diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSArray.java b/jso/apis/src/main/java/org/teavm/jso/core/JSArray.java
index 206a36e90..1ea7c5e51 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSArray.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSArray.java
@@ -18,10 +18,9 @@ package org.teavm.jso.core;
 import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSIndexer;
-import org.teavm.jso.JSObject;
 import org.teavm.jso.JSProperty;
 
-public abstract class JSArray<T extends JSObject> implements JSArrayReader<T> {
+public abstract class JSArray<T> implements JSArrayReader<T> {
     private JSArray() {
     }
 
@@ -85,18 +84,18 @@ public abstract class JSArray<T extends JSObject> implements JSArrayReader<T> {
 
     @JSBody(script = "return new Array();")
     @NoSideEffects
-    public static native <T extends JSObject> JSArray<T> create();
+    public static native <T> JSArray<T> create();
 
     @JSBody(params = "size", script = "return new Array(size);")
     @NoSideEffects
-    public static native <T extends JSObject> JSArray<T> create(int size);
+    public static native <T> JSArray<T> create(int size);
 
     @JSBody(params = "object", script = "return Array.isArray(object);")
     @NoSideEffects
-    public static native boolean isArray(JSObject object);
+    public static native boolean isArray(Object object);
 
     @SafeVarargs
-    public static <S extends JSObject> JSArray<S> of(S... items) {
+    public static <S> JSArray<S> of(S... items) {
         JSArray<S> array = create(items.length);
         for (int i = 0; i < items.length; ++i) {
             array.set(i, items[i]);
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSArrayReader.java b/jso/apis/src/main/java/org/teavm/jso/core/JSArrayReader.java
index 706d92e09..16c69449d 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSArrayReader.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSArrayReader.java
@@ -20,7 +20,7 @@ import org.teavm.jso.JSIndexer;
 import org.teavm.jso.JSObject;
 import org.teavm.jso.JSProperty;
 
-public interface JSArrayReader<T extends JSObject> extends JSObject {
+public interface JSArrayReader<T> extends JSObject {
     @JSProperty
     @NoSideEffects
     int getLength();
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSError.java b/jso/apis/src/main/java/org/teavm/jso/core/JSError.java
index 5883eb14e..77e86ec44 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSError.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSError.java
@@ -27,7 +27,7 @@ public abstract class JSError implements JSObject {
             + "} catch (e) {"
                 + "return catchClause(e);"
             + "}")
-    public static native <T extends JSObject> T catchNative(TryClause<T> tryClause, CatchClause<T> catchClause);
+    public static native <T> T catchNative(TryClause<T> tryClause, CatchClause<T> catchClause);
 
     @JSBody(params = "object", script = "return object instanceof Error;")
     public static native boolean isError(JSObject object);
@@ -42,12 +42,12 @@ public abstract class JSError implements JSObject {
     public abstract String getName();
 
     @JSFunctor
-    public interface TryClause<T extends JSObject> extends JSObject {
+    public interface TryClause<T> extends JSObject {
         T run();
     }
 
     @JSFunctor
-    public interface CatchClause<T extends JSObject> extends JSObject {
+    public interface CatchClause<T> extends JSObject {
         T accept(JSObject e);
     }
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSFinalizationRegistry.java b/jso/apis/src/main/java/org/teavm/jso/core/JSFinalizationRegistry.java
index 413fcc443..53eeca28a 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSFinalizationRegistry.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSFinalizationRegistry.java
@@ -15,12 +15,18 @@
  */
 package org.teavm.jso.core;
 
+import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 
 public abstract class JSFinalizationRegistry implements JSObject {
-    public abstract void register(JSObject obj, JSObject token);
+    public abstract void register(Object obj, Object token);
 
     @JSBody(params = "consumer", script = "return new FinalizationRegistry(consumer);")
     public static native JSFinalizationRegistry create(JSFinalizationRegistryConsumer consumer);
+
+
+    @JSBody(script = "return typeof FinalizationRegistry !== 'undefined';")
+    @NoSideEffects
+    public static native boolean isSupported();
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSFinalizationRegistryConsumer.java b/jso/apis/src/main/java/org/teavm/jso/core/JSFinalizationRegistryConsumer.java
index 1201dc8a3..91c91df1f 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSFinalizationRegistryConsumer.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSFinalizationRegistryConsumer.java
@@ -20,5 +20,5 @@ import org.teavm.jso.JSObject;
 
