Optional Properties & strictNullChecks

Type Inference & Literal Types (const vs. let)

Discover how TypeScript infers types automatically and how let versus const declarations shape wide types versus narrow literal types.

1. The Billion-Dollar Mistake

Tony Hoare called the invention of null references "my billion-dollar mistake" due to endless null pointer crashes. TypeScript solves this problem using the strictNullChecks compiler flag.

2. What strictNullChecks Does

When strictNullChecks: true is enabled in tsconfig.json, null and undefined are NOT assignable to other types unless explicitly included in a union:

let username: string = "Alex";
// username = null; // Error: Type 'null' is not assignable to type 'string'

let optionalEmail: string | null = null; // Explicit union required!

3. Optional Properties (?)

Adding a ? to a property or parameter marks it as optional, automatically making its type type | undefined:

interface Config {
  apiKey: string;
  timeout?: number; // Inferred as 'number | undefined'
}

function connect(config: Config) {
  // Safe default fallback using Nullish Coalescing (??)
  const timeoutMs = config.timeout ?? 5000;
  console.log(`Connecting with timeout: ${timeoutMs}ms`);
}

4. Safe Optional Navigation

type UserResponse = {
  profile?: {
    avatarUrl?: string;
  };
};

function getAvatar(res: UserResponse): string {
  // Safe optional chaining ?.
  return res.profile?.avatarUrl ?? "/default-avatar.png";
}

5. Common Mistakes

• Disabling strictNullChecks: Setting strictNullChecks: false turns off null checking globally, re-introducing silent runtime TypeError: Cannot read properties of undefined crashes. Always keep strictNullChecks: true enabled!

6. Next Steps

Keep strict null checks enabled across all projects. Next, let's enter Phase 2 and master Functions, Overloads, and Narrowing!