var testData = {
	name: 'filtered',
       data: 'This should be filtered',
	extras: {
         headers: {
           name: 'update',
           categpory: 'computing',
           location: 'London',
	    product: '"Laptop"',
    quantity: '10' 
         },
       },
     };
     var filterOption = {
       filter: 'headers.category == `"Computing"` && headers.location == `London`',
     };
...
/* set up realtime */
....
var rtChannel = realtime.channels.getDerived('chan', filterOption);
/* subscribe to event */
 rtChannel.subscribe('filtered', function (msg) {