TypeScript Best Practices for 2026

Keep types close to their data

One of the most common mistakes is defining types in a central types.ts file and importing them everywhere. This works early on, but creates tight coupling as your project grows.

Instead, define types next to the data they describe:

// user/types.ts
export interface User {
  id: string;
  email: string;
  createdAt: Date;
}

// user/api.ts
import type { User } from './types';

export async function fetchUser(id: string): Promise<User> {
  const res = await fetch(`/api/users/${id}`);
  return res.json();
}

Prefer type over interface for unions

When you’re working with union types, type aliases are more expressive:

// Prefer this
type Status = 'idle' | 'loading' | 'success' | 'error';

// Over a fragile string enum
enum Status {
  Idle = 'idle',
  Loading = 'loading',
}

Use satisfies for config objects

The satisfies operator, introduced in TypeScript 4.9, is underused. It lets you check a value against a type without widening it:

type Config = {
  theme: 'light' | 'dark';
  language: string;
};

const config = {
  theme: 'dark',
  language: 'en',
} satisfies Config;

// config.theme is still 'dark', not 'light' | 'dark'

Narrow early, stay narrow

Type guards should live at the boundaries of your system. Once data enters your app and is narrowed to the right type, don’t widen it again.

function isApiError(error: unknown): error is ApiError {
  return typeof error === 'object' && error !== null && 'code' in error;
}

These patterns won’t make your code shorter, but they’ll make it far easier to change safely.