TypeScript patterns that catch bugs before runtime

Types are most valuable when they make an invalid state impossible to write. A few small patterns do most of that work.

Model states as a union

Instead of a bag of booleans, describe each state exactly once. TypeScript then refuses to let you read fields that do not exist in that state.

type RequestState =
  | { status: "idle" }
  | { status: "loading" }
  | { status: "success"; data: string[] }
  | { status: "error"; message: string };

Ask the compiler for exhaustiveness

A `never` check turns a forgotten branch into a build error rather than a mysterious runtime bug.

function assertNever(value: never): never {
  throw new Error(`Unhandled case: ${JSON.stringify(value)}`);
}

Prefer one source of truth

• Derive related values instead of storing them twice.
• Keep optional fields truly optional, not empty strings.
• Use narrow types at the boundaries, then trust them inside.

These patterns cost almost nothing at runtime and remove bugs that are otherwise found in production.