// Case 1: toString() from prototype
class Prop {
  toString() {
    return 1 // Number
  }
}
style.setProperty('opacity', new Prop) // Ok, new value: 1

// Case 2: toString() from instance
const Prop = {
  toString() {
    return 1
  }
}
style.setProperty('opacity', Prop) // Ok, new value: 1

// Case 3: recursive toString()
class RecursiveProp {
  toString() {
    return new Prop()
  }
}
style.setProperty('opacity', new RecursiveProp) // Error