typescript-forms
Katara's TypeScript forms, practiced until every move is exact. Use for typescript-best-practices, /typescript-best-practices, "TypeScript best practices", and whenever reading or editing any .ts or .
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GitHubKatara's TypeScript forms
A waterbender drills each form until her hands cannot get it wrong. Then, in a real fight, the form holds without thought. Types are forms for code. Shape them so the wrong move cannot be made, and the compiler holds the line when you are not looking.
Apply Type System Discipline (Earth in four-elements) first.
| Form | Summary |
|---|---|
| Discriminated unions | Model variants with a kind literal discriminant so impossible states cannot be represented. No optional-field bags. |
| Branded types | Brand primitives with & { readonly __brand: "X" } so they cannot be mixed up. Validate once at the boundary. |
| Constructive modeling | Build the shape so the illegal value cannot be constructed. [T, ...T[]] for non-empty, [T, T][] for even length, start plus duration for a range. Not a runtime guard, and 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 to happen. Cast only after validation. |
| Narrowing hierarchy | Discriminant switch, then the in operator, then typeof or instanceof, then a user-defined type guard, and as last. |
| Type guards | A guard must verify its claim. A lying guard is worse than as, because the bug hides behind a name that says it is 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 the types inside. See Boundary Discipline (Earth in four-elements). |
| Schema-derived types | Reach for Pick, Omit, Parameters, ReturnType, Awaited, and typeof before declaring a new interface. |
| Object args | Pass objects, not positional arguments, so argument order documents itself. Skip this on hot paths (per-frame render, tokenizers, parsers). |
| Real tests | Do not mock what you can run. Prefer the framework's real test primitives with leak and disposable checks, and verify UI in a running build. Mock only what you cannot run locally. See push-and-pull. |
| Structured telemetry | Prefer structured logger diagnostics with enough context to debug from an id. No console.log in shipped code. |
Examples for every form: references/patterns.md.