Object Types

TypeScript · Reference cheat sheet

📖 Overview

Object types describe property names, optionality, readonly-ness, index signatures, and call/construct signatures. Use interface or type for named shapes; anonymous object types work inline.

🧩 Core concepts

• Properties — required by default; ? makes optional; readonly prevents reassignment.
• Index signatures — [key: string]: T / [key: number]: T for dynamic keys.
• Excess property checks — apply to fresh object literals assigned to typed targets.
• Intersection (&) — combine shapes; conflicting properties become never when incompatible.
• Record<K, V> — mapped object with keys K and values V.
• object vs \{\} vs Record<string, unknown> — prefer explicit shapes; object means non-primitive.

💡 Examples

interface Product {
  readonly id: string;
  name: string;
  price?: number;
}

const p: Product = { id: "sku-1", name: "Mug" };
// p.id = "x"; // error: readonly

type Dict = { [key: string]: number };
const scores: Dict = { alice: 10, bob: 8 };

// Nested + optional chaining-friendly shapes
type Config = {
  server: { host: string; port: number };
  features?: { darkMode?: boolean };
};

// Intersection
type Timestamped = { createdAt: Date };
type Entity = Product & Timestamped;

const item: Entity = {
  id: "1",
  name: "Tea",
  createdAt: new Date(),
};

// Prefer explicit over empty object
type JsonObject = Record<string, unknown>;

⚠️ Pitfalls

• Optional props may be missing or explicitly undefined depending on exactOptionalPropertyTypes.
• String index signatures force all named props to be assignable to the index type.
• \{\} accepts almost everything except null/undefined — rarely what you want.
• Methods vs function properties differ under strictFunctionTypes / this typing.

🔗 Related

• Types
• Class
• Mapped types
• Utility types
• Module