend/BytecodeCompiler.cpp b/js/src/frontend/BytecodeCompiler.cpp
index 7760c4b2e..ccfe3cd2e 100644
--- a/js/src/frontend/BytecodeCompiler.cpp
+++ b/js/src/frontend/BytecodeCompiler.cpp
@@ -729,3 +729,16 @@ frontend::CompileStandaloneAsyncFunction(JSContext* cx, MutableHandleFunction fu
                               TraceLogger_ParserCompileFunction);
     return compiler.compileStandaloneFunction(fun, NotGenerator, AsyncFunction, parameterListEnd);
 }
+
+bool
+frontend::CompileStandaloneAsyncGenerator(JSContext* cx, MutableHandleFunction fun,
+                                          const ReadOnlyCompileOptions& options,
+                                          JS::SourceBufferHolder& srcBuf,
+                                          Maybe<uint32_t> parameterListEnd)
+{
+    RootedScope emptyGlobalScope(cx, &cx->global()->emptyGlobalScope());
+
+    BytecodeCompiler compiler(cx, cx->tempLifoAlloc(), options, srcBuf, emptyGlobalScope,
+                              TraceLogger_ParserCompileFunction);
+    return compiler.compileStandaloneFunction(fun, StarGenerator, AsyncFunction, parameterListEnd);
+}
diff --git a/js/src/frontend/BytecodeCompiler.h b/js/src/frontend/BytecodeCompiler.h
index 0bc1ab2ab..029fbe632 100644
--- a/js/src/frontend/BytecodeCompiler.h
+++ b/js/src/frontend/BytecodeCompiler.h
@@ -85,6 +85,12 @@ CompileStandaloneAsyncFunction(JSContext* cx, MutableHandleFunction fun,
                                mozilla::Maybe<uint32_t> parameterListEnd);
 
 MOZ_MUST_USE bool
+CompileStandaloneAsyncGenerator(JSContext* cx, MutableHandleFunction fun,
+                                const ReadOnlyCompileOptions& options,
+                                JS::SourceBufferHolder& srcBuf,
+                                mozilla::Maybe<uint32_t> parameterListEnd);
+
+MOZ_MUST_USE bool
 CompileAsyncFunctionBody(JSContext* cx, MutableHandleFunction fun,
                          const ReadOnlyCompileOptions& options,
                          Handle<PropertyNameVector> formals, JS::SourceBufferHolder& srcBuf);
diff --git a/js/src/frontend/BytecodeEmitter.cpp b/js/src/frontend/BytecodeEmitter.cpp
index a98016d63..9d6a68609 100644
--- a/js/src/frontend/BytecodeEmitter.cpp
+++ b/js/src/frontend/BytecodeEmitter.cpp
@@ -8067,7 +8067,8 @@ BytecodeEmitter::emitFunction(ParseNode* pn, bool needsProto)
         MOZ_ASSERT(fun->isArrow() == (pn->getOp() == JSOP_LAMBDA_ARROW));
         if (funbox->isAsync()) {
             MOZ_ASSERT(!needsProto);
-            return emitAsyncWrapper(index, funbox->needsHomeObject(), fun->isArrow());
+            return emitAsyncWrapper(index, funbox->needsHomeObject(), fun->isArrow(),
+                                    fun->isStarGenerator());
         }
 
         if (fun->isArrow()) {
@@ -8122,8 +8123,11 @@ BytecodeEmitter::emitFunction(ParseNode* pn, bool needsProto)
             MOZ_ASSERT(pn->getOp() == JSOP_NOP);
             switchToPrologue();
             if (funbox->isAsync()) {
-                if (!emitAsyncWrapper(index, fun->isMethod(), fun->isArrow()))
+                if (!emitAsyncWrapper(index, fun->isMethod(), fun->isArrow(),
+                                      fun->isStarGenerator()))
+                {
                     return false;
+                }
             } else {
                 if (!emitIndex32(JSOP_LAMBDA, index))
                     return false;
@@ -8139,10 +8143,12 @@ BytecodeEmitter::emitFunction(ParseNode* pn, bool needsProto)
         // initialize the binding name of the function in the current scope.
 
         bool isAsync = funbox->isAsync();
-        auto emitLambda = [index, isAsync](BytecodeEmitter* bce, const NameLocation&, bool) {
+        bool isStarGenerator = funbox->isStarGenerator();
+        auto emitLambda = [index, isAsync, isStarGenerator](BytecodeEmitter* bce,
+                                                            const NameLocation&, bool) {
             if (isAsync) {
                 return bce->emitAsyncWrapper(index, /* needsHomeObject = */ false,
-                                             /* isArrow = */ false);
+                                             /* isArrow = */ false, isStarGenerator);
             }
             return bce->emitIndexOp(JSOP_LAMBDA, index);
         };
@@ -8177,7 +8183,8 @@ BytecodeEmitter::emitAsyncWrapperLambda(unsigned index, bool isArrow) {
 }
 
 bool
-BytecodeEmitter::emitAsyncWrapper(unsigned index, bool needsHomeObject, bool isArrow)
+BytecodeEmitter::emitAsyncWrapper(unsigned index, bool needsHomeObject, bool isArrow,
+                                  bool isStarGenerator)
 {
     // needsHomeObject can be true for propertyList for extended class.
     // In that case push both unwrapped and wrapped function, in order to
@@ -8209,8 +8216,13 @@ BytecodeEmitter::emitAsyncWrapper(unsigned index, bool needsHomeObject, bool isA
         if (!emit1(JSOP_DUP))
             return false;
     }
-    if (!emit1(JSOP_TOASYNC))
-        return false;
+    if (isStarGenerator) {
+        if (!emit1(JSOP_TOASYNCGEN))
+            return false;
+    } else {
+        if (!emit1(JSOP_TOASYNC))
+            return false;
+    }
     return true;
 }
 
diff --git a/js/src/frontend/BytecodeEmitter.h b/js/src/frontend/BytecodeEmitter.h
index f3f78df16..67e329489 100644
--- a/js/src/frontend/BytecodeEmitter.h
+++ b/js/src/frontend/BytecodeEmitter.h
@@ -620,7 +620,8 @@ struct MOZ_STACK_CLASS BytecodeEmitter
     MOZ_MUST_USE bool emitPropIncDec(ParseNode* pn);
 
     MOZ_MUST_USE bool emitAsyncWrapperLambda(unsigned index, bool isArrow);
-    MOZ_MUST_USE bool emitAsyncWrapper(unsigned index, bool needsHomeObject, bool isArrow);
+    MOZ_MUST_USE bool emitAsyncWrapper(unsigned index, bool needsHomeObject, bool isArrow,
+                                       bool isStarGenerator);
 
     MOZ_MUST_USE bool emitComputedPropertyName(ParseNode* computedPropName);
 
diff --git a/js/src/frontend/Parser.cpp b/js/src/frontend/Parser.cpp
index 3ae5890ac..7704cf65a 100644
--- a/js/src/frontend/Parser.cpp
+++ b/js/src/frontend/Parser.cpp
@@ -3785,10 +3785,12 @@ Parser<ParseHandler>::functionStmt(uint32_t toStringStart, YieldHandling yieldHa
 
