tsc TS18048: 'x' is possibly 'undefined' (strictNullChecks) in CI

TS18048 fires under strictNullChecks when you read a value whose type includes undefined without first narrowing it. CI commonly hits this when its tsconfig is stricter than the local one.

What this error means

tsc fails with "error TS18048: 'x' is possibly 'undefined'" at a property access or call on a value typed as optional or T | undefined.

tsc

src/parse.ts:5:10 - error TS18048: 'match' is possibly 'undefined'.

5   return match[1].trim();
           ~~~~~

Common causes

A value typed T | undefined is used without a guard

Optional properties, Array.find, and regex match return T | undefined. Using the result directly is rejected under strictNullChecks.

CI enables strict where the local config did not

The error did not appear locally because the developer's tsconfig had strictNullChecks off, while the CI config (or a stricter base config) turns it on.

How to fix it

Guard the value before using it

Check whether the value is undefined before accessing members.
Return early, throw, or provide a default for the undefined case.
Re-run tsc to confirm the access is now safe.

src/parse.ts

const match = re.exec(input);
if (!match) throw new Error('no match');
return match[1].trim();

Use optional chaining or a nullish default

When undefined is acceptable, use ?. or ?? to express the fallback explicitly instead of a non-null assertion.