Virtuals in mongoose, 'this' is empty object
The issue is the arrow function used in virtual function, same issue could be found here ES6 anonymous function and schema methods, the reason is the Lexical this feature of arrow function
To solve it, please change your codes as below
objSchema.virtual('hour').get(function(){ console.log(this.created); var d = new Date(this.created); return d.getHours();});