diff --git a/jso/apis/pom.xml b/jso/apis/pom.xml
index d4fed183e..87ae78109 100644
--- a/jso/apis/pom.xml
+++ b/jso/apis/pom.xml
@@ -34,6 +34,11 @@ xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xs
       <artifactId>teavm-jso</artifactId>
       <version>${project.version}</version>
     </dependency>
+    <dependency>
+      <groupId>org.teavm</groupId>
+      <artifactId>teavm-interop</artifactId>
+      <version>${project.version}</version>
+    </dependency>
     <dependency>
       <groupId>junit</groupId>
       <artifactId>junit</artifactId>
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 b735c8c9a..b62178a51 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
@@ -15,6 +15,7 @@
  */
 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;
@@ -83,9 +84,11 @@ public abstract class JSArray<T extends JSObject> implements JSArrayReader<T> {
     public abstract void setLength(int len);
 
     @JSBody(script = "return new Array();")
+    @NoSideEffects
     public static native <T extends JSObject> JSArray<T> create();
 
     @JSBody(params = "size", script = "return new Array(size);")
+    @NoSideEffects
     public static native <T extends JSObject> JSArray<T> create(int size);
 
     @SafeVarargs
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSBoolean.java b/jso/apis/src/main/java/org/teavm/jso/core/JSBoolean.java
index d2cf33d6f..d9c445eb4 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSBoolean.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSBoolean.java
@@ -15,6 +15,7 @@
  */
 package org.teavm.jso.core;
 
+import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 
@@ -27,8 +28,10 @@ public abstract class JSBoolean implements JSObject {
     }
 
     @JSBody(params = "value", script = "return value;")
+    @NoSideEffects
     private static native boolean booleanValue(JSBoolean value);
 
     @JSBody(params = "value", script = "return value;")
+    @NoSideEffects
     public static native JSBoolean valueOf(boolean value);
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSDate.java b/jso/apis/src/main/java/org/teavm/jso/core/JSDate.java
index e9afd6fb4..ee7069e99 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSDate.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSDate.java
@@ -15,63 +15,80 @@
  */
 package org.teavm.jso.core;
 
+import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSMethod;
 import org.teavm.jso.JSObject;
 
 public abstract class JSDate implements JSObject {
     @JSBody(script = "return new Date();")
+    @NoSideEffects
     public static native JSDate create();
 
     @JSBody(params = "millis", script = "return new Date(millis);")
+    @NoSideEffects
     public static native JSDate create(double millis);
 
     @JSBody(params = { "year", "month" }, script = "return new Date(year, month);")
+    @NoSideEffects
     public static native JSDate create(int year, int month);
 
     @JSBody(params = { "year", "month", "day" }, script = "return new Date(year, month, day);")
+    @NoSideEffects
     public static native JSDate create(int year, int month, int day);
 
     @JSBody(params = { "year", "month", "day", "hour" }, script = "return new Date(year, month, day, hour);")
+    @NoSideEffects
     public static native JSDate create(int year, int month, int day, int hour);
 
     @JSBody(params = { "year", "month", "day", "hour", "minute" },
             script = "return new Date(year, month, day, hour, minute);")
+    @NoSideEffects
     public static native JSDate create(int year, int month, int day, int hour, int minute);
 
     @JSBody(params = { "year", "month", "day", "hour", "minute", "second" },
             script = "return new Date(year, month, day, hour, minute, second);")
+    @NoSideEffects
     public static native JSDate create(int year, int month, int day, int hour, int minute, int second);
 
     @JSBody(params = { "year", "month", "day", "hour", "minute", "second", "millisecond" },
             script = "return new Date(year, month, day, hour, minute, second, millisecond);")
+    @NoSideEffects
     public static native JSDate create(int year, int month, int day, int hour, int minute, int second, int millisecond);
 
     @JSBody(params = {}, script = "return Date.now();")
+    @NoSideEffects
     public static native double now();
 
     @JSBody(params = "stringValue", script = "return Date.parse(stringValue);")
+    @NoSideEffects
     public static native double parse(String stringValue);
 
     @JSBody(params = { "year", "month" }, script = "return Date.UTC(year, month);")
+    @NoSideEffects
     public static native double UTC(int year, int month);
 
     @JSBody(params = { "year", "month", "day" }, script = "return Date.UTC(year, month, day);")
+    @NoSideEffects
     public static native double UTC(int year, int month, int day);
 
     @JSBody(params = { "year", "month", "day", "hour" }, script = "return Date.UTC(year, month, day, hour);")
+    @NoSideEffects
     public static native double UTC(int year, int month, int day, int hour);
 
     @JSBody(params = { "year", "month", "day", "hour", "minute" },
             script = "return Date.UTC(year, month, day, hour, minute);")
+    @NoSideEffects
     public static native double UTC(int year, int month, int day, int hour, int minute);
 
     @JSBody(params = { "year", "month", "day", "hour", "minute", "second" },
             script = "return Date.UTC(year, month, day, hour, minute, second);")
+    @NoSideEffects
     public static native double UTC(int year, int month, int day, int hour, int minute, int second);
 
     @JSBody(params = { "year", "month", "day", "hour", "minute", "second", "millisecond" },
             script = "return Date.UTC(year, month, day, hour, minute, second, millisecond);")
+    @NoSideEffects
     public static native double UTC(int year, int month, int day, int hour, int minute, int second, int millisecond);
 
     public abstract int getDate();
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSNumber.java b/jso/apis/src/main/java/org/teavm/jso/core/JSNumber.java
index 52257a8e6..5e8a49fbf 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSNumber.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSNumber.java
@@ -15,6 +15,7 @@
  */
 package org.teavm.jso.core;
 
+import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 
@@ -27,6 +28,7 @@ public abstract class JSNumber implements JSObject {
     }
 
