typescript-best-practices
TypeScript best practices. Use when reading or editing any .ts or .tsx file, designing a module's public types, fixing a type error, choosing between type and interface, or setting up tsconfig, and wh
SKILL.md.claude/skills/typescript-best-practices/SKILL.md
GitHubTypeScript best practices
One idea underneath every rule: a wrong program should not compile. Every any, as, ! and @ts-ignore is a place where you took that back.
| 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 the types inside. A schema at every trust boundary, never a typeof chain behind an as cast. |
| 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. |
| Strict by default | strict, noUncheckedIndexedAccess, exactOptionalPropertyTypes, verbatimModuleSyntax, isolatedModules. Below that floor the rest of this table is decoration. A project missing one raises it rather than working around it. |
| Errors as values | Expected failure returns a Result with a union of literal codes. Throw only for a violated invariant. catch (error: unknown), never a silent catch {}. |
| No floating promises | Every promise is awaited, returned, or handed to an owner. Independent work runs under one Promise.all. Long work takes an AbortSignal. |
| Named exports | A default export renames itself at every call site. import type for types, no barrel over a package's internals. |
Examples: references/patterns.md, which covers the rules that need one.
Read the project's tsconfig and lint config before deciding a rule here does not
apply. Where a project also writes Rust, the same table lives one language over in
rust-best-practices.