Objects

Objects are the fundamental way in which data and functions are grouped and passed around.

In the example above, the person object has an implicit typing with a name and age key corresponding to string and number. However, in virtually all cases, it is more useful to define the data structure of JavaScript objects with an explicit TypeScript type.

Interfaces

The most common way to define the keys and values of an object is an interface declaration.

Optional keys

Add a ? after a key name to mark it as optional.

If a key is not marked as optional, it is required. An object that is missing a required key will generate a TypeScript error.

Required

Convert all keys to required Required

Partial

Convert keys to optional

Readonly

Index signatures

You can define dynamic keys for index signatures.

interface Dictionary {
  [key: string]: string;
}
const translations: Dictionary = {
  hello: "Hola",
  goodbye: "Adiós",
};

Using Record K, V for Object Types A utility type to define objects with fixed key-value types:

type UserRoles = Record<string, "admin" | "user">;
const roles: UserRoles = {
  alice: "admin",
  bob: "user",
};

Extending and Merging Object Types

interface Animal {
  name: string;
}
interface Dog extends Animal {
  breed: string;
}
const myDog: Dog = { name: "Buddy", breed: "Golden Retriever" };

Type Composition

type Animal = { name: string };
type Dog = Animal & { breed: string };
const myDog: Dog & { friend: string } = { 
    name: "Buddy", 
    breed: "Golden Retriever", 
    friend: "best"
}

Excess property checks

Indexed Access

• https://www.typescriptlang.org/docs/handbook/2/indexed-access-types.html

Mapped Types

• https://www.typescriptlang.org/docs/handbook/2/mapped-types.html