Private Fields and Methods ship with Firefox 90

Firefox will ship Private Fields and Methods in Firefox 90. This new language syntax allows programmers to have strict access control over their class internals. A private field can only be accessed by code inside the class declaration.

class PrivateDetails {
  #private_data = "I shouldn't be seen by others";

  #private_method { return "private data" }

  useData() {
    /.../.test(this.#private_data);

    var p = this.#private_method();
  }
}

var p = new PrivateDetails();
p.useData(); // OK
p.#private_data; // SyntaxError

This is the last remaining piece of the Stage 4 Proposal, Class field declarations for JavaScript, which has many more details about the design of private data.