     GeneratorKind generatorKind = NotGenerator;
     if (tt == TOK_MUL) {
+#ifdef RELEASE_OR_BETA
         if (asyncKind != SyncFunction) {
             error(JSMSG_ASYNC_GENERATOR);
             return null();
         }
+#endif
         generatorKind = StarGenerator;
         if (!tokenStream.getToken(&tt))
             return null();
@@ -3857,10 +3859,12 @@ Parser<ParseHandler>::functionExpr(uint32_t toStringStart, InvokedPrediction inv
         return null();
 
     if (tt == TOK_MUL) {
+#ifdef RELEASE_OR_BETA
         if (asyncKind != SyncFunction) {
             error(JSMSG_ASYNC_GENERATOR);
             return null();
         }
+#endif
         generatorKind = StarGenerator;
         if (!tokenStream.getToken(&tt))
             return null();
@@ -9468,11 +9472,6 @@ Parser<ParseHandler>::propertyName(YieldHandling yieldHandling,
 
     bool isGenerator = false;
     bool isAsync = false;
-    if (ltok == TOK_MUL) {
-        isGenerator = true;
-        if (!tokenStream.getToken(&ltok))
-            return null();
-    }
 
     if (ltok == TOK_ASYNC) {
         // AsyncMethod[Yield, Await]:
@@ -9500,9 +9499,16 @@ Parser<ParseHandler>::propertyName(YieldHandling yieldHandling,
         }
     }
 
-    if (isAsync && isGenerator) {
-        error(JSMSG_ASYNC_GENERATOR);
-        return null();
+    if (ltok == TOK_MUL) {
+#ifdef RELEASE_OR_BETA
+        if (isAsync) {
+            error(JSMSG_ASYNC_GENERATOR);
+            return null();
+        }
+#endif
+        isGenerator = true;
+        if (!tokenStream.getToken(&ltok))
+            return null();
     }
 
     propAtom.set(nullptr);
diff --git a/js/src/js.msg b/js/src/js.msg
index 7465129bf..2778085c7 100644
--- a/js/src/js.msg
+++ b/js/src/js.msg
@@ -593,3 +593,6 @@ MSG_DEF(JSMSG_PROMISE_ERROR_IN_WRAPPED_REJECTION_REASON,0, JSEXN_INTERNALERR, "P
 // Iterator
 MSG_DEF(JSMSG_RETURN_NOT_CALLABLE,     0, JSEXN_TYPEERR, "property 'return' of iterator is not callable")
 MSG_DEF(JSMSG_ITERATOR_NO_THROW,       0, JSEXN_TYPEERR, "iterator does not have a 'throw' method")
+
+// Async Iteration
+MSG_DEF(JSMSG_NOT_AN_ASYNC_GENERATOR,  0, JSEXN_TYPEERR, "Not an async generator")
diff --git a/js/src/jsapi.cpp b/js/src/jsapi.cpp
index f9a0c6a6b..6483641f3 100644
--- a/js/src/jsapi.cpp
+++ b/js/src/jsapi.cpp
@@ -70,6 +70,7 @@
 #include "js/StructuredClone.h"
 #include "js/Utility.h"
 #include "vm/AsyncFunction.h"
+#include "vm/AsyncIteration.h"
 #include "vm/DateObject.h"
 #include "vm/Debugger.h"
 #include "vm/EnvironmentObject.h"
@@ -3591,6 +3592,11 @@ CloneFunctionObject(JSContext* cx, HandleObject funobj, HandleObject env, Handle
         return nullptr;
     }
 
+    if (IsWrappedAsyncGenerator(fun)) {
+        JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_CANT_CLONE_OBJECT);
+        return nullptr;
+    }
+
     if (CanReuseScriptForClone(cx->compartment(), fun, env)) {
         // If the script is to be reused, either the script can already handle
         // non-syntactic scopes, or there is only the standard global lexical
diff --git a/js/src/jsfun.cpp b/js/src/jsfun.cpp
index 65d44cba7..ce1ca33fe 100644
--- a/js/src/jsfun.cpp
+++ b/js/src/jsfun.cpp
@@ -41,6 +41,7 @@
 #include "js/CallNonGenericMethod.h"
 #include "js/Proxy.h"
 #include "vm/AsyncFunction.h"
+#include "vm/AsyncIteration.h"
 #include "vm/Debugger.h"
 #include "vm/GlobalObject.h"
 #include "vm/Interpreter.h"
@@ -312,6 +313,8 @@ CallerGetterImpl(JSContext* cx, const CallArgs& args)
         JSFunction* callerFun = &callerObj->as<JSFunction>();
         if (IsWrappedAsyncFunction(callerFun))
             callerFun = GetUnwrappedAsyncFunction(callerFun);
+        else if (IsWrappedAsyncGenerator(callerFun))
+            callerFun = GetUnwrappedAsyncGenerator(callerFun);
         MOZ_ASSERT(!callerFun->isBuiltin(), "non-builtin iterator returned a builtin?");
 
         if (callerFun->strict()) {
@@ -364,13 +367,15 @@ static const JSPropertySpec function_properties[] = {
 static bool
 ResolveInterpretedFunctionPrototype(JSContext* cx, HandleFunction fun, HandleId id)
 {
-    MOZ_ASSERT(fun->isInterpreted() || fun->isAsmJSNative());
+    bool isAsyncGenerator = IsWrappedAsyncGenerator(fun);
+
+    MOZ_ASSERT_IF(!isAsyncGenerator, fun->isInterpreted() || fun->isAsmJSNative());
     MOZ_ASSERT(id == NameToId(cx->names().prototype));
 
     // Assert that fun is not a compiler-created function object, which
     // must never leak to script or embedding code and then be mutated.
     // Also assert that fun is not bound, per the ES5 15.3.4.5 ref above.
-    MOZ_ASSERT(!IsInternalFunctionObject(*fun));
+    MOZ_ASSERT_IF(!isAsyncGenerator, !IsInternalFunctionObject(*fun));
     MOZ_ASSERT(!fun->isBoundFunction());
 
     // Make the prototype object an instance of Object with the same parent as
@@ -380,7 +385,9 @@ ResolveInterpretedFunctionPrototype(JSContext* cx, HandleFunction fun, HandleId
     bool isStarGenerator = fun->isStarGenerator();
     Rooted<GlobalObject*> global(cx, &fun->global());
     RootedObject objProto(cx);
-    if (isStarGenerator)
+    if (isAsyncGenerator)
+        objProto = GlobalObject::getOrCreateAsyncGeneratorPrototype(cx, global);
+    else if (isStarGenerator)
         objProto = GlobalObject::getOrCreateStarGeneratorObjectPrototype(cx, global);
     else
         objProto = GlobalObject::getOrCreateObjectPrototype(cx, global);
@@ -396,7 +403,7 @@ ResolveInterpretedFunctionPrototype(JSContext* cx, HandleFunction fun, HandleId
     // non-enumerable, and writable.  However, per the 15 July 2013 ES6 draft,
     // section 15.19.3, the .prototype of a generator function does not link
     // back with a .constructor.
-    if (!isStarGenerator) {
+    if (!isStarGenerator && !isAsyncGenerator) {
         RootedValue objVal(cx, ObjectValue(*fun));
         if (!DefineProperty(cx, proto, cx->names().constructor, objVal, nullptr, nullptr, 0))
             return false;
@@ -446,11 +453,13 @@ fun_resolve(JSContext* cx, HandleObject obj, HandleId id, bool* resolvedp)
          * - Arrow functions
          * - Function.prototype
          */
-        if (fun->isBuiltin())
-            return true;
-        if (!fun->isConstructor()) {
-            if (!fun->isStarGenerator() && !fun->isLegacyGenerator() && !fun->isAsync())
+        if (!IsWrappedAsyncGenerator(fun)) {
+            if (fun->isBuiltin())
                 return true;
+            if (!fun->isConstructor()) {
+                if (!fun->isStarGenerator() && !fun->isLegacyGenerator() && !fun->isAsync())
+                    return true;
+            }
         }
 
         if (!ResolveInterpretedFunctionPrototype(cx, fun, id))
@@ -946,6 +955,11 @@ js::FunctionToString(JSContext* cx, HandleFunction fun, bool prettyPrint)
         return FunctionToString(cx, unwrapped, prettyPrint);
     }
 
