TypeScript Best Practices

Quick Start

Always enable strict mode and use explicit types for public APIs. Prefer type-only imports (import type) and named exports over default exports. Use discriminated unions for state management and type guards for runtime validation.

Type Safety Fundamentals

Strict Mode Configuration

Enable all strict flags in tsconfig.json:

{
  "strict": true,
  "noImplicitAny": true,
  "noImplicitReturns": true,
  "strictNullChecks": true,
  "strictFunctionTypes": true
}

Null Safety

// Explicit null handling
export function findById<T>(items: Map<string, T>, id: string): T | null {
  return items.get(id) ?? null;
}

// Optional chaining and nullish coalescing
export function getName(item?: Item): string {
  return item?.name ?? 'Unknown';
}

Explicit Return Types

Use explicit return types for public APIs:

// Good
export function validateConfig(data: unknown): ValidatedConfig | null {
  const result = ConfigSchema.safeParse(data);
  return result.success ? result.data : null;
}

// Bad - implicit return type
export function validateConfig(data) {
  return data;
}

Core Patterns

Discriminated Unions for State

export type ConnectionState =
  | { status: 'disconnected' }
  | { status: 'connecting'; progress: number }
  | { status: 'connected'; connectionId: string }
  | { status: 'error'; message: string };

// TypeScript knows which properties are available in each branch

Result Type for Error Handling

export type Result<T, E = Error> =
  | { success: true; data: T }
  | { success: false; error: E };

const result = await connectToService(options);
if (result.success) {
  console.log('Connected:', result.data.id);
} else {
  console.error('Failed:', result.error.message);
}

Readonly Properties

Use readonly for immutable data:

export interface Config {
  readonly enableFeature: boolean;
  readonly port: number;
}

Type Guards and Assertions

// Type guard
export function isUserData(data: unknown): data is UserData {
  return typeof data === 'object' && data !== null && 'name' in data;
}

// Assertion function
export function assertIsDefined<T>(value: T): asserts value is NonNullable<T> {
  if (value === undefined || value === null) throw new Error('Value is undefined or null');
}

Import/Export Conventions

// Type-only imports (preferred)
import type { Config } from '../types/config';

// Named exports (avoid default exports)
export class DataService {}
export const getService = () => DataService.getInstance();

// Grouped imports
import { External } from 'external';           // External deps
import { internalUtil } from './utils';         // Internal utils
import type { MyType } from './types';          // Types

When to Use References

Reference File	When to Load
tsconfig.md	Setting up TypeScript configuration
interfaces-types.md	Defining interfaces, generic types, extending external types
unions-discriminated.md	Working with discriminated unions, result types, exhaustive checks
type-guards-assertions.md	Runtime validation, branded types, assertion functions
utility-types.md	Using built-in utilities, const assertions, template literal types
error-handling.md	Implementing structured errors, error factories, retry logic
generics.md	Building generic services, repositories, factories, event emitters
import-export.md	Organizing imports/exports, avoiding circular dependencies

Common Anti-Patterns to Avoid

1. Using any - Use unknown with type guards instead
2. Default exports - Harder to refactor and tree-shake
3. Implicit return types on public APIs
4. Non-readonly interfaces for immutable data
5. Nested optionals ({ a?: { b?: string } }) - use discriminated unions instead