--- a/js/src/builtin/Promise.cpp
+++ b/js/src/builtin/Promise.cpp
@@ -2339,18 +2339,18 @@ js::PromiseResolve(JSContext* cx, Handle
 {
     RootedValue C(cx, ObjectValue(*constructor));
     return CommonStaticResolveRejectImpl(cx, C, value, ResolveMode);
 }
 
 /**
  * ES2016, 25.4.4.4, Promise.reject.
  */
-bool
-js::Promise_reject(JSContext* cx, unsigned argc, Value* vp)
+static bool
+Promise_reject(JSContext* cx, unsigned argc, Value* vp)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
     RootedValue thisVal(cx, args.thisv());
     RootedValue argVal(cx, args.get(0));
     JSObject* result = CommonStaticResolveRejectImpl(cx, thisVal, argVal, RejectMode);
     if (!result)
         return false;
     args.rval().setObject(*result);
@@ -2368,18 +2368,18 @@ PromiseObject::unforgeableReject(JSConte
         return nullptr;
     RootedValue cVal(cx, ObjectValue(*promiseCtor));
     return CommonStaticResolveRejectImpl(cx, cVal, value, RejectMode);
 }
 
 /**
  * ES2016, 25.4.4.5, Promise.resolve.
  */
-bool
-js::Promise_static_resolve(JSContext* cx, unsigned argc, Value* vp)
+static bool
+Promise_static_resolve(JSContext* cx, unsigned argc, Value* vp)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
     RootedValue thisVal(cx, args.thisv());
     RootedValue argVal(cx, args.get(0));
     JSObject* result = CommonStaticResolveRejectImpl(cx, thisVal, argVal, ResolveMode);
     if (!result)
         return false;
     args.rval().setObject(*result);
@@ -3023,26 +3023,61 @@ js::AsyncGeneratorEnqueue(JSContext* cx,
             return false;
     }
 
     // Step 9.
     result.setObject(*resultPromise);
     return true;
 }
 
-bool Promise_then_impl(JSContext* cx, unsigned argc, Value* vp, bool rvalUsed)
+static bool Promise_then(JSContext* cx, unsigned argc, Value* vp);
+static bool Promise_then_impl(JSContext* cx, HandleValue promiseVal, HandleValue onFulfilled,
+                              HandleValue onRejected, MutableHandleValue rval, bool rvalUsed);
+
+static bool
+Promise_catch_impl(JSContext* cx, unsigned argc, Value* vp, bool rvalUsed)
 {
     CallArgs args = CallArgsFromVp(argc, vp);
 
     // Step 1.
-    RootedValue promiseVal(cx, args.thisv());
-
-    RootedValue onFulfilled(cx, args.get(0));
-    RootedValue onRejected(cx, args.get(1));
-
+    RootedValue thenVal(cx);
+    if (!GetProperty(cx, args.thisv(), cx->names().then, &thenVal))
+        return false;
+
+    if (IsNativeFunction(thenVal, &Promise_then)) {
+        return Promise_then_impl(cx, args.thisv(), UndefinedHandleValue, args.get(0),
+                                 args.rval(), rvalUsed);
+    }
+
+    FixedInvokeArgs<2> iargs(cx);
+    iargs[0].setUndefined();
+    iargs[1].set(args.get(0));
+
+    return Call(cx, thenVal, args.thisv(), iargs, args.rval());
+}
+
+// ES2016, 25.4.5.3.
+static bool
+Promise_catch_noRetVal(JSContext* cx, unsigned argc, Value* vp)
+{
+    return Promise_catch_impl(cx, argc, vp, false);
+}
+
+// ES2016, 25.4.5.3.
+static bool
+Promise_catch(JSContext* cx, unsigned argc, Value* vp)
+{
+    return Promise_catch_impl(cx, argc, vp, true);
+}
+
+static bool
+Promise_then_impl(JSContext* cx, HandleValue promiseVal, HandleValue onFulfilled,
+                  HandleValue onRejected, MutableHandleValue rval, bool rvalUsed)
+{
+    // Step 1 (implicit).
     // Step 2.
     if (!promiseVal.isObject()) {
         JS_ReportErrorNumberASCII(cx, GetErrorMessage, nullptr, JSMSG_NOT_NONNULL_OBJECT,
                                   "Receiver of Promise.prototype.then call");
         return false;
     }
     RootedObject promiseObj(cx, &promiseVal.toObject());
     Rooted<PromiseObject*> promise(cx);
@@ -3071,34 +3106,36 @@ bool Promise_then_impl(JSContext* cx, un
     RootedObject resultPromise(cx);
     if (!OriginalPromiseThen(cx, promise, onFulfilled, onRejected, &resultPromise,
                              createDependent))
     {
         return false;
     }
 
