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@studnicky/v8/prototype-modification ​

Disallows whole-prototype and prototype-property assignments, __proto__ assignments, and calls that pass a prototype to Object.assign, Object.defineProperty, Object.defineProperties, Object.setPrototypeOf, Reflect.set, or Reflect.setPrototypeOf. Computed forms of the Object methods resolve through their TypeScript identity, so Object['assign'](...) is covered as well.

A module-top-level mutation outside every function and loop is exempt because that shape is provably one-shot before an instance exists. Mutations nested in a function or a loop, including a per-element iteration callback, are reported. The measured hazard is post-instantiation mutation: after V8 optimized a hot method call, adding a prototype member cleared its optimized status and forced recompilation. That is a one-time latency or startup stall; sustained calls can re-optimize.

Fixable: No · Options: No · Suggested severity: error

✗ Incorrect ​

ts
function addMethod(): void {
  Object.assign(Worker.prototype, {
    describe(): string {
      return 'worker';
    }
  });
}
ts
for (const target of targets) {
  target.__proto__ = replacement;
}

✓ Correct ​

ts
class Worker {
  public describe(): string {
    return 'worker';
  }
}
ts
class Worker {
  public value = 1;
}

Object.assign(Worker.prototype, {
  describe(): string {
    return String(this.value);
  }
});