ts-r
  • Error
  • Reported by the type checker

TS18049

'{0}' is possibly 'null' or 'undefined'.

A value that may be `null` or `undefined` is used without a check. Handle both cases first.

What it means

With strictNullChecks, a value whose type includes both null and undefined can't be dereferenced directly. A loose != null check covers both.

Example

index.ts
export function upper(s: string | null | undefined) {
  return s.toUpperCase();
}
ts-r-check -p tsconfig.json --pretty false ts-r output
index.ts(2,10): error TS18049: 's' is possibly 'null' or '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 both at once: if (s == null) return "";.
  • Use optional chaining and a default: s?.toUpperCase() ?? "".