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Private and Static Blocks

Private Members and Static Blocks

Since ES2022 a # prefix makes a field, method or accessor private, on instances or on the class (static #x). Using obj.#count outside the class body is a SyntaxError; subclasses cannot see it either, and #count in obj tests for the field safely. A static { } block runs once while the class is defined and may use try...catch and private statics. Symbols vs Private Fields compares private fields with Symbol keys.

Private fields and methods, a brand check and a static blockJavaScriptLive
class Counter {
  #count = 0;                                            // private field
  static { console.log('static block runs once'); }      // at class-definition time
  #step() { return 1; }                                  // private method
  increment() { this.#count += this.#step(); return this; }
  get value() { return this.#count; }
  static isCounter(obj) { return #count in obj; }        // ergonomic brand check
}
const c = new Counter().increment().increment();
console.log(c.value, Counter.isCounter(c), Counter.isCounter({}), JSON.stringify(c));
try { c.increment.call({}); } catch (e) { console.log(e.message); }