TS Design Patterns

TypeScript Design Patterns

Effect-inspired, without an Effect dependency.

• Make illegal states unrepresentable. _tag discriminated unions, not optional-field + boolean soup.
• Errors as values. Tagged Ok | Err for expected domain failures, so signatures show them. Unexpected and infra errors propagate; prefer .catch() over try/catch when handling is needed.
• No silent catches. .catch(() => null) hides failures. Handle (log, surface, fallback with reason) or propagate. Swallow only genuinely ignorable failures, with a comment saying so.
• Parse, don't validate. Parse untrusted input once at the boundary into a precise type; trust it inward.
• Explicit dependencies. Pass clients, config, clock as args. No hidden singletons, no import-time side effects.
• Pure core, effectful shell. Side effects at the edges, decision logic pure data-in/data-out.
• Design out the bug. After a production error, find the design that kills the whole category. Prefer a type or structural change; guard at the failure site only when no design change exists.

As a review rubric

When reviewing TS, score against each principle and report only violations that change behaviour or block testing. A class that wraps state with no illegal-state risk is a style note, not a finding; a catch that swallows a network failure is a finding.