Iron-Clad Encapsulation

Private fields (starting with #) are not just a naming convention — they are physically enforced by the JavaScript engine.

How # Fields Differ From _ Convention

Feature	_field (convention)	#field (private)
Actually private?	No — just a warning sign	Yes — enforced by engine
Accessible via string?	Yes: obj['_field']	No — obj['#field'] returns undefined
Visible in for..in?	Yes	No
Error timing	Runtime (if accessed wrong)	Compile-time (SyntaxError)

class User {
  #password = '12345';

  checkPassword(input) {
    return this.#password === input;
  }
}

const user = new User();
console.log(user.checkPassword('12345')); // true
console.log(user.#password); // SyntaxError — blocked at parse time!