'dette' er udefineret i en Mongoose pre save hook

problemet er, at din pilefunktion bruger leksikalsk denne https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Functions/Arrow_functions

ændre

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

til

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