+    if (IsWrappedAsyncGenerator(fun)) {
+        RootedFunction unwrapped(cx, GetUnwrappedAsyncGenerator(fun));
+        return FunctionToString(cx, unwrapped, prettyPrint);
+    }
+
     StringBuffer out(cx);
     RootedScript script(cx);
 
@@ -1606,10 +1620,14 @@ FunctionConstructor(JSContext* cx, const CallArgs& args, GeneratorKind generator
                                          &mutedErrors);
 
     const char* introductionType = "Function";
-    if (isAsync)
-        introductionType = "AsyncFunction";
-    else if (generatorKind != NotGenerator)
+    if (isAsync) {
+        if (isStarGenerator)
+            introductionType = "AsyncGenerator";
+        else
+            introductionType = "AsyncFunction";
+    } else if (generatorKind != NotGenerator) {
         introductionType = "GeneratorFunction";
+    }
 
     const char* introducerFilename = filename;
     if (maybeScript && maybeScript->scriptSource()->introducerFilename())
@@ -1737,12 +1755,20 @@ FunctionConstructor(JSContext* cx, const CallArgs& args, GeneratorKind generator
                                               : SourceBufferHolder::NoOwnership;
     bool ok;
     SourceBufferHolder srcBuf(chars.begin().get(), chars.length(), ownership);
-    if (isAsync)
-        ok = frontend::CompileStandaloneAsyncFunction(cx, &fun, options, srcBuf, parameterListEnd);
-    else if (isStarGenerator)
-        ok = frontend::CompileStandaloneGenerator(cx, &fun, options, srcBuf, parameterListEnd);
-    else
-        ok = frontend::CompileStandaloneFunction(cx, &fun, options, srcBuf, parameterListEnd);
+    if (isAsync) {
+        if (isStarGenerator) {
+            ok = frontend::CompileStandaloneAsyncGenerator(cx, &fun, options, srcBuf,
+                                                           parameterListEnd);
+        } else {
+            ok = frontend::CompileStandaloneAsyncFunction(cx, &fun, options, srcBuf,
+                                                          parameterListEnd);
+        }
+    } else {
+        if (isStarGenerator)
+            ok = frontend::CompileStandaloneGenerator(cx, &fun, options, srcBuf, parameterListEnd);
+        else
+            ok = frontend::CompileStandaloneFunction(cx, &fun, options, srcBuf, parameterListEnd);
+    }
 
     // Step 33.
     args.rval().setObject(*fun);
@@ -1768,7 +1794,7 @@ js::AsyncFunctionConstructor(JSContext* cx, unsigned argc, Value* vp)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
 
-    // Save the callee before its reset in FunctionConstructor().
+    // Save the callee before it's reset in FunctionConstructor().
     RootedObject newTarget(cx);
     if (args.isConstructing())
         newTarget = &args.newTarget().toObject();
@@ -1801,6 +1827,40 @@ js::AsyncFunctionConstructor(JSContext* cx, unsigned argc, Value* vp)
 }
 
 bool
+js::AsyncGeneratorConstructor(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Save the callee before its reset in FunctionConstructor().
+    RootedObject newTarget(cx);
+    if (args.isConstructing())
+        newTarget = &args.newTarget().toObject();
+    else
+        newTarget = &args.callee();
+
+    if (!FunctionConstructor(cx, args, StarGenerator, AsyncFunction))
+        return false;
+
+    RootedObject proto(cx);
+    if (!GetPrototypeFromConstructor(cx, newTarget, &proto))
+        return false;
+
+    if (!proto) {
+        proto = GlobalObject::getOrCreateAsyncGenerator(cx, cx->global());
+        if (!proto)
+            return false;
+    }
+
+    RootedFunction unwrapped(cx, &args.rval().toObject().as<JSFunction>());
+    RootedObject wrapped(cx, WrapAsyncGeneratorWithProto(cx, unwrapped, proto));
+    if (!wrapped)
+        return false;
+
+    args.rval().setObject(*wrapped);
+    return true;
+}
+
+bool
 JSFunction::isBuiltinFunctionConstructor()
 {
     return maybeNative() == Function || maybeNative() == Generator;
diff --git a/js/src/jsfun.h b/js/src/jsfun.h
index 9e50387d6..ea9e7af9b 100644
--- a/js/src/jsfun.h
+++ b/js/src/jsfun.h
@@ -660,6 +660,9 @@ Generator(JSContext* cx, unsigned argc, Value* vp);
 extern bool
 AsyncFunctionConstructor(JSContext* cx, unsigned argc, Value* vp);
 
+extern bool
+AsyncGeneratorConstructor(JSContext* cx, unsigned argc, Value* vp);
+
 // Allocate a new function backed by a JSNative.  Note that by default this
 // creates a singleton object.
 extern JSFunction*
diff --git a/js/src/jsiter.cpp b/js/src/jsiter.cpp
index c4da86bdb..c58f32382 100644
--- a/js/src/jsiter.cpp
+++ b/js/src/jsiter.cpp
@@ -916,28 +916,30 @@ js::GetIteratorObject(JSContext* cx, HandleObject obj, uint32_t flags)
     return iterator;
 }
 
+// ES 2017 draft 7.4.7.
 JSObject*
-js::CreateItrResultObject(JSContext* cx, HandleValue value, bool done)
+js::CreateIterResultObject(JSContext* cx, HandleValue value, bool done)
 {
-    // FIXME: We can cache the iterator result object shape somewhere.
-    AssertHeapIsIdle(cx);
+    // Step 1 (implicit).
 
-    RootedObject proto(cx, GlobalObject::getOrCreateObjectPrototype(cx, cx->global()));
-    if (!proto)
+    // Step 2.
+    RootedObject resultObj(cx, NewBuiltinClassInstance<PlainObject>(cx));
+    if (!resultObj)
         return nullptr;
 
-    RootedPlainObject obj(cx, NewObjectWithGivenProto<PlainObject>(cx, proto));
-    if (!obj)
-        return nullptr;
-
-    if (!DefineProperty(cx, obj, cx->names().value, value))
+    // Step 3.
+    if (!DefineProperty(cx, resultObj, cx->names().value, value))
         return nullptr;
 
-    RootedValue doneBool(cx, BooleanValue(done));
-    if (!DefineProperty(cx, obj, cx->names().done, doneBool))
+    // Step 4.
+    if (!DefineProperty(cx, resultObj, cx->names().done,
+                        done ? TrueHandleValue : FalseHandleValue))
+    {
         return nullptr;
+    }
 
-    return obj;
+    // Step 5.
+    return resultObj;
 }
 
 bool
diff --git a/js/src/jsiter.h b/js/src/jsiter.h
index 52eb045c5..17b7df38b 100644
--- a/js/src/jsiter.h
+++ b/js/src/jsiter.h
@@ -216,10 +216,10 @@ ThrowStopIteration(JSContext* cx);
 
