typescript-best-practices
TypeScript coding guidelines with dos and don'ts for type design and patterns. Use when writing, reviewing, or refactoring TypeScript code in projects with tsconfig.json or .ts/.tsx files. Trigger when the user asks to "review TypeScript code", "check my TS code", "write TypeScript", or when creating or modifying any .ts/.tsx file. Also applies when discussing type design, generics, naming conventions, interface vs type decisions, or TypeScript patterns. Boundary: not for JavaScript-only projects without TypeScript configuration.
What this skill does
# TypeScript Best Practices
Guidelines for writing clean, type-safe, and maintainable TypeScript code.
> **Note:** If the repository has established code style conventions, follow those first. These guidelines serve as defaults.
## Core Principles
1. **Type-First Design** - Define types before implementation; minimize reliance on inference
2. **Interface for Structure** - Use `interface` for objects, `type` for unions/mapped/conditional
3. **Namespace for Type Organization** - Group related types with namespaces (types only, not runtime)
4. **Generic Const for Strictness** - Use `<const TConfig>` for strict literal inference
5. **Extract, Don't Redefine** - Get types from existing definitions instead of duplicating
6. **Strictest Config** - Use strictest tsconfig base; install `ts-reset` for saner built-in types
## Quick Reference
### interface vs type
| Use | When |
|-----|------|
| `interface` | Object structures, class contracts, extensible APIs |
| `type` | Union types, mapped types, conditional types, tuples |
### Naming Conventions
| Element | Convention | Example |
|---------|------------|---------|
| Interface/Type | PascalCase | `UserProfile`, `ResponseData` |
| Generic parameters | `T` prefix | `TUser`, `TConfig` (never bare `T`, `K`, `V`) |
| Acronyms | First cap only | `userId`, `ApiResponse` (NOT `userID`, `APIResponse`) |
| Constants | UPPER_SNAKE | `MAX_RETRY_COUNT` |
| Variables/Functions | camelCase | `getUserById`, `isActive` |
### Array Syntax
| DO | DON'T |
|----|-------|
| `Array<TItem>` | `TItem[]` |
| `ReadonlyArray<TItem>` | `readonly TItem[]` |
### Object Types
| Use Case | DO | DON'T |
|----------|-----|-------|
| Empty object | `Record<string, never>` | `{}` |
| Any object (extends) | `Record<string, any>` | `Record<string, unknown>` |
| Any object (annotation) | `Record<string, unknown>` | `Record<string, any>` |
| Non-primitive | `object` | `{}` |
### Assertions
| DO | DON'T |
|----|-------|
| Zod/arktype for runtime validation | `response as User` |
| `satisfies` for compile-time checks | `value as unknown as Type` |
| Type guards (`if ('prop' in obj)`) | `as any` to silence errors |
| Explicit null checks | `x!` non-null assertion |
### Function Declarations
```typescript
// DO: Type on the const
const myFunction: myFunction.Type = (options) => {
// implementation
};
// DO: satisfies when namespace doesn't exist
const onClick = ((event) => {
// implementation
}) satisfies React.ComponentProps<'button'>['onClick'];
```
### Type Extraction
```typescript
// DO: Extract from existing definitions
type OnClick = React.ComponentProps<'button'>['onClick'];
type ItemIds = Array<Item['id']>;
type TimeoutType = NonNullable<typeof config['timeout']>;
// DON'T: Manually redefine types
type BadItemIds = Array<number>; // Won't update if Item.id changes
```
## Summary Checklist
Before committing TypeScript code, verify:
- [ ] Used `interface` for object types, `type` for unions/mapped/conditional
- [ ] No `as` or `!` assertions — use Zod, `satisfies`, type guards, or explicit null checks
- [ ] Branded types use Zod `.brand()` or type-fest `Tagged` (not manual casting)
- [ ] Naming follows conventions (PascalCase types, `T` prefix for generics, `Id` not `ID`)
- [ ] Types extracted from existing definitions where possible
- [ ] Functions use namespace pattern for complex type organization
- [ ] Arrow functions for const declarations
- [ ] Complex generics have type tests
## Reference Files
For detailed patterns and examples, see:
- **[type-patterns.md](references/type-patterns.md)** - Type syntax, assertions, namespace pattern, generics
- **[code-style.md](references/code-style.md)** - Safe array access, early return, avoid destructuring, avoid enum
- **[union-exhaustive.md](references/union-exhaustive.md)** - Discriminated unions + exhaustive handling (e.g., for state, events, API responses)
- **[branded-types.md](references/branded-types.md)** - Nominal types for ID/unit safety (e.g., UserId vs OrderId)
- **[template-literals.md](references/template-literals.md)** - String pattern types (e.g., event names, CSS values, route parameters)
- **[type-testing.md](references/type-testing.md)** - Type-level testing with `*.test-d.ts` files
- **[setup.md](references/setup.md)** - tsconfig, strict options, ts-reset configuration
## Notes
- These guidelines complement, not replace, project-specific conventions
- When in doubt, prioritize readability and maintainability
- Runtime type validation (zod, arktype) is recommended for external data
- Avoid over-engineering types; simple is better than clever
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