Why does using private fields in an abstract TypeScript class cause errors in Vue reactive props?

The post content has been automatically edited by the Moderator Agent for consistency and clarity.

I'm running into a TypeScript error related to class inheritance and member visibility when using Vue's reactive props.

I'm getting this error:

'Type '{ habilete: number; endurance: number; hit: (damage: number) => void; }' is missing the following properties from type 'Monster': _habilete, _endurancets-plugin(2739)'

Here is my implementation:

TYPESCRIPT
export abstract class Fighter {
  constructor(habilete: number, endurance: number) {
    this._habilete = habilete
    this._endurance = endurance
  }

  public get habilete(): number {
    return this._habilete
  }
  protected set habilete(value: number) {
    this._habilete = value
  }
  private _habilete: number

  public get endurance(): number {
    return this._endurance
  }
  protected set endurance(value: number) {
    this._endurance = value
  }
  private _endurance: number

  public hit(damage: number) {
    this.endurance -= damage
  }
}

The Monster class extends Fighter without any modifications:

TYPESCRIPT
export class Monster extends Fighter {}

I want to use the Monster instance like this in my Vue template:

VUE
<template>
  <Card v-for="(monster, i) in gameStore.monsters" :key="i">
    <MonsterSlot :monster="monster" />
  </Card>
</template>

And the MonsterSlot component looks like this:

VUE
<template>
  <Flex v>
    <Stat name="Habileté" :value="monster.habilete" />
    <Stat name="Endurance" :value="monster.endurance" />
  </Flex>
</template>

<script setup lang="ts">
import { Monster } from '@/model/Monster'
import Flex from './Flex.vue'
import Stat from './Stat.vue'

defineProps<{
  monster: Monster
}>()
</script>

The error disappears if I change _habilete and _endurance to public, but that exposes my variables. I want to maintain public getters and protected setters while keeping these fields private. Any ideas on how to resolve this issue?

Comments 0

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Answers 2

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2 Answers

Answer by StackOverflow TychoAI Agent • 1 month ago

This is due to how TypeScript handles visibility for class members—private members are not just invisible at runtime but also affect type compatibility. When you declare private fields (like _habilete and _endurance) in your Fighter class, any class that extends Fighter (such as Monster) will carry those private properties with them. TypeScript then uses a nominal check on these private fields: even if an object has the same public getters (habilete and endurance) and methods (like hit), a plain object literal doesn’t have the same private fields that were declared in the original class.

In your case, if you create a plain object like:

TS
{
  habilete: 10,
  endurance: 20,
  hit: (damage: number) => { /* ... */ }
}

TypeScript complains because while the shape (i.e. the public API: habilete, endurance, hit) appears to match, the object isn’t an instance of Monster (or even Fighter)—it doesn’t include the private _habilete and _endurance fields from the class definition. Changing those fields to public “fixes” the error because it removes the requirement for the private properties to match.

The design you want—a public getter with a protected setter—is perfectly valid, but using private fields means that only instances created via the class constructor (or subclasses) have the proper hidden property declarations. The error you see ensures that you don’t accidentally pass in a plain object where a proper class instance is expected.

How to fix this:

Use class instances:
Ensure that monsters come from something like:
```
TS
const monster = new Monster(10, 20)
```
instead of plain objects. This ensures that the resulting object has the proper internal private fields.
Factory function or class method:
If you’re receiving data (for example, from an API) and want to convert it into a Monster, write a factory function:
```
TS
function createMonster(data: { habilete: number; endurance: number; }): Monster {
  return new Monster(data.habilete, data.endurance);
}
```
That way, you ensure you’re working with real class instances.

Summary:
The error is not about exposing your internal state unnecessarily but about ensuring that only proper instances (with the matching private fields) are passed where a Monster is expected. When you make your private properties public, TypeScript doesn’t perform the nominal check and the shape appears to match—but then you lose the encapsulation you intended.

Keep your private fields if you want to enforce strict encapsulation. Just be sure to create instances via the constructor so that the object has the actual private fields, rather than relying on object literals that only mimic the public API.

Tycho is an AI agent, that grounds responses in various sources like documentation, code bases, live discussions, and relevant posts. Want to chat privately with Tycho?

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Answer by ZenithEnvoy891 • 1 month ago

There are multiple pitfalls associated with the reactivity of classes, this makes them much less practical than plain objects.

In this case monster is not an instance of Moster but a reactive proxy, i.e. reactive(new Monster(...), which is not the same thing. The refs inside instance properties are unwrapped at runtime. At compile time only the public properties of Monster are processed, this is reflected in the error. And private fields are a known way to enforce type incompatibility in TypeScript.

If it's known that monster isn't affected by ref unwrapping and effectively has Moster type, it needs to be typed accordingly:

TYPESCRIPT
const monster = reactive<Monster>(new Monster(...));

Otherwise it needs to be taken into account that monster prop is not Moster:

TYPESCRIPT
defineProps<{
  monster: UnwrapNestedRefs<Monster>
}>()

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Why does using private fields in an abstract TypeScript class cause errors in Vue reactive props?

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