 /*
  * Create an object of the form { value: VALUE, done: DONE }.
- * ES6 draft from 2013-09-05, section 25.4.3.4.
+ * ES 2017 draft 7.4.7.
  */
 extern JSObject*
-CreateItrResultObject(JSContext* cx, HandleValue value, bool done);
+CreateIterResultObject(JSContext* cx, HandleValue value, bool done);
 
 extern JSObject*
 InitLegacyIteratorClass(JSContext* cx, HandleObject obj);
diff --git a/js/src/moz.build b/js/src/moz.build
index 1546b8351..d6c2a426b 100644
--- a/js/src/moz.build
+++ b/js/src/moz.build
@@ -305,6 +305,7 @@ UNIFIED_SOURCES += [
     'vm/ArgumentsObject.cpp',
     'vm/ArrayBufferObject.cpp',
     'vm/AsyncFunction.cpp',
+    'vm/AsyncIteration.cpp',
     'vm/Caches.cpp',
     'vm/CallNonGenericMethod.cpp',
     'vm/CharacterEncoding.cpp',
@@ -747,6 +748,7 @@ selfhosted.inputs = [
     'builtin/SelfHostingDefines.h',
     'builtin/Utilities.js',
     'builtin/Array.js',
+    'builtin/AsyncIteration.js',
     'builtin/Classes.js',
     'builtin/Date.js',
     'builtin/Error.js',
diff --git a/js/src/shell/js.cpp b/js/src/shell/js.cpp
index 51cd11fe8..33f7d6bdc 100644
--- a/js/src/shell/js.cpp
+++ b/js/src/shell/js.cpp
@@ -85,6 +85,7 @@
 #include "threading/Thread.h"
 #include "vm/ArgumentsObject.h"
 #include "vm/AsyncFunction.h"
+#include "vm/AsyncIteration.h"
 #include "vm/Compression.h"
 #include "vm/Debugger.h"
 #include "vm/HelperThreads.h"
@@ -2304,6 +2305,8 @@ ValueToScript(JSContext* cx, HandleValue v, JSFunction** funp = nullptr)
     // Get unwrapped async function.
     if (IsWrappedAsyncFunction(fun))
         fun = GetUnwrappedAsyncFunction(fun);
+    if (IsWrappedAsyncGenerator(fun))
+        fun = GetUnwrappedAsyncGenerator(fun);
 
     if (!fun->isInterpreted()) {
         JS_ReportErrorNumberASCII(cx, my_GetErrorMessage, nullptr, JSSMSG_SCRIPTS_ONLY);
diff --git a/js/src/tests/ecma_7/AsyncFunctions/syntax.js b/js/src/tests/ecma_7/AsyncFunctions/syntax.js
index 28e5febc1..0b4b50af4 100644
--- a/js/src/tests/ecma_7/AsyncFunctions/syntax.js
+++ b/js/src/tests/ecma_7/AsyncFunctions/syntax.js
@@ -9,9 +9,6 @@ if (typeof Reflect !== "undefined" && Reflect.parse) {
     assertEq(Reflect.parse("async function a() {}").body[0].async, true);
     assertEq(Reflect.parse("() => {}").body[0].async, undefined);
 
-    // Async generators are not allowed (with regards to spec)
-    assertThrows(() => Reflect.parse("async function* a() {}"), SyntaxError);
-
     // No line terminator after async
     assertEq(Reflect.parse("async\nfunction a(){}").body[0].expression.name, "async");
 
