TypeScript Best Practices
Code Style
| Rule |
Guideline |
| Formatter |
Prettier |
| Linter |
ESLint + @typescript-eslint |
| Strict mode |
strict: true in tsconfig |
Type Safety
| Rule |
Guideline |
Avoid any |
Use unknown + narrowing |
| Type guards |
Custom predicates |
| Discriminated unions |
For variants |
type Result<T> = { success: true; data: T } | { success: false; error: Error };
function isUser(v: unknown): v is User {
return typeof v === 'object' && v !== null && 'id' in v;
}
Error Handling
class ApiError extends Error {
constructor(message: string, public status: number) {
super(message);
this.name = 'ApiError';
}
}
async function fetchUser(id: string): Promise<User> {
const res = await fetch(`/api/users/${id}`);
if (!res.ok) throw new ApiError('Failed', res.status);
return res.json();
}
Project Structure
src/{index.ts, types/, services/, utils/}
tests/
package.json, tsconfig.json
Async Patterns
| Pattern |
Usage |
Promise.all |
Parallel operations |
AbortController |
Cancellation |
const [user, settings] = await Promise.all([fetchUser(id), fetchSettings(id)]);
Anti-Patterns
| Avoid |
Use Instead |
as assertions |
Type guards |
! non-null |
?. and ?? |
any |
unknown + narrowing |
// Bad: data as User, user!.name
// Good:
if (isUser(data)) { /* data is User */ }
const name = user?.name ?? 'Unknown';
Testing (Vitest)
describe('Service', () => {
it('should create', async () => {
const mock = { save: vi.fn().mockResolvedValue({ id: '1' }) };
const result = await new Service(mock).create({ name: 'Test' });
expect(result.id).toBe('1');
});
});
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