// Prefer staticAssertUnreachable in the default branch of switches that must cover every variant of a union, so exhaustiveness is checked at compile time. Use when writing or reviewing switch on discriminated unions, string literal unions, enums, or any switch meant to be exhaustive in the Tachi TypeScript server.
Patterns for writing actions (MakeAction/MakeAnonAction), using the Postgres DB (Kysely), and migrating Express routes from MongoDB to Postgres in the Tachi server. Use when adding a new mutation, migrating a Mongo-backed router to Postgres, writing action files, or writing tests for actions or routers.
Aggregates Vitest v8/Istanbul coverage across Tachi workspaces via tachi-coverage-tools (manifest, CLI, optional programmatic API). Use when measuring test coverage, reviewing coverage after changes, adding CI gates, registering a new Vitest package, or when the user mentions coverage reports, coverage-final.json, or just coverage-report.
Guidance on using db-formats/ column lists (SELECT_*) and document mappers (To*Document) instead of selectAll() when writing Postgres-backed endpoints in the Tachi server. Use when writing a new endpoint that reads from a Postgres table, adding a new db-formats file, or when working with Kysely selects that need to return API-compatible (v1-mongodb-like) response shapes.
Migrates server code from MongoDB to Postgres without modifying the legacy Mongo service. Never edits typescript/server/src/services/mongo/ (especially db.ts). Requires removing MONGODB_KILL imports and replacing usage with Postgres (Kysely) in migrated files. Use during Mongo-to-Postgres migration, when migrating routers/actions/utils off
Prefer `return` over `return await` in async functions when the await is redundant. Use when writing or reviewing async TypeScript/JavaScript, simplifying control flow, or fixing redundant awaits on returned promises.
Preserves existing code comments (line, block, JSDoc, region markers) when editing files; does not delete or collapse comments for brevity unless the user asked to remove or update them. Use when editing any source file, refactoring, or applying fixes where comments already exist.
| name | exhaustive-switch |
| description | Prefer staticAssertUnreachable in the default branch of switches that must cover every variant of a union, so exhaustiveness is checked at compile time. Use when writing or reviewing switch on discriminated unions, string literal unions, enums, or any switch meant to be exhaustive in the Tachi TypeScript server. |
staticAssertUnreachableUse this pattern when the switch is meant to be exhaustive: every value of the discriminant type should have a case, and adding a new variant later should cause a TypeScript error until a new case is added.
Import staticAssertUnreachable from #utils/misc.js (server code).
Bind the switched value in a variable if needed so the default branch receives a narrowed type.
In default, call staticAssertUnreachable(discriminant) with the same value you are switching on (after all cases, TypeScript should infer its type as never).
import { staticAssertUnreachable } from "#utils/misc.js";
switch (value) {
case "a":
return 1;
case "b":
return 2;
default:
staticAssertUnreachable(value);
}
For a block body:
default: {
staticAssertUnreachable(gpt);
}
Reference: staticAssertUnreachable in typescript/server/src/utils/misc.ts - it takes never and throws at runtime if execution reaches it.
default no longer receives never, and TypeScript reports an error.default that only throw new Error("unreachable") without passing a never-typed value - you lose exhaustiveness checking.default: break or empty default on switches that are supposed to be exhaustive - same problem.default that handles “everything else” with correct typing, not staticAssertUnreachable.string), default will not be never and the pattern will not typecheck - narrow the type first or handle the real domain of values.In typescript/server, prefer #utils/misc.js. Relative imports to ./misc are acceptable next to utils/misc.ts (see conversion.ts).