     if (rvalUsed)
-        args.rval().setObject(*resultPromise);
+        rval.setObject(*resultPromise);
     else
-        args.rval().setUndefined();
+        rval.setUndefined();
     return true;
 }
 
 // ES2016, 25.4.5.3.
 bool
 Promise_then_noRetVal(JSContext* cx, unsigned argc, Value* vp)
 {
-    return Promise_then_impl(cx, argc, vp, false);
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return Promise_then_impl(cx, args.thisv(), args.get(0), args.get(1), args.rval(), false);
 }
 
 // ES2016, 25.4.5.3.
-bool
-js::Promise_then(JSContext* cx, unsigned argc, Value* vp)
+static bool
+Promise_then(JSContext* cx, unsigned argc, Value* vp)
 {
-    return Promise_then_impl(cx, argc, vp, true);
+    CallArgs args = CallArgsFromVp(argc, vp);
+    return Promise_then_impl(cx, args.thisv(), args.get(0), args.get(1), args.rval(), true);
 }
 
 // ES2016, 25.4.5.3.1.
 static MOZ_MUST_USE bool
 PerformPromiseThen(JSContext* cx, Handle<PromiseObject*> promise, HandleValue onFulfilled_,
                    HandleValue onRejected_, HandleObject resultPromise,
                    HandleObject resolve, HandleObject reject)
 {
@@ -3766,19 +3803,28 @@ const JSJitInfo promise_then_info = {
   { (JSJitGetterOp)Promise_then_noRetVal },
   { 0 }, /* unused */
   { 0 }, /* unused */
   JSJitInfo::IgnoresReturnValueNative,
   JSJitInfo::AliasEverything,
   JSVAL_TYPE_UNDEFINED,
 };
 
+const JSJitInfo promise_catch_info = {
+  { (JSJitGetterOp)Promise_catch_noRetVal },
+  { 0 }, /* unused */
+  { 0 }, /* unused */
+  JSJitInfo::IgnoresReturnValueNative,
+  JSJitInfo::AliasEverything,
+  JSVAL_TYPE_UNDEFINED,
+};
+
 static const JSFunctionSpec promise_methods[] = {
-    JS_SELF_HOSTED_FN("catch", "Promise_catch", 1, 0),
     JS_FNINFO("then", Promise_then, &promise_then_info, 2, 0),
+    JS_FNINFO("catch", Promise_catch, &promise_catch_info, 1, 0),
     JS_SELF_HOSTED_FN("finally", "Promise_finally", 1, 0),
     JS_FS_END
 };
 
 static const JSPropertySpec promise_properties[] = {
     JS_STRING_SYM_PS(toStringTag, "Promise", JSPROP_READONLY),
     JS_PS_END
 };

--- a/js/src/builtin/Promise.h
+++ b/js/src/builtin/Promise.h
@@ -254,18 +254,11 @@ class OffThreadPromiseRuntimeState
     // called to periodically drain the dispatch queue before shutdown.
     void internalDrain(JSContext* cx);
     bool internalHasPending();
 
     // shutdown() must be called by the JSRuntime while the JSRuntime is valid.
     void shutdown(JSContext* cx);
 };
 
-bool
-Promise_static_resolve(JSContext* cx, unsigned argc, Value* vp);
-bool
-Promise_reject(JSContext* cx, unsigned argc, Value* vp);
-bool
-Promise_then(JSContext* cx, unsigned argc, Value* vp);
-
 } // namespace js
 
 #endif /* builtin_Promise_h */