TypeScript Strict Mode Best Practices in 2026: Write Bulletproof Code

Enabling TypeScript's strict mode is the single biggest quality improvement you can make to a JavaScript codebase. This guide covers patterns every TS developer should know in 2026.

🔒 Enable Strict Mode

{
  "compilerOptions": {
    "strict": true,
    "noUncheckedIndexedAccess": true,
    "noImplicitOverride": true,
    "exactOptionalPropertyTypes": true
  }
}

🎯 Pattern 1: Discriminated Unions

type Result<T> =
  | { ok: true; data: T }
  | { ok: false; error: string };

function handle(r: Result<User>) {
  if (r.ok) return r.data.name;  // narrowed to User
  return r.error;                 // narrowed to string
}

🏷️ Pattern 2: Branded Types

type UserId = string & { __brand: 'UserId' };
type Email  = string & { __brand: 'Email' };

function sendEmail(to: Email) { /* ... */ }
sendEmail("foo@bar.com" as Email);  // OK
sendEmail("not-validated");         // ❌ Type error

🚫 Banish any

Use unknown for truly unknown values, then narrow with type guards. Configure ESLint with @typescript-eslint/no-explicit-any to enforce this team-wide.