diff --git a/js/src/vm/AsyncIteration.cpp b/js/src/vm/AsyncIteration.cpp
new file mode 100644
index 000000000..04effe1b1
--- /dev/null
+++ b/js/src/vm/AsyncIteration.cpp
@@ -0,0 +1,564 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#include "vm/AsyncIteration.h"
+
+#include "jsarray.h"
+#include "jscompartment.h"
+
+#include "builtin/Promise.h"
+#include "vm/GeneratorObject.h"
+#include "vm/GlobalObject.h"
+#include "vm/Interpreter.h"
+#include "vm/SelfHosting.h"
+
+#include "jscntxtinlines.h"
+#include "jsobjinlines.h"
+
+#include "vm/NativeObject-inl.h"
+
+using namespace js;
+using namespace js::gc;
+
+#define UNWRAPPED_ASYNC_WRAPPED_SLOT 1
+#define WRAPPED_ASYNC_UNWRAPPED_SLOT 0
+
+// Async Iteration proposal 2.3.10 Runtime Semantics: EvaluateBody.
+static bool
+WrappedAsyncGenerator(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    RootedFunction wrapped(cx, &args.callee().as<JSFunction>());
+    RootedValue unwrappedVal(cx, wrapped->getExtendedSlot(WRAPPED_ASYNC_UNWRAPPED_SLOT));
+    RootedFunction unwrapped(cx, &unwrappedVal.toObject().as<JSFunction>());
+    RootedValue thisValue(cx, args.thisv());
+
+    // Step 1.
+    RootedValue generatorVal(cx);
+    InvokeArgs args2(cx);
+    if (!args2.init(cx, argc))
+        return false;
+    for (size_t i = 0, len = argc; i < len; i++)
+        args2[i].set(args[i]);
+    if (!Call(cx, unwrappedVal, thisValue, args2, &generatorVal))
+        return false;
+
+    // Step 2.
+    Rooted<AsyncGeneratorObject*> asyncGenObj(
+        cx, AsyncGeneratorObject::create(cx, wrapped, generatorVal));
+    if (!asyncGenObj)
+        return false;
+
+    // Step 3 (skipped).
+    // Done in AsyncGeneratorObject::create and generator.
+
+    // Step 4.
+    args.rval().setObject(*asyncGenObj);
+    return true;
+}
+
+JSObject*
+js::WrapAsyncGeneratorWithProto(JSContext* cx, HandleFunction unwrapped, HandleObject proto)
+{
+    MOZ_ASSERT(unwrapped->isAsync());
+    MOZ_ASSERT(proto, "We need an explicit prototype to avoid the default"
+                      "%FunctionPrototype% fallback in NewFunctionWithProto().");
+
+    // Create a new function with AsyncGeneratorPrototype, reusing the name and
+    // the length of `unwrapped`.
+
+    RootedAtom funName(cx, unwrapped->explicitName());
+    uint16_t length;
+    if (!JSFunction::getLength(cx, unwrapped, &length))
+        return nullptr;
+
+    RootedFunction wrapped(cx, NewFunctionWithProto(cx, WrappedAsyncGenerator, length,
+                                                    JSFunction::NATIVE_FUN, nullptr,
+                                                    funName, proto,
+                                                    AllocKind::FUNCTION_EXTENDED,
+                                                    TenuredObject));
+    if (!wrapped)
+        return nullptr;
+
+    if (unwrapped->hasCompileTimeName())
+        wrapped->setCompileTimeName(unwrapped->compileTimeName());
+
+    // Link them to each other to make GetWrappedAsyncGenerator and
+    // GetUnwrappedAsyncGenerator work.
+    unwrapped->setExtendedSlot(UNWRAPPED_ASYNC_WRAPPED_SLOT, ObjectValue(*wrapped));
+    wrapped->setExtendedSlot(WRAPPED_ASYNC_UNWRAPPED_SLOT, ObjectValue(*unwrapped));
+
+    return wrapped;
+}
+
+JSObject*
+js::WrapAsyncGenerator(JSContext* cx, HandleFunction unwrapped)
+{
+    RootedObject proto(cx, GlobalObject::getOrCreateAsyncGenerator(cx, cx->global()));
+    if (!proto)
+        return nullptr;
+
+    return WrapAsyncGeneratorWithProto(cx, unwrapped, proto);
+}
+
+bool
+js::IsWrappedAsyncGenerator(JSFunction* fun)
+{
+    return fun->maybeNative() == WrappedAsyncGenerator;
+}
+
+JSFunction*
+js::GetWrappedAsyncGenerator(JSFunction* unwrapped)
+{
+    MOZ_ASSERT(unwrapped->isAsync());
+    return &unwrapped->getExtendedSlot(UNWRAPPED_ASYNC_WRAPPED_SLOT).toObject().as<JSFunction>();
+}
+
+JSFunction*
+js::GetUnwrappedAsyncGenerator(JSFunction* wrapped)
+{
+    MOZ_ASSERT(IsWrappedAsyncGenerator(wrapped));
+    JSFunction* unwrapped = &wrapped->getExtendedSlot(WRAPPED_ASYNC_UNWRAPPED_SLOT)
+                            .toObject().as<JSFunction>();
+    MOZ_ASSERT(unwrapped->isAsync());
+    return unwrapped;
+}
+
+static MOZ_MUST_USE bool
+AsyncGeneratorResume(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                     CompletionKind completionKind, HandleValue argument);
+
+// Async Iteration proposal 5.1.1 Await Fulfilled Functions.
+MOZ_MUST_USE bool
+js::AsyncGeneratorAwaitedFulfilled(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                                   HandleValue value)
+{
+    return AsyncGeneratorResume(cx, asyncGenObj, CompletionKind::Normal, value);
+}
+
+// Async Iteration proposal 5.1.2 Await Rejected Functions.
+MOZ_MUST_USE bool
+js::AsyncGeneratorAwaitedRejected(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                                  HandleValue reason)
+{
+    return AsyncGeneratorResume(cx, asyncGenObj, CompletionKind::Throw, reason);
+}
+
+// Async Iteration proposal 6.4.1.2 AsyncGenerator.prototype.next.
+static bool
+AsyncGeneratorNext(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Steps 1-3.
+    return AsyncGeneratorEnqueue(cx, args.thisv(), CompletionKind::Normal, args.get(0),
+                                 args.rval());
+}
+
+// Async Iteration proposal 6.4.1.3 AsyncGenerator.prototype.return.
+static bool
+AsyncGeneratorReturn(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Steps 1-3.
+    return AsyncGeneratorEnqueue(cx, args.thisv(), CompletionKind::Return, args.get(0),
+                                 args.rval());
+}
+
+// Async Iteration proposal 6.4.1.4 AsyncGenerator.prototype.throw.
+static bool
+AsyncGeneratorThrow(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+
+    // Steps 1-3.
+    return AsyncGeneratorEnqueue(cx, args.thisv(), CompletionKind::Throw, args.get(0),
+                                 args.rval());
+}
+
+const Class AsyncGeneratorObject::class_ = {
+    "AsyncGenerator",
+    JSCLASS_HAS_RESERVED_SLOTS(AsyncGeneratorObject::Slots)
+};
+
+// ES 2017 draft 9.1.13.
+template <typename ProtoGetter>
+static JSObject*
+OrdinaryCreateFromConstructor(JSContext* cx, HandleFunction fun,
+                              ProtoGetter protoGetter, const Class* clasp)
+{
+    // Step 1 (skipped).
+
+    // Step 2.
+    RootedValue protoVal(cx);
+    if (!GetProperty(cx, fun, fun, cx->names().prototype, &protoVal))
+        return nullptr;
+
+    RootedObject proto(cx, protoVal.isObject() ? &protoVal.toObject() : nullptr);
+    if (!proto) {
+        proto = protoGetter(cx, cx->global());
+        if (!proto)
+            return nullptr;
+    }
+
+    // Step 3.
+    return NewNativeObjectWithGivenProto(cx, clasp, proto);
+}
+
+/* static */ AsyncGeneratorObject*
+AsyncGeneratorObject::create(JSContext* cx, HandleFunction asyncGen, HandleValue generatorVal)
+{
+    MOZ_ASSERT(generatorVal.isObject());
+    MOZ_ASSERT(generatorVal.toObject().is<GeneratorObject>());
+
+    RootedObject obj(
+        cx, OrdinaryCreateFromConstructor(cx, asyncGen,
+                                          GlobalObject::getOrCreateAsyncGeneratorPrototype,
+                                          &class_));
+    if (!obj)
+        return nullptr;
+
+    Handle<AsyncGeneratorObject*> asyncGenObj = obj.as<AsyncGeneratorObject>();
+
+    // Async Iteration proposal 6.4.3.2 AsyncGeneratorStart.
+    // Step 6.
+    asyncGenObj->setGenerator(generatorVal);
+
+    // Step 7.
+    asyncGenObj->setSuspendedStart();
+
+    // Step 8.
+    asyncGenObj->clearSingleQueueRequest();
+
+    return asyncGenObj;
+}
+
+/* static */ MOZ_MUST_USE bool
+AsyncGeneratorObject::enqueueRequest(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                                     Handle<AsyncGeneratorRequest*> request)
+{
+    if (asyncGenObj->isSingleQueue()) {
+        if (asyncGenObj->isSingleQueueEmpty()) {
+            asyncGenObj->setSingleQueueRequest(request);
+            return true;
+        }
+
+        RootedArrayObject queue(cx, NewDenseEmptyArray(cx));
+        if (!queue)
+            return false;
+
+        if (!NewbornArrayPush(cx, queue, ObjectValue(*asyncGenObj->singleQueueRequest())))
+            return false;
+        if (!NewbornArrayPush(cx, queue, ObjectValue(*request)))
+            return false;
+
+        asyncGenObj->setQueue(queue);
+        return true;
+    }
+
+    RootedArrayObject queue(cx, asyncGenObj->queue());
+
+    FixedInvokeArgs<1> args(cx);
+    args[0].setObject(*request);
+    args.setThis(ObjectValue(*queue));
+    return CallJSNative(cx, array_push, args);
+}
+
+/* static */ AsyncGeneratorRequest*
+AsyncGeneratorObject::dequeueRequest(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj)
+{
+    if (asyncGenObj->isSingleQueue()) {
+        AsyncGeneratorRequest* request = asyncGenObj->singleQueueRequest();
+        asyncGenObj->clearSingleQueueRequest();
+        return request;
+    }
+
+    RootedArrayObject queue(cx, asyncGenObj->queue());
+
+    FixedInvokeArgs<0> args(cx);
+    args.setThis(ObjectValue(*queue));
+    if (!CallJSNative(cx, array_shift, args))
+        return nullptr;
+
+    return &args.rval().toObject().as<AsyncGeneratorRequest>();
+}
+
+/* static */ AsyncGeneratorRequest*
+AsyncGeneratorObject::peekRequest(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj)
+{
+    if (asyncGenObj->isSingleQueue())
+        return asyncGenObj->singleQueueRequest();
+
+    RootedArrayObject queue(cx, asyncGenObj->queue());
+
+    RootedValue requestVal(cx);
+    if (!GetElement(cx, queue, queue, 0, &requestVal))
+        return nullptr;
+
+    return &requestVal.toObject().as<AsyncGeneratorRequest>();
+}
+
+const Class AsyncGeneratorRequest::class_ = {
+    "AsyncGeneratorRequest",
+    JSCLASS_HAS_RESERVED_SLOTS(AsyncGeneratorRequest::Slots)
+};
+
+// Async Iteration proposal 6.4.3.1.
+/* static */ AsyncGeneratorRequest*
+AsyncGeneratorRequest::create(JSContext* cx, CompletionKind completionKind_,
+                              HandleValue completionValue_, HandleObject promise_)
+{
+    RootedObject obj(cx, NewNativeObjectWithGivenProto(cx, &class_, nullptr));
+    if (!obj)
+        return nullptr;
+
+    Handle<AsyncGeneratorRequest*> request = obj.as<AsyncGeneratorRequest>();
+    request->setCompletionKind(completionKind_);
+    request->setCompletionValue(completionValue_);
+    request->setPromise(promise_);
+    return request;
+}
+
+// Async Iteration proposal 6.4.3.2 steps 5.d-g.
+static MOZ_MUST_USE bool
+AsyncGeneratorReturned(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                       HandleValue value)
+{
+    // Step 5.d.
+    asyncGenObj->setCompleted();
+
+    // Step 5.e (done in bytecode).
+    // Step 5.f.i (implicit).
+
+    // Step 5.g.
+    return AsyncGeneratorResolve(cx, asyncGenObj, value, true);
+}
+
+// Async Iteration proposal 6.4.3.2 steps 5.d, f.
+static MOZ_MUST_USE bool
+AsyncGeneratorThrown(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj)
+{
+    // Step 5.d.
+    asyncGenObj->setCompleted();
+
+    // Not much we can do about uncatchable exceptions, so just bail.
+    if (!cx->isExceptionPending())
+        return false;
+
+    // Step 5.f.i.
+    RootedValue value(cx);
+    if (!GetAndClearException(cx, &value))
+        return false;
+
+    // Step 5.f.ii.
+    return AsyncGeneratorReject(cx, asyncGenObj, value);
+}
+
+// Async Iteration proposal 6.4.3.5.
+MOZ_MUST_USE bool
+js::AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj)
+{
+    // Step 1 (implicit).
+
+    // Steps 2-3.
+    MOZ_ASSERT(!asyncGenObj->isExecuting());
+
+    // Steps 4-5.
+    if (asyncGenObj->isQueueEmpty())
+        return true;
+
+    // Steps 6-7.
+    Rooted<AsyncGeneratorRequest*> request(
+        cx, AsyncGeneratorObject::peekRequest(cx, asyncGenObj));
+    if (!request)
+        return false;
+
+    // Step 8.
+    CompletionKind completionKind = request->completionKind();
+
+    // Step 9.
+    if (completionKind != CompletionKind::Normal) {
+        // Step 9.a.
+        if (asyncGenObj->isSuspendedStart())
+            asyncGenObj->setCompleted();
+
+        // Step 9.b.
+        if (asyncGenObj->isCompleted()) {
+            // Step 9.b.i.
+            RootedValue value(cx, request->completionValue());
+            if (completionKind == CompletionKind::Return)
+                return AsyncGeneratorResolve(cx, asyncGenObj, value, true);
+            // Step 9.b.ii.
+            return AsyncGeneratorReject(cx, asyncGenObj, value);
+        }
+    } else if (asyncGenObj->isCompleted()) {
+        // Step 10.
+        return AsyncGeneratorResolve(cx, asyncGenObj, UndefinedHandleValue, true);
+    }
+
+    // Step 11.
+    MOZ_ASSERT(asyncGenObj->isSuspendedStart() || asyncGenObj->isSuspendedYield());
+
+    // Step 15 (reordered).
+    asyncGenObj->setExecuting();
+
+    RootedValue argument(cx, request->completionValue());
+
+    // Steps 12-14, 16-20.
+    return AsyncGeneratorResume(cx, asyncGenObj, completionKind, argument);
+}
+
+// Async Iteration proposal 6.2.1.2 (partially).
+// Most steps are done in generator.
+static MOZ_MUST_USE bool
+AsyncGeneratorYield(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                    HandleValue value, bool done)
+{
+    // Step 6.
+    asyncGenObj->setSuspendedYield();
+
+    // Step 10.c.
+    return AsyncGeneratorResolve(cx, asyncGenObj, value, done);
+}
+
+// Async Iteration proposal 6.4.3.5 steps 12-14, 16-20.
