TS18048

Possibly undefined

'maybe.a' is possibly 'undefined'.

The compiler’s own words. Not translated — this is the string you pasted into a search box.

An optional property was used without checking it. Under `strictNullChecks`, "optional" means the absence is part of the type and has to be handled.

Reproduction

declare const maybe: { a?: { b: number } }

maybe.a.b

The build asserts this emits exactly this code.

Why the compiler says this

Without `strictNullChecks` this compiles and throws at run time; with it, the gap becomes visible where it can still be fixed. The error is not asking you to prove the value exists — it is asking you to say what should happen when it does not, which is a question the code was silently answering with "crash".

Fixes

01
```
declare const maybe: { a?: { b: number } }

const value = maybe.a?.b
```
Optional chaining answers "nothing happens, the result is `undefined`" — which is right when the caller can cope with that.
02
```
declare const maybe: { a?: { b: number } }

if (maybe.a) {
  const value = maybe.a.b
}
```
A guard answers "we do nothing at all in that case", and narrows the type for the whole block rather than one access.

Takeaway

The non-null assertion `!` also silences this, and it is the one fix that answers the question with a promise instead of code. Keep it for the cases you can actually prove.

Where to go next

Modifiers Try it in the Lab

Errors