 @JSFunctor
 public interface JSFinalizationRegistryConsumer extends JSObject {
-    void accept(JSObject obj);
+    void accept(Object obj);
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSFunction.java b/jso/apis/src/main/java/org/teavm/jso/core/JSFunction.java
index 9a5df7a23..33eb3d1e3 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSFunction.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSFunction.java
@@ -19,33 +19,31 @@ import org.teavm.jso.JSByRef;
 import org.teavm.jso.JSObject;
 import org.teavm.jso.JSProperty;
 
-public interface JSFunction extends JSObject {
+public abstract class JSFunction implements JSObject {
     @JSProperty
-    int getLength();
+    public abstract int getLength();
 
     @JSProperty
-    String getName();
+    public abstract String getName();
 
-    JSObject call(JSObject thisArg);
+    public abstract Object call(Object thisArg);
 
-    JSObject call(JSObject thisArg, JSObject a);
+    public abstract Object call(Object thisArg, Object a);
 
-    JSObject call(JSObject thisArg, JSObject a, JSObject b);
+    public abstract Object call(Object thisArg, Object a, Object b);
 
-    JSObject call(JSObject thisArg, JSObject a, JSObject b, JSObject c);
+    public abstract Object call(Object thisArg, Object a, Object b, Object c);
 
-    JSObject call(JSObject thisArg, JSObject a, JSObject b, JSObject c,
-            JSObject d);
+    public abstract Object call(Object thisArg, Object a, Object b, Object c, Object d);
 
-    JSObject call(JSObject thisArg, JSObject a, JSObject b, JSObject c, JSObject d, JSObject e);
+    public abstract Object call(Object thisArg, Object a, Object b, Object c, Object d, Object e);
 
-    JSObject call(JSObject thisArg, JSObject a, JSObject b, JSObject c, JSObject d, JSObject e, JSObject f);
+    public abstract Object call(Object thisArg, Object a, Object b, Object c, Object d, Object e, Object f);
 
-    JSObject call(JSObject thisArg, JSObject a, JSObject b, JSObject c, JSObject d, JSObject e, JSObject f,
-            JSObject g);
+    public abstract Object call(Object thisArg, Object a, Object b, Object c, Object d, Object e, Object f, Object g);
 
-    JSObject call(JSObject thisArg, JSObject a, JSObject b, JSObject c, JSObject d, JSObject e, JSObject f,
-            JSObject g, JSObject h);
+    public abstract Object call(Object thisArg, Object a, Object b, Object c, Object d, Object e, Object f, Object g,
+            Object h);
 
-    JSObject apply(JSObject thisArg, @JSByRef JSObject[] arguments);
+    public abstract Object apply(Object thisArg, @JSByRef Object[] arguments);
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSMapLike.java b/jso/apis/src/main/java/org/teavm/jso/core/JSMapLike.java
index 7fb0808d7..49705a2d2 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSMapLike.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSMapLike.java
@@ -18,7 +18,7 @@ package org.teavm.jso.core;
 import org.teavm.jso.JSIndexer;
 import org.teavm.jso.JSObject;
 
-public interface JSMapLike<T extends JSObject> extends JSObject {
+public interface JSMapLike<T> extends JSObject {
     @JSIndexer
     T get(String key);
 
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSObjects.java b/jso/apis/src/main/java/org/teavm/jso/core/JSObjects.java
index b5d3dcc4f..e33d08aa3 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSObjects.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSObjects.java
@@ -25,11 +25,11 @@ public final class JSObjects {
 
     @JSBody(params = "object", script = "return Object.getOwnPropertyNames(object);")
     @NoSideEffects
-    public static native String[] getOwnPropertyNames(JSObject object);
+    public static native String[] getOwnPropertyNames(Object object);
 
     @JSBody(params = "object", script = "return Object.keys(object);")
     @NoSideEffects
-    public static native String[] keys(JSObject object);
+    public static native String[] keys(Object object);
 