+// Async Iteration proposal 6.2.1.2 step 10.
+// Async Iteration proposal 6.4.3.2 step 5.f-g.
+// Async Iteration proposal 5.1 steps 2-9.
+// Execution context switching is handled in generator.
+static MOZ_MUST_USE bool
+AsyncGeneratorResume(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                     CompletionKind completionKind, HandleValue argument)
+{
+    RootedValue generatorVal(cx, asyncGenObj->generatorVal());
+
+    // 6.4.3.5 steps 12-14, 16-20.
+    HandlePropertyName funName = completionKind == CompletionKind::Normal
+                                 ? cx->names().StarGeneratorNext
+                                 : completionKind == CompletionKind::Throw
+                                 ? cx->names().StarGeneratorThrow
+                                 : cx->names().StarGeneratorReturn;
+    FixedInvokeArgs<1> args(cx);
+    args[0].set(argument);
+    RootedValue result(cx);
+    if (!CallSelfHostedFunction(cx, funName, generatorVal, args, &result)) {
+        // 6.4.3.2 step 5.d, f.
+        return AsyncGeneratorThrown(cx, asyncGenObj);
+    }
+
+    // The following code corresponds to the following 3 cases:
+    //   * yield
+    //   * await
+    //   * return
+    // For await and return, property access is done on an internal result
+    // object and it's not observable.
+    // For yield, it's done on an user-provided result object, and it's
+    // observable, so perform in that order in all cases.
+
+    RootedObject resultObj(cx, &result.toObject());
+    RootedValue value(cx);
+    RootedValue doneVal(cx);
+
+    // 6.2.1.2 step 10.a.
+    if (!GetProperty(cx, resultObj, resultObj, cx->names().value, &value))
+        return false;
+
+    // 6.2.1.2 step 10.b.
+    if (!GetProperty(cx, resultObj, resultObj, cx->names().done, &doneVal))
+        return false;
+
+    // 6.2.1.2 step 10.c.
+    if (asyncGenObj->generatorObj()->isAfterYield())
+        return AsyncGeneratorYield(cx, asyncGenObj, value, ToBoolean(doneVal));
+
+    // 6.4.3.2 step 5.d-g.
+    if (ToBoolean(doneVal)) {
+        MOZ_ASSERT(!asyncGenObj->generatorObj()->isAfterAwait());
+        return AsyncGeneratorReturned(cx, asyncGenObj, value);
+    }
+
+    MOZ_ASSERT(asyncGenObj->generatorObj()->isAfterAwait());
+
+    // 5.1 steps 2-9.
+    return AsyncGeneratorAwait(cx, asyncGenObj, value);
+}
+
+static const JSFunctionSpec async_iterator_proto_methods[] = {
+    JS_SELF_HOSTED_SYM_FN(asyncIterator, "AsyncIteratorIdentity", 0, 0),
+    JS_FS_END
+};
+
+static const JSFunctionSpec async_generator_methods[] = {
+    JS_FN("next", AsyncGeneratorNext, 1, 0),
+    JS_FN("throw", AsyncGeneratorThrow, 1, 0),
+    JS_FN("return", AsyncGeneratorReturn, 1, 0),
+    JS_FS_END
+};
+
+/* static */ MOZ_MUST_USE bool
+GlobalObject::initAsyncGenerators(JSContext* cx, Handle<GlobalObject*> global)
+{
+    if (global->getReservedSlot(ASYNC_ITERATOR_PROTO).isObject())
+        return true;
+
+    // Async Iteration proposal 6.1.2 %AsyncIteratorPrototype%.
+    RootedObject asyncIterProto(cx, GlobalObject::createBlankPrototype<PlainObject>(cx, global));
+    if (!asyncIterProto)
+        return false;
+    if (!DefinePropertiesAndFunctions(cx, asyncIterProto, nullptr, async_iterator_proto_methods))
+        return false;
+
+    // Async Iteration proposal 6.4.1 %AsyncGeneratorPrototype%.
+    RootedObject asyncGenProto(
+        cx, GlobalObject::createBlankPrototypeInheriting(cx, global, &PlainObject::class_,
+                                                         asyncIterProto));
+    if (!asyncGenProto)
+        return false;
+    if (!DefinePropertiesAndFunctions(cx, asyncGenProto, nullptr, async_generator_methods) ||
+        !DefineToStringTag(cx, asyncGenProto, cx->names().AsyncGenerator))
+    {
+        return false;
+    }
+
+    // Async Iteration proposal 6.3.3 %AsyncGenerator%.
+    RootedObject asyncGenerator(cx, NewSingletonObjectWithFunctionPrototype(cx, global));
+    if (!asyncGenerator)
+        return false;
+    if (!JSObject::setDelegate(cx, asyncGenerator))
+        return false;
+    if (!LinkConstructorAndPrototype(cx, asyncGenerator, asyncGenProto, JSPROP_READONLY,
+                                     JSPROP_READONLY) ||
+        !DefineToStringTag(cx, asyncGenerator, cx->names().AsyncGeneratorFunction))
+    {
+        return false;
+    }
+
+    RootedValue function(cx, global->getConstructor(JSProto_Function));
+    if (!function.toObjectOrNull())
+        return false;
+    RootedObject proto(cx, &function.toObject());
+    RootedAtom name(cx, cx->names().AsyncGeneratorFunction);
+
+    // Async Iteration proposal 6.3.2 %AsyncGeneratorFunction%.
+    RootedObject asyncGenFunction(
+        cx, NewFunctionWithProto(cx, AsyncGeneratorConstructor, 1, JSFunction::NATIVE_CTOR,
+                                 nullptr, name, proto, gc::AllocKind::FUNCTION, SingletonObject));
+    if (!asyncGenFunction)
+        return false;
+    if (!LinkConstructorAndPrototype(cx, asyncGenFunction, asyncGenerator,
+                                     JSPROP_PERMANENT | JSPROP_READONLY, JSPROP_READONLY))
+    {
+        return false;
+    }
+
+    global->setReservedSlot(ASYNC_ITERATOR_PROTO, ObjectValue(*asyncIterProto));
+    global->setReservedSlot(ASYNC_GENERATOR, ObjectValue(*asyncGenerator));
+    global->setReservedSlot(ASYNC_GENERATOR_FUNCTION, ObjectValue(*asyncGenFunction));
+    global->setReservedSlot(ASYNC_GENERATOR_PROTO, ObjectValue(*asyncGenProto));
+    return true;
+}
diff --git a/js/src/vm/AsyncIteration.h b/js/src/vm/AsyncIteration.h
new file mode 100644
index 000000000..c0135283f
--- /dev/null
+++ b/js/src/vm/AsyncIteration.h
@@ -0,0 +1,204 @@
+/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
+ * vim: set ts=8 sts=4 et sw=4 tw=99:
+ * This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+#ifndef vm_AsyncIteration_h
+#define vm_AsyncIteration_h
+
+#include "jscntxt.h"
+#include "jsobj.h"
+
+#include "builtin/Promise.h"
+#include "vm/GeneratorObject.h"
+
+namespace js {
+
+// Async generator consists of 2 functions, |wrapped| and |unwrapped|.
+// |unwrapped| is a generator function compiled from async generator script,
+// |await| behaves just like |yield| there.  |unwrapped| isn't exposed to user
+// script.
+// |wrapped| is a native function that is the value of async generator.
+
+JSObject*
+WrapAsyncGeneratorWithProto(JSContext* cx, HandleFunction unwrapped, HandleObject proto);
+
+JSObject*
+WrapAsyncGenerator(JSContext* cx, HandleFunction unwrapped);
+
+bool
+IsWrappedAsyncGenerator(JSFunction* fun);
+
+JSFunction*
+GetWrappedAsyncGenerator(JSFunction* unwrapped);
+
+JSFunction*
+GetUnwrappedAsyncGenerator(JSFunction* wrapped);
+
+MOZ_MUST_USE bool
+AsyncGeneratorAwaitedFulfilled(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                              HandleValue value);
+
+MOZ_MUST_USE bool
+AsyncGeneratorAwaitedRejected(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                             HandleValue reason);
+
+class AsyncGeneratorRequest : public NativeObject
+{
+  private:
+    enum AsyncGeneratorRequestSlots {
+        Slot_CompletionKind = 0,
+        Slot_CompletionValue,
+        Slot_Promise,
+        Slots,
+    };
+
+    void setCompletionKind(CompletionKind completionKind_) {
+        setFixedSlot(Slot_CompletionKind,
+                     Int32Value(static_cast<int32_t>(completionKind_)));
+    }
+    void setCompletionValue(HandleValue completionValue_) {
+        setFixedSlot(Slot_CompletionValue, completionValue_);
+    }
+    void setPromise(HandleObject promise_) {
+        setFixedSlot(Slot_Promise, ObjectValue(*promise_));
+    }
+
+  public:
+    static const Class class_;
+
+    static AsyncGeneratorRequest*
+    create(JSContext* cx, CompletionKind completionKind, HandleValue completionValue,
+           HandleObject promise);
+
+    CompletionKind completionKind() const {
+        return static_cast<CompletionKind>(getFixedSlot(Slot_CompletionKind).toInt32());
+    }
+    JS::Value completionValue() const {
+        return getFixedSlot(Slot_CompletionValue);
+    }
+    JSObject* promise() const {
+        return &getFixedSlot(Slot_Promise).toObject();
+    }
+};
+
+class AsyncGeneratorObject : public NativeObject
+{
+  private:
+    enum AsyncGeneratorObjectSlots {
+        Slot_State = 0,
+        Slot_Generator,
+        Slot_QueueOrRequest,
+        Slots
+    };
+
+    enum State {
+        State_SuspendedStart,
+        State_SuspendedYield,
+        State_Executing,
+        State_Completed
+    };
+
+    State state() const {
+        return static_cast<State>(getFixedSlot(Slot_State).toInt32());
+    }
+    void setState(State state_) {
+        setFixedSlot(Slot_State, Int32Value(state_));
+    }
+
+    void setGenerator(const Value& value) {
+        setFixedSlot(Slot_Generator, value);
+    }
+
+    // Queue is implemented in 2 ways.  If only one request is queued ever,
+    // request is stored directly to the slot.  Once 2 requests are queued, an
+    // array is created and requests are pushed into it, and the array is
+    // stored to the slot.
+
+    bool isSingleQueue() const {
+        return getFixedSlot(Slot_QueueOrRequest).isNull() ||
+               getFixedSlot(Slot_QueueOrRequest).toObject().is<AsyncGeneratorRequest>();
+    }
+    bool isSingleQueueEmpty() const {
+        return getFixedSlot(Slot_QueueOrRequest).isNull();
+    }
+    void setSingleQueueRequest(AsyncGeneratorRequest* request) {
+        setFixedSlot(Slot_QueueOrRequest, ObjectValue(*request));
+    }
+    void clearSingleQueueRequest() {
+        setFixedSlot(Slot_QueueOrRequest, NullHandleValue);
+    }
+    AsyncGeneratorRequest* singleQueueRequest() const {
+        return &getFixedSlot(Slot_QueueOrRequest).toObject().as<AsyncGeneratorRequest>();
+    }
+
+    ArrayObject* queue() const {
+        return &getFixedSlot(Slot_QueueOrRequest).toObject().as<ArrayObject>();
+    }
+    void setQueue(JSObject* queue_) {
+        setFixedSlot(Slot_QueueOrRequest, ObjectValue(*queue_));
+    }
+
+  public:
+    static const Class class_;
+
+    static AsyncGeneratorObject*
+    create(JSContext* cx, HandleFunction asyncGen, HandleValue generatorVal);
+
+    bool isSuspendedStart() const {
+        return state() == State_SuspendedStart;
+    }
+    bool isSuspendedYield() const {
+        return state() == State_SuspendedYield;
+    }
+    bool isExecuting() const {
+        return state() == State_Executing;
+    }
+    bool isCompleted() const {
+        return state() == State_Completed;
+    }
+
+    void setSuspendedStart() {
+        setState(State_SuspendedStart);
+    }
+    void setSuspendedYield() {
+        setState(State_SuspendedYield);
+    }
+    void setExecuting() {
+        setState(State_Executing);
+    }
+    void setCompleted() {
+        setState(State_Completed);
+    }
+
+    JS::Value generatorVal() const {
+        return getFixedSlot(Slot_Generator);
+    }
+    GeneratorObject* generatorObj() const {
+        return &getFixedSlot(Slot_Generator).toObject().as<GeneratorObject>();
+    }
+
+    static MOZ_MUST_USE bool
+    enqueueRequest(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj,
+                   Handle<AsyncGeneratorRequest*> request);
+
+    static AsyncGeneratorRequest*
+    dequeueRequest(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj);
+
+    static AsyncGeneratorRequest*
+    peekRequest(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj);
+
+    bool isQueueEmpty() const {
+        if (isSingleQueue())
+            return isSingleQueueEmpty();
+        return queue()->length() == 0;
+    }
+};
+
+MOZ_MUST_USE bool
+AsyncGeneratorResumeNext(JSContext* cx, Handle<AsyncGeneratorObject*> asyncGenObj);
+
+} // namespace js
+
+#endif /* vm_AsyncIteration_h */
diff --git a/js/src/vm/CommonPropertyNames.h b/js/src/vm/CommonPropertyNames.h
index 99cb02e58..4a9c03e88 100644
--- a/js/src/vm/CommonPropertyNames.h
+++ b/js/src/vm/CommonPropertyNames.h
@@ -30,6 +30,8 @@
     macro(as, as, "as") \
     macro(Async, Async, "Async") \
     macro(AsyncFunction, AsyncFunction, "AsyncFunction") \
+    macro(AsyncGenerator, AsyncGenerator, "AsyncGenerator") \
+    macro(AsyncGeneratorFunction, AsyncGeneratorFunction, "AsyncGeneratorFunction") \
     macro(AsyncWrapped, AsyncWrapped, "AsyncWrapped") \
     macro(async, async, "async") \
     macro(await, await, "await") \
@@ -311,6 +313,7 @@
     macro(star, star, "*") \
     macro(starDefaultStar, starDefaultStar, "*default*") \
     macro(StarGeneratorNext, StarGeneratorNext, "StarGeneratorNext") \
+    macro(StarGeneratorReturn, StarGeneratorReturn, "StarGeneratorReturn") \
     macro(StarGeneratorThrow, StarGeneratorThrow, "StarGeneratorThrow") \
     macro(startTimestamp, startTimestamp, "startTimestamp") \
     macro(state, state, "state") \
diff --git a/js/src/vm/GlobalObject.h b/js/src/vm/GlobalObject.h
index dd3f23151..ce802daba 100644
--- a/js/src/vm/GlobalObject.h
+++ b/js/src/vm/GlobalObject.h
@@ -100,6 +100,10 @@ class GlobalObject : public NativeObject
         STAR_GENERATOR_FUNCTION,
         ASYNC_FUNCTION_PROTO,
         ASYNC_FUNCTION,
+        ASYNC_ITERATOR_PROTO,
+        ASYNC_GENERATOR,
+        ASYNC_GENERATOR_FUNCTION,
+        ASYNC_GENERATOR_PROTO,
         MAP_ITERATOR_PROTO,
         SET_ITERATOR_PROTO,
         COLLATOR_PROTO,
@@ -624,6 +628,30 @@ class GlobalObject : public NativeObject
         return getOrCreateObject(cx, global, ASYNC_FUNCTION, initAsyncFunction);
     }
 
