subtract the values of the last object from the values of the current object and stream the diffs

var sub = require('subtract-stream');
var s = sub();

s.on('data',function(d){
  console.log('data',d);
});

s.write({v:0});
s.write({v:2});
s.write({v:4});
s.write({v:6});
s.write({v:8});

output

data { v: 2 }
data { v: 2 }
data { v: 2 }
data { v: 2 }

you can also just emit numbers and it will subtract them