     @JSBody(params = "object", script = "return Object.values(object);")
     @NoSideEffects
@@ -45,7 +45,7 @@ public final class JSObjects {
 
     @JSBody(params = "object", script = "return typeof object === 'undefined';")
     @NoSideEffects
-    public static native boolean isUndefined(JSObject object);
+    public static native boolean isUndefined(Object object);
 
     @JSBody(script = "return void 0;")
     @NoSideEffects
@@ -53,16 +53,16 @@ public final class JSObjects {
 
     @JSBody(params = "object", script = "return typeof object;")
     @NoSideEffects
-    public static native String typeOf(JSObject object);
+    public static native String typeOf(Object object);
 
     @JSBody(params = "object", script = "return object.toString();")
-    public static native String toString(JSObject object);
+    public static native String toString(Object object);
 
     @JSBody(params = { "object", "name" }, script = "return name in object;")
     @NoSideEffects
-    public static native boolean hasProperty(JSObject object, String name);
+    public static native boolean hasProperty(Object object, String name);
 
     @JSBody(params = "object", script = "return Object.getPrototypeOf(object);")
     @NoSideEffects
-    public static native JSObject getPrototypeOf(JSObject object);
+    public static native JSObject getPrototypeOf(Object object);
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSSortFunction.java b/jso/apis/src/main/java/org/teavm/jso/core/JSSortFunction.java
index 09cefca0f..5bd567497 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSSortFunction.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSSortFunction.java
@@ -16,15 +16,9 @@
 package org.teavm.jso.core;
 
 import org.teavm.jso.JSFunctor;
-import org.teavm.jso.JSObject;
 
-/**
- *
- * @author Alexey Andreev
- * @param <T>
- */
 @JSFunctor
 @FunctionalInterface
-public interface JSSortFunction<T extends JSObject> {
+public interface JSSortFunction<T> {
     int compare(T a, T b);
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSSymbol.java b/jso/apis/src/main/java/org/teavm/jso/core/JSSymbol.java
index 07de90884..d528693e2 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSSymbol.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSSymbol.java
@@ -19,14 +19,14 @@ import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 
-public abstract class JSSymbol<T extends JSObject> implements JSObject {
+public abstract class JSSymbol<T> implements JSObject {
     @JSBody(params = "name", script = "return Symbol(name);")
-    public static native <T extends JSObject> JSSymbol<T> create(String name);
+    public static native <T> JSSymbol<T> create(String name);
 
     @JSBody(params = "obj", script = "return obj[this];")
-    public native T get(JSObject obj);
+    public native T get(Object obj);
 
     @JSBody(params = { "obj", "value" }, script = "obj[this] = value;")
     @NoSideEffects
-    public native void set(JSObject obj, T value);
+    public native void set(Object obj, T value);
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSWeakMap.java b/jso/apis/src/main/java/org/teavm/jso/core/JSWeakMap.java
index 8f00bd8e7..196ffe72b 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSWeakMap.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSWeakMap.java
@@ -19,7 +19,7 @@ import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 
-public abstract class JSWeakMap<K extends JSObject, V extends JSObject> implements JSObject {
+public abstract class JSWeakMap<K, V> implements JSObject {
     public abstract V get(K key);
 
     public abstract boolean has(K key);
@@ -30,5 +30,9 @@ public abstract class JSWeakMap<K extends JSObject, V extends JSObject> implemen
 
     @JSBody(script = "return new WeakMap();")
     @NoSideEffects
-    public static native <K extends JSObject, V extends JSObject> JSWeakMap<K, V> create();
+    public static native <K, V> JSWeakMap<K, V> create();
+
+    @JSBody(script = "return typeof WeakMap !== 'undefined';")
+    @NoSideEffects
+    public static native boolean isSupported();
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSWeakRef.java b/jso/apis/src/main/java/org/teavm/jso/core/JSWeakRef.java
index b90df6ba7..98680ff04 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSWeakRef.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSWeakRef.java
@@ -19,12 +19,12 @@ import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 
-public abstract class JSWeakRef<T extends JSObject> implements JSObject {
+public abstract class JSWeakRef<T> implements JSObject {
     public abstract T deref();
 
