Hi,   Thank you for pointing that out. It has certaintly speed up my development.   This is a short snippet to my solution:   var StreamCallback = Java.type("org.apache.nifi.processor.io.StreamCallback"); var IOUtils = Java.type("org.apache.commons.io.IOUtils"); var StandardCharsets = Java.type("java.nio.charset.StandardCharsets"); var flowFile = session.get(); if(flowFile != null) { try { // Create a new StreamCallback, passing in a function to define the interface method flowFile = session.write(flowFile, new StreamCallback(function(inputStream, outputStream) { var text = IOUtils.toString(inputStream, StandardCharsets.UTF_8); var obj = JSON.parse(text); obj["gender"] = "Male"; outputStream.write(JSON.stringify(obj).getBytes(StandardCharsets.UTF_8)); })); session.transfer(flowFile, REL_SUCCESS); } catch (e) { log.error(e.message) session.transfer(flowFile, REL_FAILURE); } }   Something to point out for those using javascript. If your using GenerateFlowFile processor to generate sample JSON content, the JSON content in Custom Text must use double quotes ( " ), not single quotes ( ' ). I spent an hour debugging why my JSON.parse(text) didn't work.   Custom Text   {'name':'myName'}   It should be   {"name":"myName"}           ... View more