how resolve error in Adding javaScript to wso2?

I'm trying to add this script (for grouping json response) , it is works in online jsvascript test but in wso2 it returns 202 Accepted in postman and return empty (the "field" is the name of json array and "Libraryid" the key I want to grouped by :

  <api xmlns="http://ws.apache.org/ns/synapse" name="obj" context="/obj">
   <resource methods="POST GET" url-mapping="/get">
      <inSequence>
         <log/>
         <script language="js">payload = mc.getPayloadJSON(); var result= payload.field.reduce(function(result, current) {    result[current.Libraryid] = result[current.Libraryid] || [];    result[current.Libraryid].push(current);    return result;}, {});  mc.setPayloadJSON(result);</script>
         <property name="messageType" value="application/json" scope="axis2" type="STRING"/>
         <respond/>
      </inSequence>
   </resource>
</api>

the error is :

ERROR - obj The script engine returned an error executing the inlined js script function mediate
com.sun.phobos.script.util.ExtendedScriptException: org.mozilla.javascript.WrappedException: Wrapped java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String (<Unknown Source>#1) in <Unknown Source> at line number 1
        at com.sun.phobos.script.javascript.RhinoCompiledScript.eval(RhinoCompiledScript.java:68)
        at javax.script.CompiledScript.eval(CompiledScript.java:92)
        at org.apache.synapse.mediators.bsf.ScriptMediator.mediateForInlineScript(ScriptMediator.java:395)
        at org.apache.synapse.mediators.bsf.ScriptMediator.invokeScript(ScriptMediator.java:290)
        at org.apache.synapse.mediators.bsf.ScriptMediator.mediate(ScriptMediator.java:258)
        at org.apache.synapse.mediators.AbstractListMediator.mediate(AbstractListMediator.java:109)
        at org.apache.synapse.mediators.AbstractListMediator.mediate(AbstractListMediator.java:71)
        at org.apache.synapse.mediators.base.SequenceMediator.mediate(SequenceMediator.java:158)
        at org.apache.synapse.rest.Resource.process(Resource.java:363)
        at org.apache.synapse.rest.API.process(API.java:431)
        at org.apache.synapse.rest.RESTRequestHandler.apiProcess(RESTRequestHandler.java:135)
        at org.apache.synapse.rest.RESTRequestHandler.dispatchToAPI(RESTRequestHandler.java:113)
        at org.apache.synapse.rest.RESTRequestHandler.process(RESTRequestHandler.java:71)
        at org.apache.synapse.core.axis2.Axis2SynapseEnvironment.injectMessage(Axis2SynapseEnvironment.java:325)
        at org.apache.synapse.core.axis2.SynapseMessageReceiver.receive(SynapseMessageReceiver.java:92)
        at org.apache.axis2.engine.AxisEngine.receive(AxisEngine.java:180)
        at org.apache.synapse.transport.passthru.ServerWorker.processNonEntityEnclosingRESTHandler(ServerWorker.java:338)
        at org.apache.synapse.transport.passthru.ServerWorker.processEntityEnclosingRequest(ServerWorker.java:383)
        at org.apache.synapse.transport.passthru.ServerWorker.run(ServerWorker.java:152)
        at org.apache.axis2.transport.base.threads.NativeWorkerPool$1.run(NativeWorkerPool.java:172)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
        at java.lang.Thread.run(Thread.java:748)
Caused by: org.mozilla.javascript.WrappedException: Wrapped java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String (<Unknown Source>#1)
        at org.mozilla.javascript.Context.throwAsScriptRuntimeEx(Context.java:1754)
        at org.mozilla.javascript.MemberBox.invoke(MemberBox.java:148)
        at org.mozilla.javascript.NativeJavaMethod.call(NativeJavaMethod.java:225)
        at org.mozilla.javascript.optimizer.OptRuntime.call1(OptRuntime.java:32)
        at org.mozilla.javascript.gen._Unknown_Source__480._c_script_0(<Unknown Source>:1)
        at org.mozilla.javascript.gen._Unknown_Source__480.call(<Unknown Source>)
        at org.mozilla.javascript.ContextFactory.doTopCall(ContextFactory.java:394)
        at org.mozilla.javascript.ScriptRuntime.doTopCall(ScriptRuntime.java:3091)
        at org.mozilla.javascript.gen._Unknown_Source__480.call(<Unknown Source>)
        at org.mozilla.javascript.gen._Unknown_Source__480.exec(<Unknown Source>)
        at com.sun.phobos.script.javascript.RhinoCompiledScript.eval(RhinoCompiledScript.java:55)
        ... 22 more
Caused by: java.lang.ClassCastException: java.lang.Integer cannot be cast to java.lang.String
        at org.apache.synapse.mediators.bsf.CommonScriptMessageContext.serializeJson(CommonScriptMessageContext.java:996)
        at org.apache.synapse.mediators.bsf.CommonScriptMessageContext.setPayloadJSON(CommonScriptMessageContext.java:957)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at org.mozilla.javascript.MemberBox.invoke(MemberBox.java:126)
        ... 31 more

can anyone help me ? I spent 2 weeks to solve the problem ..

Because Wso2 does not accept any methods in javascript , so you should write javascript code in basic format without any methods .