Read USB Serial data and write to SD card

Hello,
How do I go about reading Serial data coming from a PC via USB and write that to the SD card?
I've tried the code below but nothing gets written to data.txt and LED2 doesn't flash.

    function onInit() {
    digitalWrite(LED2,1);
    var cmd="";
    var fs = require("fs");

    Serial1.setup(9600/*baud*/);
    Serial1.on('data', function () { 
      digitalWrite(LED2,0);
  
      cmd+=data;
      var idx = cmd.indexOf("\r");

      while (idx>=0) {
        var line = cmd.substr(0,idx);
        cmd = cmd.substr(idx+1);
        var s = "'"+line+"' = "+eval(line); 
        idx = cmd.indexOf("\r");
    
        fs.appendFile("data.txt", s+"\r");
      }
      digitalWrite(LED2,1);
  
    });  
    }