     @JSBody(params = "number", script = "return number;")
+    @NoSideEffects
     private static native double doubleValue(JSNumber number);
 
     public final int intValue() {
@@ -34,6 +36,7 @@ public abstract class JSNumber implements JSObject {
     }
 
     @JSBody(params = "number", script = "return number;")
+    @NoSideEffects
     private static native int intValue(JSNumber number);
 
     public final char charValue() {
@@ -41,6 +44,7 @@ public abstract class JSNumber implements JSObject {
     }
 
     @JSBody(params = "number", script = "return number;")
+    @NoSideEffects
     private static native char charValue(JSNumber number);
 
     public final byte byteValue() {
@@ -48,6 +52,7 @@ public abstract class JSNumber implements JSObject {
     }
 
     @JSBody(params = "number", script = "return number;")
+    @NoSideEffects
     private static native byte byteValue(JSNumber number);
 
     public final short shortValue() {
@@ -55,6 +60,7 @@ public abstract class JSNumber implements JSObject {
     }
 
     @JSBody(params = "number", script = "return number;")
+    @NoSideEffects
     private static native short shortValue(JSNumber number);
 
     public final float floatValue() {
@@ -62,23 +68,30 @@ public abstract class JSNumber implements JSObject {
     }
 
     @JSBody(params = "number", script = "return number;")
+    @NoSideEffects
     private static native float floatValue(JSNumber number);
 
     @JSBody(params = "value", script = "return value;")
+    @NoSideEffects
     public static native JSNumber valueOf(byte value);
 
     @JSBody(params = "value", script = "return value;")
+    @NoSideEffects
     public static native JSNumber valueOf(short value);
 
     @JSBody(params = "value", script = "return value;")
+    @NoSideEffects
     public static native JSNumber valueOf(int value);
 
     @JSBody(params = "value", script = "return value;")
+    @NoSideEffects
     public static native JSNumber valueOf(char value);
 
     @JSBody(params = "value", script = "return value;")
+    @NoSideEffects
     public static native JSNumber valueOf(float value);
 
     @JSBody(params = "value", script = "return value;")
+    @NoSideEffects
     public static native JSNumber valueOf(double value);
 }
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 da4b3e875..0939c08b7 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
@@ -15,6 +15,7 @@
  */
 package org.teavm.jso.core;
 
+import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 
@@ -23,26 +24,33 @@ public final class JSObjects {
     }
 
     @JSBody(params = "object", script = "return Object.getOwnPropertyNames(object);")
+    @NoSideEffects
     public static native String[] getOwnPropertyNames(JSObject object);
 
     @JSBody(script = "return {};")
+    @NoSideEffects
     public static native <T extends JSObject> T create();
 
     @JSBody(script = "return Object.create(null);")
+    @NoSideEffects
     public static native <T extends JSObject> T createWithoutProto();
 
     @JSBody(params = "object", script = "return typeof object === 'undefined';")
+    @NoSideEffects
     public static native boolean isUndefined(JSObject object);
 
     @JSBody(script = "return void 0;")
+    @NoSideEffects
     public static native JSObject undefined();
 
     @JSBody(params = "object", script = "return typeof object;")
+    @NoSideEffects
     public static native String typeOf(JSObject object);
 
     @JSBody(params = "object", script = "return object.toString();")
     public static native String toString(JSObject object);
 
     @JSBody(params = { "object", "name" }, script = "return name in object;")
+    @NoSideEffects
     public static native boolean hasProperty(JSObject object, String name);
 }
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSRegExp.java b/jso/apis/src/main/java/org/teavm/jso/core/JSRegExp.java
index d60dd4da0..9f34c8552 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSRegExp.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSRegExp.java
@@ -15,15 +15,18 @@
  */
 package org.teavm.jso.core;
 
+import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 import org.teavm.jso.JSProperty;
 
 public abstract class JSRegExp implements JSObject {
     @JSBody(params = "pattern", script = "return new RegExp(pattern);")
+    @NoSideEffects
     public static native JSRegExp create(String pattern);
 
     @JSBody(params = { "pattern", "flags" }, script = "return new RegExp(pattern, flags);")
+    @NoSideEffects
     public static native JSRegExp create(String pattern, String flags);
 
     public static JSRegExp create(String pattern, JSRegExpFlag... flags) {
diff --git a/jso/apis/src/main/java/org/teavm/jso/core/JSString.java b/jso/apis/src/main/java/org/teavm/jso/core/JSString.java
index 9c7752064..837e498ab 100644
--- a/jso/apis/src/main/java/org/teavm/jso/core/JSString.java
+++ b/jso/apis/src/main/java/org/teavm/jso/core/JSString.java
@@ -15,6 +15,7 @@
  */
 package org.teavm.jso.core;
 
+import org.teavm.interop.NoSideEffects;
 import org.teavm.jso.JSBody;
 import org.teavm.jso.JSObject;
 import org.teavm.jso.JSProperty;
@@ -28,12 +29,15 @@ public abstract class JSString implements JSObject {
     }
 
     @JSBody(params = "str", script = "return str;")
+    @NoSideEffects
     private static native String stringValue(JSString str);
 
     @JSBody(params = "str", script = "return str;")
+    @NoSideEffects
     public static native JSString valueOf(String str);
 
     @JSBody(params = "code", script = "return String.fromCharCode(code)")
+    @NoSideEffects
     public static native JSString fromCharCode(int code);
 
     @JSProperty
@@ -96,5 +100,6 @@ public abstract class JSString implements JSObject {
     public abstract JSString trim();
 
     @JSBody(params = "obj", script = "return typeof obj === 'string';")
+    @NoSideEffects
     public static native boolean isInstance(JSObject obj);
 }