TypeScript

Concrete TypeScript idioms that make the architecture skill's principles correct-by-construction: the how, not the agnostic why or the enforceable config - see routing.

Routing - who owns what

Rule: state the idiom and its why here, point out for the rest, and never copy their tables.

Adapt first

Read the repo first. These are defaults for greenfield or where the repo has no convention - not a migration mandate.

Does the repo already have a convention for this concern?
|-- errors      -> its Result type or Effect; no rival
|-- schema      -> its parser (effect Schema / zod / valibot); no second one
|-- modules     -> its file layout and import style before "fixing" it
|-- concurrency -> its cancellation convention (signal parameter or none) before adding one
|-- tests       -> its runner and double strategy
`-- none        -> the defaults here; integrate, don't migrate

Priority when rules pull apart: correctness/safety > existing conventions > better local design > avoiding broad migrations > documenting the trade-off. A new code path follows these standards; an unrelated change migrates nothing.

Effect boundary. Use Effect when the repo already depends on it. In another repo, typed errors, dependency injection, retry policy and structured concurrency together justify proposing adoption for discussion, never introducing it silently; one or two alone is a Result plus the idioms below. Discuss the target version, then read the effect skill for everything inside an Effect program. This skill owns the seam only: see references/errors.md.

Runtime floor. The compiler is not the runtime: Node strips types and checks nothing, so only erasable syntax runs, and a global the lib types (Temporal, DisposableStack) may not exist where the code runs. references/toolchain.md owns the floor (Node 24), the capability facts, and every dated library fact - no other page carries a version number.

Core idioms

Errors as values

Expected failures - domain, parsing, auth, I/O, persistence - belong in the return type, not a thrown exception, and a rejected promise is a throw.

Promise<Result<User, UserNotFound | UserStoreUnavailable>>  // not Promise<User>

Throwing is for defects only; a boundary exception translates at the shell. See references/errors.md.

Composing fallible steps

Chain with map and flatMap so the first error short-circuits, map every step's error into one declared channel, and collapse a Result[] with all, never a hand-rolled loop. Fail fast for dependent steps; accumulate for independent validations. See references/errors.md.

Make signatures total and honest

Constrain the input to a parsed or branded value, or widen the output to Result; constraining is better, deleting the branch for every caller. A void return from core logic hides a mutation. See references/parsing.md.

Parse, don't validate

Turn unknown into domain types once at the boundary and keep the refined type. Name parsers parseX, smart constructors makeX, and predicates isX(value): value is X - a boolean narrows nothing. See references/parsing.md.

Make illegal states unrepresentable

A lifecycle is a tagged union, not a bag of booleans, and assertNever on the default arm turns a new variant into a compile error. Named options replace boolean behaviour flags. See references/modeling.md.

Records, entities, and collections

interface for object shapes, type for unions and mapped types; a class only for nominality through a #private field; brands for primitives; readonly is compile-only and Object.freeze is its shallow runtime half. JS objects have no structural value equality, so entities compare and key by branded id. See references/modeling.md.

Deep, cohesive modules

Centre a module on one concept and depend on the narrowest structural shape a caller needs, often a single function type. tsc never reports an import cycle; the failure is a runtime ReferenceError on one import order, which is why the barrel rule exists. See references/modules.md.

The composition root is a scope

await using stack = new AsyncDisposableStack() releases in exact reverse of acquisition, and ownership leaves the root only through an eager stack.move(). See references/resources.md; config lifetime belongs to architecture.

Transaction boundaries

A transaction is a Result-aware closure that commits only an ok; an err rolls back, while infrastructure failures receive stable tags. See references/resources.md.

Cancellation and task ownership

A deadline is unenforceable unless the port type takes signal: AbortSignal. Promise.all rejects on the first failure and leaves siblings running with their later rejections swallowed - it is not a task group; a scope that spawns must also abort and await. See references/concurrency.md.

Bounded fan-out and back-pressure

Promise.all(items.map(fn)) sets concurrency to the input size; bound it with a worker pool whose limit is a checked finite number. A port that streams returns AsyncIterable<T>, and await inside for await is the back-pressure. See references/concurrency.md.

Exhaustiveness and the cast discipline

Construct branded values only through parsers, never an as cast; avoid any and !; satisfies T checks a literal without widening it, and as const satisfies T keeps its keys as a literal union. Any other cast carries a // SAFETY: comment, and @ts-expect-error with a reason is the only suppression that expires. See references/conventions.md.

References

• references/errors.md - Result, the ladder and the Effect seam, tagged errors, panics, Redacted.
• references/parsing.md - schema ladder, narrowing predicates, brands, optionality.
• references/modeling.md - record-type chooser, illegal states, entities, collections.
• references/modules.md - deep modules, ports, layout, import cycles, batching port.
• references/resources.md - using, composition root, transactions, retries, clock.
• references/concurrency.md - cancellation, combinators, task ownership, fan-out, back-pressure.
• references/conventions.md - casts, suppressions, satisfies, NoInfer, JSDoc, testing.
• references/toolchain.md - TS 7, lib ceiling, erasable syntax, runtimes, dated facts.