TypeScript/JavaScript Development Skill

@./../_shared/development-workflow.md @./../_shared/typescript-commands.md @./../_shared/react-redux-patterns.md

No any — hard rule

• New code must not introduce any, explicit or implicit. No any annotations, no as any / as unknown as, no untyped parameters or returns that infer any, no implicitly-any destructures or array/object literals.
• Untyped third-party / boundary values must be typed at the boundary (a declared type, unknown + type guard, or a small typed wrapper) — never let any propagate inward.
• Mandatory type verification. Before finishing a TS/TSX change, run bun run type-check-pure. If TypeScript LSP tools are available, also inspect changed symbols with hover and go-to-definition; otherwise skip LSP check.

Type tightening

• Avoid type casts and loose unknown — fix the signature instead.
• If a function only needs one field of a wide object, accept that field — not the wide object. The cast often disappears once the signature is right.
• Reach for Partial<T>, Pick<T, K>, Record<K, V>, and generics before reaching for a cast.
• Prefer making props/components generic (<T>) when a value flows through unchanged and the caller knows the type.
• Prefer unknown over loose typing and narrow before use — an unknown value forces a guard at the point of use.
• satisfies for object literals that must conform without widening (config objects, lookup maps, discriminated literals) — better than : T (widens) or as T (unsafe).
• Avoid non-null assertions (!). Prefer a guard, early return, or ?.. Use ! only when non-nullness is provably true and localized, with a comment.
• No redundant runtime coercion — don't wrap already-typed values in Number() / String() / Boolean().
• Type guards belong in frontend/src/metabase-types/guards/. Do not redefine them locally.
• A cast you can't avoid needs a real justification comment. The metabase/no-unjustified-type-casts rule accepts any preceding comment — state the actual reason the cast is safe. NEVER write the legacy // Unjustified type cast. FIXME placeholder; it exists only on casts that predated the rule, and copying it sneaks an unjustified cast past the linter. If you can't articulate why the cast is correct, the cast is wrong — fix the types.

Type modeling

• Reuse existing types; don't re-declare them. Use canonical IDs and domain entity types from metabase-types/api (FieldId, TableId, ConcreteTableId, SchemaName, …) and key data structures by them (new Map<ConcreteTableId, …>()). Don't duplicate generated/API types — compose or derive (Pick, Omit, indexed access SomeType["field"], ReturnType).
• Use generics to allow TypeScript to infer correct types when creating functions and components that need to be reusable and type-safe. Don't hesitate to introduce complex generics if they allow to derive types automatically instead of manual narrowing.
• Model the actual data contract; keep types narrow. Optional field?: T for a key that may be absent, field: T | undefined only when the key is always present but the value may be undefined, | null for explicit API nulls. Prefer domain unions over broad string / number / loose Record.
• Refer to API implementation when defining or refining types to ensure they match the actual data structure. When considering a type cast, first consider if the type should be refined to match the actual data structure.
• Discriminated unions for variant state, with exhaustive checks. Model "one of N shapes" as a union with a literal discriminant rather than a bag of optional fields, and exhaust it with ts-pattern's .exhaustive() so adding a variant becomes a compile error:

  import { match } from "ts-pattern";
  
  const result = match(status)
    .with({ type: "loading" }, () => <Spinner />)
    .with({ type: "error", error: P.select() }, (error) => <Error message={error.message} />)
    .with({ type: "success", data: P.select() }, (data) => <Content data={data} />)
    .exhaustive(); // Compile-time guarantee all cases handled
  

• Derive union types from constants (as const + typeof/keyof) so the type and the values can't drift.
• readonly / immutability where mutation isn't intended — component props, shared constants, exported config. Prefer readonly T[] / ReadonlyArray<T> for inputs you don't mutate.
• Type async and error states explicitly (a discriminated union or the data-layer's typed result) — never leave loading/error/empty implicit.

Null and undefined

• Narrow at the source. If a value is optional only in a corner case, don't thread undefined through every layer — guard at the producer.
• Sensible defaults for optional values. Use ?. and ?? at the consumer.
• Lists should be filtered before being used in a map or other iteration.
• Avoid non-strict null comparisons (X != null) when X can never be null — use a strict check or narrow the type. Use checkNotNull where necessary.
• Check actual nullability against API implementation. Find the API endpoint implementation and check if the field can actually be null.

Naming

• Names describe the entity, not the mechanism. A name must reflect what the value holds.
• Align sibling concepts: keep verb conventions consistent across a related API.
• No names that encode implementation history rather than current meaning. Suffixes like Base, New, Old, Initial need a real semantic distinction, otherwise drop them.
• Avoid cryptic identifiers (v, n, $n) for domain values; short names are fine only in tiny conventional contexts (loop index i, coordinates x/y, generic params T/K/V).

Code structure and organization

• Prioritize reusability over duplication. The codebase already has many utility functions — leverage them. If you introduce duplicated logic, extract it to a shared utility.
• Generic helpers do not belong in feature folders. Promote to a shared utility.
• Keep functions small and single-purpose. A 100+ line function is hard to review — split into focused named helpers, each with one responsibility and a minimal dependency surface. When necessary, cover with unit tests.
• Extract distinct complex JSX into named components. Choose same-file vs separate-file by reuse, coupling, testability, and readability.

Comments

• No comments by default. Well-named identifiers carry the what.
• Comments should be concise. Add a short, concise comment only when the why is non-obvious: a workaround, a hidden invariant, a subtle ordering constraint, a clever reduction. Never document the actual implementation, focus on the intent and the why.

TypeScript Migration

When touching existing JavaScript files, propose to convert them to TypeScript first. Create a separate PR for the conversion, then implement the changes.

Verify before done

• Run the project type-check when finished (see the shared TypeScript commands above).