Guarding type when class property is symbol?

Question

I was looking to use a symbol for a property, but I realize because I\'m using this[SYM] syntax, I can\'t get guarding to work properly. I was wondering if there wa

Answer 1

Narrowing the type of an accessed property does not narrow the type of the parent object. So checking this[SYM] doesn't tell TS anything, because it's not in a standalone variable. That's why extracting the value into v works - it's a standalone variable, so it can be narrowed.

A more concise version would be:

const SYM = Symbol('SYM');
class Example {
  [SYM]?: string;
  get prop() {
    return this[SYM] ?? 'default';
  }
}

No need to explicitly annotate the return value of prop once your typings are correct.