Compare files and create a new file containing the missing lines. Used like: node File1 File2 FileWithLinesMissingInFile2.txt

CompareFilesFindMissingLines.js

var fs = require('fs');
var oldReport = [];
var newReport = [];
var missingLines = [];

fs.readFile(process.argv[2], {encoding: "utf8"}, function (err, data) {
  if (err) throw err;
  oldReport = data.split('\n');
  readNewReportFile()
});

function readNewReportFile() {
  fs.readFile(process.argv[3], {encoding: "utf8"}, function (err, data) {
    if (err) throw err;
    newReport = data.split('\n');
    runComparison();
  });
}

function runComparison() {
  console.log("comparing files");
  oldReport.forEach(function(oldLine){
    if(newReport.indexOf(oldLine) == -1) missingLines.push(oldLine);
  });
  appendToFile(missingLines);
}

function appendToFile (data) {
  fs.writeFile(process.argv[4], data.join('\n'), {encoding: 'utf8'}, function (err) {
    if (err) throw err;
    console.log('Appended missing lines to file!');
  });
}