standards-typescript

name	standards-typescript
description	Use when editing *.ts or *.tsx files in this project. Enforces TypeScript conventions including Zod-derived types, Prisma type patterns, type-only imports, and shared constant extraction.
category	standards
global	false
public	true

Project Conventions

Zod → TypeScript Types

Always derive TypeScript types from Zod schemas. Never declare the same shape twice.

export const createPostSchema = z.object({ content: z.string().min(1) })
export type CreatePostInput = z.infer<typeof createPostSchema>

Prisma Types

Use Prisma.<Model>GetPayload<...> for query result shapes. Never re-declare them.

import type { Prisma } from '@prisma/client'
type Post = Prisma.PostGetPayload<Record<string, never>>
type PostWithProject = Prisma.PostGetPayload<{ include: { project: true } }>

Type-Only Imports

import type { Project, Post } from '@prisma/client'

DRY: Extract Shared Constants

When the same constant appears in more than one file, extract it to src/lib/ and import it. Client components can import shared constants directly without receiving them as props.

Interface Boundaries

• Accept unknown at the boundary and narrow internally with a type guard or Zod parse. Never any.
• For strategies where only some implementations need a capability, add it as an optional method on the base interface rather than a separate capability interface + cast.

// Good
export interface SocialProvider {
    publish(payload: PublishPayload, ctx: PublishContext): Promise<PublishOutcome>
    verifyCredentials?(credentials: unknown): Promise<{ handle: string }>
}

// Caller does provider.verifyCredentials?.(...) — no cast.

// Bad — the cast signals the interface boundary is wrong
interface BlueskyCredentialVerifier { verifyCredentials(...): ... }
const provider = registry.get('bluesky') as unknown as BlueskyCredentialVerifier

See ADR 003 — Design Patterns.

as unknown as X Casts Are a Code Smell

Any as unknown as X cast is the signal that the interface boundary is wrong. Fix the boundary (add an optional method, introduce a type guard, or narrow the return type) instead of casting. Treat these as a stop and fix item in code review.

Discriminated Union Result Types

Public functions return typed Result unions, not thrown errors:

type Result<T> =
    | { ok: true; data: T }
    | { ok: false; error: 'rate_limited'; retryAfter: number }
    | { ok: false; error: string; retryAfter?: never }

Callers branch on result.ok — TypeScript narrows the union. Throw only for unrecoverable programmer errors.

ReadonlySet for Allowed Values

When validating a string discriminator, expose allowed values as ReadonlySet<PlatformKey> derived from a config object — never hand-maintained:

export const ALLOWED_PLATFORMS: ReadonlySet<PlatformKey> = new Set(
    Object.keys(PLATFORM_CONFIGS) as PlatformKey[]
)

export function isPlatformKey(value: unknown): value is PlatformKey {
    return typeof value === 'string' && value in PLATFORM_CONFIGS
}

See ADR 003 — Design Patterns.