Node.js fs ftruncate() file truncate

var fs = require("fs");
var buf = new Buffer(1024);

console.log("Going to open an existing file");
fs.open('input.txt', 'r+', function(err, fd) {
   if (err) {//from   w  ww. j  a  va  2 s.  c  o m
       return console.error(err);
   }
   console.log("File opened successfully!");
   console.log("Going to truncate the file after 10 bytes");
   
   // Truncate the opened file.
   fs.ftruncate(fd, 10, function(err){
      if (err){
         console.log(err);
      } 
      console.log("File truncated successfully.");
      console.log("Going to read the same file"); 
      fs.read(fd, buf, 0, buf.length, 0, function(err, bytes){
         if (err){
            console.log(err);
         }

         // Print only read bytes to avoid junk.
         if(bytes > 0){
            console.log(buf.slice(0, bytes).toString());
         }

         // Close the opened file.
         fs.close(fd, function(err){
            if (err){
               console.log(err);
            } 
            console.log("File closed successfully.");
         });
      });
   });
});