+    static NativeObject*
+    getOrCreateAsyncIteratorPrototype(JSContext* cx, Handle<GlobalObject*> global)
+    {
+        return MaybeNativeObject(getOrCreateObject(cx, global, ASYNC_ITERATOR_PROTO,
+                                                   initAsyncGenerators));
+    }
+
+    static NativeObject*
+    getOrCreateAsyncGenerator(JSContext* cx, Handle<GlobalObject*> global) {
+        return MaybeNativeObject(getOrCreateObject(cx, global, ASYNC_GENERATOR,
+                                                   initAsyncGenerators));
+    }
+
+    static JSObject*
+    getOrCreateAsyncGeneratorFunction(JSContext* cx, Handle<GlobalObject*> global) {
+        return getOrCreateObject(cx, global, ASYNC_GENERATOR_FUNCTION, initAsyncGenerators);
+    }
+
+    static NativeObject*
+    getOrCreateAsyncGeneratorPrototype(JSContext* cx, Handle<GlobalObject*> global) {
+        return MaybeNativeObject(getOrCreateObject(cx, global, ASYNC_GENERATOR_PROTO,
+                                                   initAsyncGenerators));
+    }
+
     static JSObject*
     getOrCreateMapIteratorPrototype(JSContext* cx, Handle<GlobalObject*> global) {
         return getOrCreateObject(cx, global, MAP_ITERATOR_PROTO, initMapIteratorProto);
@@ -782,6 +810,8 @@ class GlobalObject : public NativeObject
 