     @JSBody(params = "value", script = "return new WeakRef(value);")
     @NoSideEffects
-    public static native <T extends JSObject> JSWeakRef<T> create(T value);
+    public static native <T> JSWeakRef<T> create(T value);
 
     @JSBody(script = "return typeof WeakRef !== 'undefined';")
     @NoSideEffects
diff --git a/jso/apis/src/main/java/org/teavm/jso/dom/events/MessageEvent.java b/jso/apis/src/main/java/org/teavm/jso/dom/events/MessageEvent.java
index 271f9b24a..64715f67e 100644
--- a/jso/apis/src/main/java/org/teavm/jso/dom/events/MessageEvent.java
+++ b/jso/apis/src/main/java/org/teavm/jso/dom/events/MessageEvent.java
@@ -15,13 +15,12 @@
  */
 package org.teavm.jso.dom.events;
 
-import org.teavm.jso.JSObject;
 import org.teavm.jso.JSProperty;
 import org.teavm.jso.typedarrays.ArrayBuffer;
 
 public interface MessageEvent extends Event {
     @JSProperty
-    JSObject getData();
+    Object getData();
 
     @JSProperty("data")
     String getDataAsString();
diff --git a/jso/apis/src/main/java/org/teavm/jso/workers/MessagePort.java b/jso/apis/src/main/java/org/teavm/jso/workers/MessagePort.java
index 2bad1ed89..be6eb97cb 100644
--- a/jso/apis/src/main/java/org/teavm/jso/workers/MessagePort.java
+++ b/jso/apis/src/main/java/org/teavm/jso/workers/MessagePort.java
@@ -15,14 +15,13 @@
  */
 package org.teavm.jso.workers;
 
-import org.teavm.jso.JSObject;
 import org.teavm.jso.JSProperty;
 import org.teavm.jso.dom.events.EventListener;
 import org.teavm.jso.dom.events.EventTarget;
 import org.teavm.jso.dom.events.MessageEvent;
 
 public interface MessagePort extends EventTarget {
-    void postMessage(JSObject message);
+    void postMessage(Object message);
 
     void start();
 
diff --git a/jso/apis/src/main/java/org/teavm/jso/workers/Worker.java b/jso/apis/src/main/java/org/teavm/jso/workers/Worker.java
index ea00af0a5..95876f9a4 100644
--- a/jso/apis/src/main/java/org/teavm/jso/workers/Worker.java
+++ b/jso/apis/src/main/java/org/teavm/jso/workers/Worker.java
@@ -16,7 +16,6 @@
 package org.teavm.jso.workers;
 
 import org.teavm.jso.JSBody;
-import org.teavm.jso.JSObject;
 import org.teavm.jso.JSProperty;
 import org.teavm.jso.dom.events.EventListener;
 import org.teavm.jso.dom.events.MessageEvent;
@@ -28,7 +27,7 @@ public abstract class Worker implements AbstractWorker {
     @JSProperty("onmessage")
     public abstract void onMessage(EventListener<MessageEvent> listener);
 
-    public abstract void postMessage(JSObject message);
+    public abstract void postMessage(Object message);
 
