When programming with Private Fields and methods, it can sometimes be desirable to check if an object has a given private field. While the semantics of private fields allow doing that check by using try...catch, the Ergonomic Brand checks proposal provides a simpler syntax, allowing one to simply write #field in o.

As an example, the following class uses ergonomic brand checks to provide a more helpful custom error.

class Scalar {
  #length = 0;

  add(s) {
    if (!(#length in s)) {
      throw new TypeError("Expected an instance of Scalar");
    }

    this.#length += s.#length;
  }
}

While the same effect could be accomplished with try...catch, it’s much uglier, and also doesn’t work reliably in the presence of private getters which may possibly throw for different reasons.

This JavaScript language feature proposal is at Stage 3 of the TC39 process, and will ship in Firefox 90.

https://spidermonkey.dev/blog/2021/05/18/ergonomic-brand-checks.html