ts-r
  • Error
  • Reported by the type checker

TS18048

'{0}' is possibly 'undefined'.

A value that may be `undefined` is used without a check, such as an optional property or parameter. Handle `undefined` first.

What it means

With strictNullChecks, a value whose type includes undefined (an optional property or parameter, a find result, an indexed read under noUncheckedIndexedAccess) can't be dereferenced directly.

Example

index.ts
interface User {
  address?: { city: string };
}
export function city(u: User) {
  return u.address.city;
}
ts-r-check -p tsconfig.json --pretty false ts-r output
index.ts(5,10): error TS18048: 'u.address' is possibly 'undefined'.

Recorded by node scripts/verify-examples.mjs with ts-r built from commit 6deaf01. The example uses the default tsconfig: strict, target: es2022, module: esnext, moduleResolution: bundler.

How to fix

  • Check it: if (!u.address) return "";.
  • Use optional chaining and a default: u.address?.city ?? "".
  • If it's always present, make the property required in the type.