0007. Enable noUncheckedIndexedAccess for the delivery-layer lookups

Status: Accepted · 2026-07-28 · #66

TL;DR We enable noUncheckedIndexedAccess. Room and sid lookups return Set | undefined, so the compiler catches the delivery layer's specific failure mode. This is a targeted safety rule for the map lookups, not a move toward general type strictness.

Decision

We turn on noUncheckedIndexedAccess. The delivery layer resolves a broadcast by looking a room up in the membership map and a sid up in the socket registry, and both lookups return Set | undefined: a room with no members, or an unknown sid, is undefined, not an empty set. Without the flag the compiler types these lookups as always present, so iterating a missing room type-checks and then throws at runtime.

The value here is narrow and deliberate. This is not a push for broad strictness across the codebase; it is that the one operation the whole project exists to get right, deciding which sockets receive an event, is exactly an indexed map lookup that can miss. With the flag on, the compiler forces the undefined case to be handled at the delivery seam, which is the project's most likely implementation mistake.

Alternatives rejected

• Leave the flag off and guard each lookup by hand. This relies on every author remembering the missing-room case at every lookup site. The flag makes the compiler enforce it once instead of trusting recall at each call.
• Adopt the full family of strict index and optionality flags. That is broader than the problem, and the cost lands on code with nothing to do with delivery. This decision wants only the check that guards the map lookups, so it enables that one flag and no more.