#Ergonomic brand checks for Private Fields S4

#Ergonomic brand checks for Private Fields

EcmaScript Proposal, specs, and reference implementation to provide brand checks without exceptions.

Spec drafted by @ljharb.

Built on top of https://github.com/tc39/ecma262/pull/1668/ pending Class Fields being stage 4 and merged into the larger spec.

This proposal is currently at stage 4 of the process.

#Rationale

Private fields have a built-in ”brand check”, in that if you try to access a private field on an object that does not have it installed, it throws an exception.

This is great! However, in order to keep the existence of a private field actually private, I often want to check if an object has a private field, and if not, have some fallback behavior (which might even be throwing a custom exception, with a message that does not reveal that I’m using a private field as my mechanism).

Using try/catch for this does work, but is quite awkward:

class C {
  #brand;

  static isC(obj) {
    try {
      obj.#brand;
      return true;
    } catch {
      return false;
    }
  }
}

This is also an issue for getters (that might throw):

class C {
  #data = null; // populated later

  get #getter() {
    if (!this.#data) {
      throw new Error('no data yet!');
    }
    return this.#data;
  }

  static isC(obj) {
    try {
      obj.#getter;
      return true;
    } catch {
      return false; // oops! might have gotten here because `#getter` threw :-(
    }
  }
}

What is desired is a simple solution to produce a boolean indicating the presence of a private field, that does not require a try/catch or exceptions.

#Solution:

#in

The most obvious solution is using the in keyword, like so:

class C {
  #brand;

  #method() {}

  get #getter() {}

  static isC(obj) {
    return #brand in obj && #method in obj && #getter in obj;
  }
}

This has no ASI hazards I am aware of; however, it does have one important tradeoff: it permanently kills any possibility for private field shorthand. Specifically, this is because if #brand (in the previous example) is a shorthand for this.#brand, then (#brand) in obj would have to mean the same as this.#brand in obj, which means "is the value of this.#brand in obj?", not "does obj have the private field #brand?".

#Rejected solutions:

#try statement

Another alternative is:

class C {
  #brand;

  static isC(obj) {
    return try obj.#brand;
  }
}

However, this strongly suggests that try <expression> would be a generic syntax for "if it throws an exception, produce false, otherwise produce true". That would be a much larger proposal, and would perhaps be a bit overkill to solve the specific problem around private fields.

This would require a lookahead restriction for a curly brace (which would mean that the try expression couldn't be an object literal, unless it was wrapped in parens).

#Spec

You can view the spec for the in solution rendered as HTML.

#Implementations

• Babel: https://babeljs.io/docs/en/babel-plugin-proposal-private-property-in-object / https://www.npmjs.com/package/@babel/plugin-proposal-private-property-in-object