Save data to file in a file system using fs in Nodejs

I want to save data to a file in a local file system.

the action what i want to implement is when a button click happens the data should pass to the back end and save the data using fs.

i have a fs code which is working fine.

 var fs = require("fs");
 var path = "D:\\Temp\\Test.txt";
 var data = "Hello";

 fs.writeFile(path, data, function(error) {
 if (error) {
   console.error("write error:  " + error.message);
 } else {
   console.log("Successful Write to " + path);
 }
 });

then i used above fs code in my developing as below.

angular front end Controller

$scope.sendToBack = function ()
{
     var csvData = {
                   dat: str
                   };           

       Event.postDataToCepFs(csvData)
           .success(function () {
               $scope.status = 'Reading the selected file';

           })
           .error(function (error) {
              $scope.status = 'Unable to insert data: ' ;
           });


    return str;

   };

FrontEnd Service Layer

  postDataToCepFs : function(event) {
        return $http.post('/api/sendDataToCepFs', event);
        $window.location.reload();
    },

Backend Route

app.post('/api/sendDataToCepFs', function(req, res) {

        var path = "D:\\Test.txt";        

          fs.writeFile(path, res.body.dat, function(error) {
               if (error) {
                 console.error("write error:  " + error.message);
               } else {
                 console.log("Successful Write to " + path);
               }
          });        
     });

The above is my approach.but it gives "Failed to load resource: the server responded with a status of 500 (Internal Server Error)"

can anyone help me this this?

Thanks