What are TypeScript Interfaces?

TypeScript interfaces define the structure of objects by specifying property types and method signatures, ensuring consistent shapes and enhancing code clarity.

• Allow for optional and read-only properties for flexibility and immutability.
• Enable interface inheritance to create reusable and extendable object structures.

interface User {
    username: string;
    email: string;
}

const newUser: User = {
    username: "john_doe",
    email: "john@example.com"
};

console.log(`User: ${newUser.username}, Email: ${newUser.email}`);

• The User interface specifies that any object of type User should have username and email properties, both of which are strings.
• The newUser object adheres to the User interface by providing the required properties with appropriate string values.

Output:

User: john_doe, Email: john@example.com

Properties of Interfaces

1. Extending Properties

Interfaces in TypeScript can extend other interfaces, allowing for property reuse and additional customization.

interface For_Array {
    var1: string;
}
interface For_List extends For_Array {
    var2: string;
}

• The For_List interface extends For_Array, inheriting the var1 property while introducing a new property var2.
• This allows a hierarchical design of interfaces, promoting modularity and reuse.

2. Read-Only Properties

Properties marked as readonly cannot be modified after their initial assignment.

Example:

interface For_class {
    readonly name: string;
    id: number;
}

• The name property in the For_class interface is immutable, ensuring its value remains constant.
• This is ideal for scenarios where certain object attributes must remain unchanged, such as IDs or constants.

3. Optional Properties

The ? symbol makes properties optional, adding flexibility to object definitions.

interface For_function {
    (key: string, value?: string): void;
}

• The value parameter is optional, meaning functions can be defined with or without it.
• This reduces constraints on function parameters, accommodating varying use cases while maintaining type safety.

More Example of TypeScript Interfaces

Defining a Product Interface

interface Product {
    id: number;
    name: string;
    price: number;
    description?: string; // Optional property
}

const item: Product = {
    id: 1,
    name: "Laptop",
    price: 999.99
};

console.log(`Product: ${item.name}, Price: $${item.price}`);

• The Product interface specifies that any object of type Product should have id, name, and price properties as required, and an optional description property.
• The item object adheres to the Product interface by providing the required properties with appropriate values.

Output:

Product: Laptop, Price: $999.99

Implementing an Interface in a Class

interface Animal {
    name: string;
    sound: () => void;
}

class Dog implements Animal {
    name: string;

    constructor(name: string) {
        this.name = name;
    }

    sound() {
        console.log(`${this.name} says: Woof!`);
    }
}

const myDog = new Dog("Buddy");
myDog.sound();

• The Animal interface defines a structure with a name property and a sound method.
• The Dog class implements the Animal interface, providing concrete implementations for the name property and the sound method.

Output:

Buddy says: Woof!

Best Practices of Using TypeScript Interfaces

• Use Interfaces to Define Object Shapes: Interfaces clearly define the structure of objects, enhancing code readability and maintainability.
• Leverage Optional and Readonly Properties: Utilize optional (?) and readonly properties to express flexibility and immutability in your data models.
• Prefer Interfaces Over Type Aliases for Object Types: Interfaces are extendable and provide better tooling support for defining object types.
• Extend Interfaces for Reusability: Create base interfaces and extend them to build complex types, promoting code reuse and consistency.