How to delete a field in a document with mongoose?

  node.js, question
User.findOne({
 activeToken: req.params.activeToken,
 activeExpires: {$gt: Date.now()}
 }, function (err, user) {
 if (err) {
 return next(err);
 bracket
 
 if (!  user) {
 Return res.status(400).end ('your activation link is invalid, please re-register!'  );
 bracket
 
 user.active = true;
 
 //Delete the useless token and Expiration Time fields
 delete user.activeToken;
 delete user.activeExpires;
 
 user.save(function (err, user) {
 if (err) {
 return next(err);
 bracket
 Res.end ('Activation Successful');
 });
 });

Why is user.active = true valid and deleting a field directly invalid? How can I delete a field?

delete user.activeToken; //mongoose without this grammar

Just show the fields you want when you findOne.

User.findOne({

activeToken: req.params.activeToken,
 activeExpires: {$gt: Date.now()}
 }, {active:1},function (err, user) {
 //xxxxxx
 })