how to parse result from MsProcess::readAllStandardOutput() ?

Hello.
I'd like to get the last line of the result for an external process execution in MuseScore. The process is called via plugin API (MsProcess), and its execution result I collect using readAllStandardOutput()

  console.log("generating MIDI file with command: "+cmd);
  proc.start(cmd);
  var val = proc.waitForFinished(30000);
  if (val) {
     var res = proc.readAllStandardOutput();
     console.log(res);
     console.log("last line of process result is: "+res.lastIndexOf("\n"));  //doesn't work, how can I parse res ?
     return(res);
  } else {
     console.log("Generation of MIDI file failed, please check (blah blah)");
     return;
  }

While the console.log(res); displays the multiple lines of execution log just fine, the next call with res.lastIndexOf("\n") fails miserably stating:

182:-1: TypeError: Property 'lastIndexOf' of object {full content of 'res' displayed} is not a function

How can I parse the content of readAllStandardOutput() or cast it into a usable (multiline) String or do anything with it a bit more complex (regexp?) than log.

Thank you!