How to update single record in mongodb from node.js

I am using mongo db on nodejs. I am trying to update record by unique key. When I prepare arguments pro grammatically, getting failed. If I hard coded the set params, working fine.

How to create $set parameters in run time on node.js server.

code:

var collection = db.collection('test', function(err, collection) {

 var args = "{field1: 5}";

 collection.update({unique_key_field: val}, {$set: args}, w:1, function(e, r) {

  //always failed.
  // If I copy {field1: 5} at args place, it's updating properly.

 });;
});