Virtuals in mongoose, 'this' is empty object

Question

ok, i'm new to mongoose and trying to understand how to use virtual properties. this is a sample code that i've been testing.

var mongoose = require('mongoose');
var Schema = mongoose.Schema;

var objSchema = new Schema({
  created: {type: Number, default: Date.now()},
});

objSchema.virtual('hour').get(()=>{
  //console.log(this);
  var d = new Date(this.created);
  return d.getHours();
});

var obj = mongoose.model('obj', objSchema);

var o = new obj();
o.toObject({virtuals: true});
console.log(o.created);
console.log(o.hour);

so i expect the log to be something like :

1457087841956
2

but the output is

1457087841956
NaN

and when i log 'this' at the beginning of the virtual getter, it prints {}. what am i doing wrong?

score 39 · Accepted Answer · answered Mar 04 '16 at 12:30

39

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();
});

answered Mar 04 '16 at 12:30

zangw

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2

thanks, couldn't figure that out my self in a million year :) – Shahriar HD Mar 04 '16 at 14:43
1

Is there an elegant way to avoid this problem all the while using `fat arrow` syntax? Thanks – theprogrammer Nov 01 '19 at 02:55
same here, this saved me hours of debugging even being from 8 years ago – sebastian-greco Jun 04 '21 at 14:22
hours and hours of Debugging saved here... Thank you for asking the question, and for the answer!!! – Martin Moss Mar 02 '23 at 17:42

Virtuals in mongoose, 'this' is empty object

1 Answers1