Typing objects

Typing objects is just as simple as typing primitive values:

type Profiler = {
    logger?: string;
    start: Date;
    done(info?: string): boolean;
    run: () => void;
}

Properties of an object can be marked as optional with the ?.

Combining object types

Those types have common fields: _id, price, and category.

type Book = {
    _id: string;
    price: number;
    category: number;
    title: string;
    description: string;
}

type Pen = {
    _id: string;
    price: number;
    category: number;
    brand: string;
    type: string;
}

Let's create a separate type for the common properties and rewrite the types:

type Product = {
    _id: string;
    price: number;
    category: number;
}

type Book = Product & {
    title: string;
    description: string;
}

type Pen = Product & {
    brand: string;
    type: string;
}

Interfaces

Interfaces are another way of creating custom types.

interface IProduct {
    _id: string;
    price: number;
    category: number;
}

interface IBook extends IProduct {
    title: string;
    description: string; 
}

interface IPen extends IProduct {
    brand: string;
    type: string;    
}

There are some very minor differences between type aliases and interfaces. In general interfaces seem to be more suitable for object-oriented code with classes.

Resources

• Handbook: Interfaces

• Handbook: Type aliases

• Interface vs Type alias