diff --git a/jso/apis/src/main/java/org/teavm/jso/browser/Navigator.java b/jso/apis/src/main/java/org/teavm/jso/browser/Navigator.java
index 854505c82..e9bda2384 100644
--- a/jso/apis/src/main/java/org/teavm/jso/browser/Navigator.java
+++ b/jso/apis/src/main/java/org/teavm/jso/browser/Navigator.java
@@ -21,6 +21,6 @@ public final class Navigator {
     private Navigator() {
     }
 
-    @JSBody(script = "return (window || self).navigator.onLine;")
+    @JSBody(script = "return navigator.onLine;")
     public static native boolean isOnline();
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/browser/Performance.java b/jso/apis/src/main/java/org/teavm/jso/browser/Performance.java
index 37c2b7009..a67c5c10e 100644
--- a/jso/apis/src/main/java/org/teavm/jso/browser/Performance.java
+++ b/jso/apis/src/main/java/org/teavm/jso/browser/Performance.java
@@ -22,9 +22,9 @@ public final class Performance implements JSObject {
     private Performance() {
     }
 
-    @JSBody(script = "return (window || self).performance.now();")
+    @JSBody(script = "return performance.now();")
     public static native double now();
 
-    @JSBody(script = "return typeof((window || self).performance) !== 'undefined';")
+    @JSBody(script = "return typeof(performance) !== 'undefined';")
     public static native boolean isSupported();
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/browser/Window.java b/jso/apis/src/main/java/org/teavm/jso/browser/Window.java
index a2a370412..0f00d8f5a 100644
--- a/jso/apis/src/main/java/org/teavm/jso/browser/Window.java
+++ b/jso/apis/src/main/java/org/teavm/jso/browser/Window.java
@@ -25,6 +25,8 @@ import org.teavm.jso.dom.html.HTMLElement;
 import org.teavm.jso.dom.html.HTMLIFrameElement;
 
 public abstract class Window implements JSObject, WindowEventTarget, StorageProvider, JSArrayReader<HTMLIFrameElement> {
+    private static Window cachedInstance;
+
     private Window() {
     }
 
@@ -159,9 +161,12 @@ public abstract class Window implements JSObject, WindowEventTarget, StorageProv
         postMessage(message, targetOrigin, JSArray.of(transfer));
     }
 
-    @JSBody(script = "return window || self;")
+    @JSBody(script = "return window;")
     public static native Window current();
 
+    @JSBody(script = "return self;")
+    public static native Window worker();
+
     @JSBody(params = "uri", script = "return encodeURI(uri);")
     public static native String encodeURI(String uri);