TypeScript Best Practices for Large Codebases

Introduction

TypeScript is essential for maintaining large JavaScript codebases. However, simply using TypeScript doesn't guarantee code quality. As projects grow, you need strict patterns to prevent "type soup" and ensure maintainability.

1. Strict Mode

Always enable "strict": true in your tsconfig.json. This enables a suite of checks, including noImplicitAny and strictNullChecks, which catch common bugs at compile time.

2. Avoid any

The any type defeats the purpose of TypeScript. If you don't know the type, use unknown.

• any: "I don't care, let me do anything."
• unknown: "I don't know what this is yet, so I must check before using it."

// Bad
function process(data: any) {
  data.doSomething(); // No error, but might crash at runtime
}

// Good
function process(data: unknown) {
  if (typeof data === 'object' && data !== null && 'doSomething' in data) {
    (data as { doSomething: () => void }).doSomething();
  }
}

3. Use Zod for Runtime Validation

TypeScript types disappear at runtime. When dealing with external data (APIs, forms), use a schema validation library like Zod.

import { z } from "zod";

const UserSchema = z.object({
  id: z.string(),
  email: z.string().email(),
});

type User = z.infer<typeof UserSchema>;

4. Utility Types

Master built-in utility types like Pick, Omit, Partial, and Record. They allow you to derive new types from existing ones, reducing duplication.

interface User {
  id: string;
  name: string;
  email: string;
  passwordHash: string;
}

// Create a type for the frontend that doesn't include the password
type UserProfile = Omit<User, "passwordHash">;

Conclusion

TypeScript is a powerful tool, but it requires discipline. By enforcing strictness, avoiding any, and validating data at the boundaries, you can build robust applications that scale.