Memory leaks when using closure

I'm using express and node, and add a middleware by myself in order to convert plain text to json object, the code is as follows:

app.use(function(req, res, next){
   var tmpJson ='';
   req.setEncoding('utf8');
   req.on('data', function(chunk){
       tmpJson += chunk;
   });
   req.on('end', function(){
       req.json = JSON.parse(tmpJson);
       next();
       tmpJson = null;
   });
});

Will this code caused a memory leak? As far as I know, when using req.json in callback function of end event, it will increase a count of usage for req, and that will make this req never released?

If it does cause a memory leak, is there anyway I could avoid using this?