/typescript-write

name: typescript-write
description: Write TypeScript and JavaScript code following Metabase coding standards and best practices. Use when developing or refactoring TypeScript/JavaScript code.

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.
• **Match dictionary keys to the data.** Use a finite key union when the keys are known. Open dictionaries can use an index signature, `Record<string, T>`, or `Map`; index signatures still constrain values, and changing their spelling to `Record<string, T>` does not make missing-key access safe.
• **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.
• **Guard indexed lookups that may miss.** Arrays and dictionaries can return `undefined` even when the inferred type omits it. Use an iteration form that preserves the key/value relationship, and only assert `keyof` when the runtime keys are known to belong to the declared type.
• **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.
• **Keep unavoidable assertions local.** Isolate a repeated or complex assertion behind a helper when that makes its invariant easier to enforce. Test nontrivial runtime assumptions that justify it; a trivial assertion does not automatically need a new helper or test. Do not weaken a public signature just to silence implementation errors.

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`).
• **Generics must make promises the implementation can keep.** A caller-selected `get<T>(): T` must not disguise an unchecked assertion about external data. Return `unknown` and validate, or accept a validator that establishes `T`. A factory such as `function empty<T>(): T[] { return []; }` is valid; judge the implementation, not how often `T` appears in the signature.
• **Prefer an honest type over false precision.** Use generics when they express a real relationship. If a complex type cannot model the behaviour accurately, choose a simpler type or `unknown` with narrowing instead of asserting an unsupported guarantee.
• **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.
• **Optionality must reflect absence.** Keep required fields required and optional fields optional. Normalise input when the application has a meaningful default, not merely to remove a type error. Preserve the actual wire shape in API types.
• **Represent related absence together when modelling internal state.** If several fields exist or disappear together, consider a nullable containing object or a discriminated union. Do not reshape a raw API declaration unless the data actually has that shape.
• **Prefer explicit special states when designing a format.** Use nullability or named union variants when sentinel values such as `-1` hide meaning. Preserve established protocol sentinel values unless the behaviour is deliberately changed, or normalise them at an explicit boundary.
• **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 opt