'this' is undefined in a Mongoose pre save hook [duplicate]

Question

I have made a Mongoose database schema for a User entity, and want to add the current date in an updated_at field. I am trying to use the .pre('save', function() {}) callback but every time I run it I get an error message telling me this is undefined. I've also decided to use ES6, which I guess could be a reason for this (everything works though). My Mongoose/Node ES6 code is below:

import mongoose from 'mongoose'

mongoose.connect("mongodb://localhost:27017/database", (err, res) => {
  if (err) {
    console.log("ERROR: " + err)
  } else {
    console.log("Connected to Mongo successfuly")
  }  
})

const userSchema = new mongoose.Schema({
  "email": { type: String, required: true, unique: true, trim: true },
  "username": { type: String, required: true, unique: true },
  "name": {
    "first": String,
    "last": String
  },
  "password": { type: String, required: true },
  "created_at": { type: Date, default: Date.now },
  "updated_at": Date
})

userSchema.pre("save", (next) => {
  const currentDate = new Date
  this.updated_at = currentDate.now
  next()
})

const user = mongoose.model("users", userSchema)
export default user

The error message is:

undefined.updated_at = currentDate.now;
                       ^
TypeError: Cannot set property 'updated_at' of undefined

EDIT: Fixed this by using @vbranden's answer and changing it from a lexical function to a standard function. However, I then had an issue where, while it wasn't showing the error anymore, it wasn't updating the updated_at field in the object. I fixed this by changing this.updated_at = currentDate.now to this.updated_at = currentDate.

Answer 1

the issue is your arrow function uses lexical this https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

change

userSchema.pre("save", (next) => {
  const currentDate = new Date
  this.updated_at = currentDate.now
  next()
})

to

userSchema.pre("save", function (next) {
  const currentDate = new Date()
  this.updated_at = currentDate.now
  next()
})