TypeScript Type System

Also Known As

TL;DR

Explanation

TypeScript's type system is structural (duck typing) not nominal — if an object has the required properties, it satisfies the type regardless of its declared class. Types include primitives, unions (string|number), intersections (A&B), literal types ('GET'|'POST'), tuples, enums, and generics. The any type disables checking; unknown is the safe alternative. Type inference reduces annotation noise — TypeScript infers types from assignments. strict mode (strictNullChecks, noImplicitAny) catches the most bugs.

Common Misconception

Why It Matters

Common Mistakes

• Using any instead of unknown — any disables all type checking; unknown forces you to narrow the type before using it.
• Not enabling strict mode — without strictNullChecks, null and undefined assignment errors are invisible.
• Type assertions (as Type) without validation — asserting a type without checking at runtime creates false confidence.
• Over-annotating when inference works — TypeScript infers const name = 'Alice' as string; no annotation needed.

Code Examples

// any disables type checking:
function process(data: any) {
    return data.user.name.toUpperCase(); // No error if data is null
}

// Type assertion without validation:
const user = response.data as User; // Unsafe if response is actually an error object
console.log(user.email.toLowerCase()); // Runtime crash if email is undefined

// unknown forces narrowing before use:
function process(data: unknown): string {
    if (typeof data !== 'object' || data === null) throw new Error('Invalid data');
    if (!('user' in data)) throw new Error('Missing user');
    const user = (data as { user: { name: string } }).user;
    return user.name.toUpperCase();
}

// Runtime validation with Zod:
const UserSchema = z.object({ email: z.string().email() });
const user = UserSchema.parse(response.data); // Throws if invalid

References

• ↗ https://www.typescriptlang.org/docs/handbook/2/types-from-types.html
• ↗ https://www.typescriptlang.org/docs/handbook/2/everyday-types.html
• ↗ https://www.typescriptlang.org/tsconfig#strict

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