typescript-best-practices

TypeScript best practices

Apply the type-system-discipline principle skill first.

| Rule | Summary |

|------|---------|

| Discriminated unions | Model variants with a kind literal discriminant so impossible states can't be represented. No optional-field bags. |

| Branded types | Brand primitives with & { readonly __brand: "X" } so they can't be mixed up. Validate once at the boundary. |

| Constructive modeling | Build the shape so the illegal value can't be constructed. [T, ...T[]] for non-empty, [T, T][] for even length, start plus duration for a range. Not a runtime guard, not a wish for refinement types. |

| Simplest total type | Keep T[] while every operation on it stays total. Strengthen to NonEmpty<T> only where the loose type forces !, a cast, or a "should never happen" throw. |

| unknown over any | External data is unknown. |

| Schemas before guards | Before hand-writing a property-by-property type guard, use the repository's runtime schema library and infer the type from the schema, such as z.infer. |

| No as casts | Every as is a runtime crash waiting. Cast only after validation. |

| Narrowing hierarchy | Discriminant switch > in operator > typeof/instanceof > user-defined type guard > as. |

| Type guards | Must verify the claim. A lying guard is worse than as because the bug hides behind a name that says it's safe. Name them isX or hasX. |

| Exhaustiveness | Inline const _exhaustive: never = x; in default arms so the compiler errors when a new variant is added. |

| satisfies over as | Validates the value without widening literal types. |

| Boundary validation | Parse where data crosses in, into a named domain type. Record<string, unknown> (however spelled) stops at that parse. Trust types inside. See the boundary-discipline principle skill. |

| Schema-derived types | Reach for Pick/Omit/Parameters/ReturnType/Awaited/typeof before declaring a new interface. |

| Object args | Pass objects, not positional, so argument order is self-documenting. Skip on hot paths (per-frame render, tokenizers, parsers). |

| Real tests | Don't mock what you can run. Prefer the framework's real test primitives with leak/disposable checks, and verify UI in a running build. Mock only what you can't run locally. |

| Structured telemetry | Prefer structured logger diagnostics with enough context to debug from an id. No console.log in shipped code. |

Examples: references/patterns.md.