Question presented to candidate: "Before the # syntax existed, developers used a leading underscore, like this._balance, to signal a field was 'private.' What's actually different about a real # private field — is it just a stricter convention, or something more?"
What a strong answer should cover:
- 📌 Interview term:
#field— a class field prefixed with#, genuinely inaccessible from outside the class body — not a naming convention developers are trusted to respect, but a real, enforced language restriction. - 📌 Interview term: the real, direct answer to the prompt — verified directly: attempting to access a
#field from OUTSIDE its class body is a genuineSyntaxError— caught at parse time, before the code even runs — fundamentally different from an underscore-prefixed field, which remains a completely ordinary, fully public, accessible property that a developer could still reach (obj._balance) despite the naming hint. - 📌 Interview term: private methods too — verified directly:
#-prefixed methods work identically — a real, genuinely inaccessible private method, callable only from within the class's own other methods. - 📌 Interview term: hidden from enumeration and serialization — verified directly:
Object.keys()andJSON.stringify()genuinely exclude private fields entirely — they do not appear in either, unlike an underscore-prefixed public field, which would appear in both. - A precise answer names the ergonomic brand-check syntax
#field in obj(an ES2022 addition) — verified directly, correctly distinguishing a genuine instance of the class from an unrelated plain object — a real, safe way to check "is this actually one of my instances" without risking a thrown error from directly accessing the private field on a potentially-wrong object type.
Clarifying questions expected:
- None — this is a definitional/comparison question; directly answering "is it enforced or just convention" with real proof of a genuine SyntaxError is the strong signal.
Code / implementation expected: Yes — a real class with a genuine #-prefixed field, verified to be inaccessible from outside via a real caught SyntaxError, is the clearest, most convincing demonstration.