How can the email of a user in MongoDB be altered using a web application built with Node, Express, EJS, and Mongoose?

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First, you would need to create a form in your web application that allows the user to input their new email address. This form would then be submitted to your Node and Express server.

On the server-side, you would use Mongoose to find the user in your MongoDB database based on their unique ID. Once you have retrieved the user object, you can simply update the email property with the new email address provided by the user.

Finally, you would save the changes to the database using Mongoose's save() method and notify the user that their email has been updated. Here's some sample code to get you started:

// Route for updating user email
app.post('/users/:id/update-email', (req, res, next) => {
  const { id } = req.params;
  const { newEmail } = req.body;

  // Find user by ID in MongoDB
  User.findById(id, (err, user) => {
    if (err) {
      return next(err);
    }

    // Update user email
    user.email = newEmail;

    // Save changes to database
    user.save((err) => {
      if (err) {
        return next(err);
      }

      // Notify user of email update
      res.send(`Your email has been updated to ${newEmail}.`);
    });
  });
});

Note that this code assumes you have already set up a Mongoose model for your user objects and have connected to your MongoDB database using Mongoose.