script mediator get from payload - wso2

Question

I used script mediator to make multi where condition ,i have a method get the script is correct when i get from query parama as below :
<script language="js"><![CDATA[var queryString = "WHERE ";
var AND =" AND ";
var id = mc.getProperty('query.param.id');
var age = mc.getProperty('query.param.age');
var note = mc.getProperty('query.param.note');
queryString += (id)? " id=" + id+" " + AND : " ";
queryString += (age)? " age=" + age+" " + AND : " ";
queryString += (note)? " note=" + note+" " + AND : "";
queryString =queryString.toString();
var n = queryString.lastIndexOf(AND);
mc.setProperty('queryString',queryString.slice(0, n));]]></script>
<property expression="get-property('queryString')" name="uri.var.queryString" scope="default" type="STRING"/>
<call>
<endpoint>
<http method="get" uri-template="http://ITMTNF3RRJSJ:8290/services/studenttest/readstudent?filtreQuery={uri.var.queryString}">
But now i have a method put with multi where i tried get from payload json then i used the iterate mediator to do the update according to the list of objects but it doesnt't work
here is my code:
<resource methods="PUT" uri-template="modif">
<inSequence>
<property expression="json-eval($)" name="JSONPayload" scope="default" type="STRING"/>
<script language="js"><![CDATA[var queryString = "WHERE ";
var AND =" AND ";
var pl_string = mc.getProperty("JSONPayload");
var newPayload="{\"event\": " + pl_string + "}";
queryString += (id)? " id=" + id+" " + AND : " ";
queryString += (age)?" age=" + age+" " + AND : " ";
queryString += (note)? " note=" + note+" " + AND : "";
queryString =queryString.toString();
var n = queryString.lastIndexOf(AND);
mc.setPayloadJSON(newPayload);]]></script>
<property expression="get-property('queryString')" name="uri.var.queryString" scope="default" type="STRING"/>
<log level="custom">
<property expression="get-property('JSONPayload')" name="msgg"/>
</log>
<call>
<endpoint>
<http method="get" uri-template="http://ITMTNF3RRJSJ:8290/services/studenttest/readstudent?filtreQuery={uri.var.queryString}">
<suspendOnFailure>
<initialDuration>-1</initialDuration>
<progressionFactor>-1</progressionFactor>
<maximumDuration>0</maximumDuration>
</suspendOnFailure>
<markForSuspension>
<retriesBeforeSuspension>0</retriesBeforeSuspension>
</markForSuspension>
</http>
</endpoint>
</call>
<iterate expression="//etuidants/etudiant">
<target>
<sequence>
<payloadFactory media-type="json">
<format>{
"_putmodifstudent": {
"id":${payload.etudiant.id},
"nom":"${payload.etudiant.nom}",
"prenom":"${payload.etudiant.prenom}",
"age":"${payload.etudiant.age}",
"note":"${payloadetudiant.note}",
}
}</format>
<args/>
</payloadFactory>
<call>
<endpoint>
<http method="put" statistics="enable" trace="enable" uri-template="http://ITMTNF3RRJSJ:8290/services/studenttest/modifstudent">