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Get Started Free →elianiva/typescript-best-practices
TypeScript best practices.
Passed 23 of 24 test cases with the skill, and 21 of 24 without it.
2 fixed21 passed either way1 still failing0 broke
Tokens, turns and time describe resource use, not a dollar price.
One of 2 test cases the skill fixed. Same model, same prompt, with and without the skill.
I'm writing a TypeScript function to parse untyped API response payloads for user accounts and organization memberships. The current code casts the response directly using `as UserResponse` and lacks validation. Please refactor this to safely validate the data structure before usage, generate derived types from existing schema shapes where appropriate, and ensure exhaustive switch handling for all account status variants.
| Checked | Without skill | With skill |
|---|---|---|
| Discriminates account variants using kind as the literal property name. | Missed | Passed |
| Inlines an exhaustive check assigned to a never variable in default switch arm. | Missed | Passed |
The instructions your model reads when the skill loads.
Apply the type-system-discipline principle skill first; this skill grounds it in TypeScript syntax.
| 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 creation. |
| 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. any disables type checking everywhere it touches. |
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 | Validate where data crosses in; 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.
test cases passed, vs 21 / 24 without
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