ts-r
  • Sound mode S-03, pre-alpha
  • Design record 0007, Accepted

TSR0301

Non-null assertion is not allowed in sound mode. Narrow '{0}' with a check, or use 'expect({0}, "why")'.

A non-null assertion '!' is used in a sound file. Narrow the value with a check, or use expect(value, "why") from @ts-r.dev/guard.

Rule S-03: No non-null assertions (level: sound). Reported by ts-r-check and ts-r-lsp in files that opt in to sound mode. See Sound mode.

What it means

The postfix e! is an unchecked claim that a value isn't null or undefined, so it isn't allowed in sound files, whatever the type of e. A ! on a value that is already non-nullable is reported too: it's harmless today but becomes a lie after a refactor. Definite assignment (x!: T) is a different rule (TSR0801).

Example

Spec case from design record 0007 (checked by the record's case harness, not by this site's build)
// Asserting an index is in range.
const names: string[] = ["a"];
export const first: string = names[0]!.trim(); // @expect TSR0301

How to fix

  • Narrow with a check: const el = document.getElementById("app"); if (el) { ... }.
  • Use expect(value, "why it is set") from @ts-r.dev/guard, which returns the non-null value and throws at runtime if it's missing.
  • Delete the ! when the operand is already non-nullable.

Sound mode is defined in design record 0007 (accepted 2026-10-15).