How do you set the Reason Code using the JavaScript API?

To set the reason code, you must use the setState() JavaScript API. Here is a code example:

var currentState = user.getState();
if (currentState == "READY") {
   var rc = { id: '5' };
   user.setState(states.NOT_READY, rc, {
      success: success,
      error: makeStateError
   });
}

Note that the id of 5 is the system generated id for the reason code, not the reason code itself.

So for the above code, if you do a REST request to get the list of NOT_READY reason codes, 

http://<FQDN>/finesse/api/User/<id>/ReasonCodes?category=NOT_READY

The response will be:

<ReasonCodes category="NOT_READY">
   <ReasonCode>
      <uri>/finesse/api/ReasonCode/5</uri>
      <category>NOT_READY</category>
      <code>10</code>
      <label>End of Shift</label>
      <forAll>true</forAll>
   </ReasonCode>
</ReasonCodes>

Notice that the value of id is 5 and not 10.