1var fs = require('fs')
2fs.readFile(someFile, 'utf8', function (err,data) {
3  if (err) {
4    return console.log(err);
5  }
6  var result = data.replace(/string to be replaced/g, 'replacement');
7
8  fs.writeFile(someFile, result, 'utf8', function (err) {
9     if (err) return console.log(err);
10  });
11});