ESNext emits optional class members

Bug Report

🔎 Search Terms

optional class members

🕗 Version & Regression Information

• This is the behavior in every version I tried, and I reviewed the FAQ for entries about optional class members

⏯ Playground Link

Playground link with relevant code

💻 Code

// Consider this class
class C { 
  x?: number; 
}

🙁 Actual behavior

// This is the emitted JS if you target ESNext and have --useDefineForClassFields
class C {
  x;
}

x is emitted as a class member, even though it was optional. This feels wrong, but to make a stronger point consider --exactOptionalPropertyTypes. With this compiler option, we should have the typing invariant !("x" in c) || typeof c.x == "number"; but with this emit, a new instance has the opposite "x" in c && typeof c.x == "undefined".

🙂 Expected behavior

Optional members should probably not be emitted:

class C { }

I am not sure / have not thought about what the implications with respect to useDefineForClassFields semantics are.

[DanielRosenwasser]

I believe that this is intended. If you want to signal that this was truly not present (i.e. “this thing will potentially not be set”), then you’ll need to use the declare modifier.

class C { 
  declare x?: number; 
}

[DanielRosenwasser]

Sounds like this is possibly a bug in --exactOptionalPropertyTypes.