I am able to retrieve value of all custom attributes, except contributor attribute, rather it returns null value. Customer is current using javascript to retrieve contributor value

Below is example of javascript

var approver = workItem.getValue("attribute.approver");

text = "Approve: " + approver;

console.log(text);

------------------------

!ENTRY com.ibm.team.rtc.common.scriptengine 1 0

!MESSAGE LOG: Approve: null

I have referred to https://jazz.net/forum/questions/76762/how-to-fecth-the-values-of-contributor-list-type-attribute and https://jazz.net/forum/questions/13795/retrieving-icontributorhandle-using-contributor-attributes
however Icontributor class seem to be used for client plugin, which mean I may not be able to use for custom attributes.

Is there any way to retrieve value of contributor attribute via javascript? Any assistance will be most appreciated