     static bool initAsyncFunction(JSContext* cx, Handle<GlobalObject*> global);
 
+    static bool initAsyncGenerators(JSContext* cx, Handle<GlobalObject*> global);
+
     // Implemented in builtin/MapObject.cpp.
     static bool initMapIteratorProto(JSContext* cx, Handle<GlobalObject*> global);
     static bool initSetIteratorProto(JSContext* cx, Handle<GlobalObject*> global);
diff --git a/js/src/vm/Interpreter.cpp b/js/src/vm/Interpreter.cpp
index 92bb18b36..4d5927324 100644
--- a/js/src/vm/Interpreter.cpp
+++ b/js/src/vm/Interpreter.cpp
@@ -39,6 +39,7 @@
 #include "jit/Ion.h"
 #include "jit/IonAnalysis.h"
 #include "vm/AsyncFunction.h"
+#include "vm/AsyncIteration.h"
 #include "vm/Debugger.h"
 #include "vm/GeneratorObject.h"
 #include "vm/Opcodes.h"
@@ -1939,7 +1940,6 @@ CASE(EnableInterruptsPseudoOpcode)
 CASE(JSOP_NOP)
 CASE(JSOP_NOP_DESTRUCTURING)
 CASE(JSOP_UNUSED126)
-CASE(JSOP_UNUSED192)
 CASE(JSOP_UNUSED210)
 CASE(JSOP_UNUSED211)
 CASE(JSOP_TRY_DESTRUCTURING_ITERCLOSE)
@@ -3584,6 +3584,18 @@ CASE(JSOP_TOASYNC)
 }
 END_CASE(JSOP_TOASYNC)
 
+CASE(JSOP_TOASYNCGEN)
+{
+    ReservedRooted<JSFunction*> unwrapped(&rootFunction0,
+                                          &REGS.sp[-1].toObject().as<JSFunction>());
+    JSObject* wrapped = WrapAsyncGenerator(cx, unwrapped);
+    if (!wrapped)
+        goto error;
+
+    REGS.sp[-1].setObject(*wrapped);
+}
+END_CASE(JSOP_TOASYNCGEN)
+
 CASE(JSOP_SETFUNNAME)
 {
     MOZ_ASSERT(REGS.stackDepth() >= 2);
diff --git a/js/src/vm/Opcodes.h b/js/src/vm/Opcodes.h
index 583d32beb..fc7fbbe66 100644
--- a/js/src/vm/Opcodes.h
+++ b/js/src/vm/Opcodes.h
@@ -1952,7 +1952,15 @@
      *   Stack: this => this
      */ \
     macro(JSOP_CHECKTHISREINIT,191,"checkthisreinit",NULL,1,  1,  1,  JOF_BYTE) \
-    macro(JSOP_UNUSED192,     192,"unused192",   NULL,    1,  0,  0,  JOF_BYTE) \
+    /*
+     * Pops the top of stack value as 'unwrapped', converts it to async
+     * generator 'wrapped', and pushes 'wrapped' back on the stack.
+     *   Category: Statements
+     *   Type: Generator
+     *   Operands:
+     *   Stack: unwrapped => wrapped
+     */ \
+    macro(JSOP_TOASYNCGEN,    192, "toasyncgen", NULL,    1,  1,  1, JOF_BYTE) \
     \
     /*
      * Pops the top two values on the stack as 'propval' and 'obj', pushes