     public abstract void terminate();
 }
diff --git a/jso/impl/src/main/java/org/teavm/jso/impl/JSWrapper.java b/jso/impl/src/main/java/org/teavm/jso/impl/JSWrapper.java
index 9c5ab12c0..1978ca723 100644
--- a/jso/impl/src/main/java/org/teavm/jso/impl/JSWrapper.java
+++ b/jso/impl/src/main/java/org/teavm/jso/impl/JSWrapper.java
@@ -62,37 +62,37 @@ public final class JSWrapper {
         if (wrappers != null) {
             var type = JSObjects.typeOf(js);
             if (type.equals("object") || type.equals("function")) {
-                var existingRef = wrappers.get(js);
-                var existing = !JSObjects.isUndefined(existingRef) ? existingRef.deref() : JSObjects.undefined();
+                var existingRef = get(wrappers, js);
+                var existing = !JSObjects.isUndefined(existingRef) ? deref(existingRef) : JSObjects.undefined();
                 if (JSObjects.isUndefined(existing)) {
                     var wrapper = new JSWrapper(js);
-                    wrappers.set(js, JSWeakRef.create(wrapperToJs(wrapper)));
+                    set(wrappers, js, createWeakRef(wrapperToJs(wrapper)));
                     return wrapper;
                 } else {
                     return jsToWrapper(existing);
                 }
             } else if (type.equals("string")) {
                 var jsString = (JSString) js;
-                var existingRef = stringWrappers.get(jsString);
-                var existing = !JSObjects.isUndefined(existingRef) ? existingRef.deref() : JSObjects.undefined();
+                var existingRef = get(stringWrappers, jsString);
+                var existing = !JSObjects.isUndefined(existingRef) ? deref(existingRef) : JSObjects.undefined();
                 if (JSObjects.isUndefined(existing)) {
                     var wrapper = new JSWrapper(js);
                     var wrapperAsJs = wrapperToJs(wrapper);
-                    stringWrappers.set(jsString, JSWeakRef.create(wrapperAsJs));
-                    stringFinalizationRegistry.register(wrapperAsJs, jsString);
+                    set(stringWrappers, jsString, createWeakRef(wrapperAsJs));
+                    register(stringFinalizationRegistry, wrapperAsJs, jsString);
                     return wrapper;
                 } else {
                     return jsToWrapper(existing);
                 }
             } else if (type.equals("number")) {
                 var jsNumber = (JSNumber) js;
-                var existingRef = numberWrappers.get(jsNumber);
-                var existing = !JSObjects.isUndefined(existingRef) ? existingRef.deref() : JSObjects.undefined();
+                var existingRef = get(numberWrappers, jsNumber);
+                var existing = !JSObjects.isUndefined(existingRef) ? deref(existingRef) : JSObjects.undefined();
                 if (JSObjects.isUndefined(existing)) {
                     var wrapper = new JSWrapper(js);
                     var wrapperAsJs = wrapperToJs(wrapper);
-                    numberWrappers.set(jsNumber, JSWeakRef.create(wrapperAsJs));
-                    numberFinalizationRegistry.register(wrapperAsJs, jsNumber);
+                    set(numberWrappers, jsNumber, createWeakRef(wrapperAsJs));
+                    register(numberFinalizationRegistry, wrapperAsJs, jsNumber);
                     return wrapper;
                 } else {
                     return jsToWrapper(existing);
@@ -102,6 +102,37 @@ public final class JSWrapper {
         return new JSWrapper(js);
     }
 
+    @JSBody(params = "target", script = "return new WeakRef(target);")
+    @NoSideEffects
+    private static native JSWeakRef<JSObject> createWeakRef(JSObject target);
+
+    @JSBody(params = "target", script = "return target.deref();")
+    @NoSideEffects
+    private static native JSObject deref(JSWeakRef<JSObject> target);
+
+    @JSBody(params = { "registry", "target", "token" }, script = "return registry.register(target, token);")
+    @NoSideEffects
+    private static native void register(JSFinalizationRegistry registry, JSObject target, JSObject token);
+
+    @JSBody(params = { "map", "key" }, script = "return map.get(key);")
+    @NoSideEffects
+    private static native JSWeakRef<JSObject> get(JSMap<? extends JSObject, JSWeakRef<JSObject>> map, JSObject key);
+
+    @JSBody(params = { "map", "key", "value" }, script = "map.set(key, value);")
+    @NoSideEffects
+    private static native void set(JSMap<? extends JSObject, JSWeakRef<JSObject>> map, JSObject key, JSObject value);
+
+    @JSBody(params = { "map", "key" }, script = "return map.get(key);")
+    @NoSideEffects
+    private static native JSWeakRef<JSObject> get(JSWeakMap<? extends JSObject, JSWeakRef<JSObject>> map,
+            JSObject key);
+
+    @JSBody(params = { "map", "key", "value" }, script = "map.set(key, value);")
+    @NoSideEffects
+    private static native void set(JSWeakMap<? extends JSObject, JSWeakRef<JSObject>> map, JSObject key,
+            JSObject value);
+
+    @NoSideEffects
     public static Object maybeWrap(Object o) {
         return o == null || isJava(o) ? o : wrap(o);
     }