What is the difference between type and interface?

In TypeScript, both `type` and `interface` are used to define the shape of objects and can be used for type checking. However, they have some key differences and use cases that can influence which one you choose for a particular scenario. Understanding these distinctions can help you write cleaner and more maintainable code.

Basic Definitions

A `type` is a way to define a type alias for a specific type or a combination of types. It can represent primitive types, union types, intersection types, tuples, and more.

An `interface`, on the other hand, is specifically designed to define the structure of an object. It can be extended and implemented by classes, making it a powerful tool for object-oriented programming in TypeScript.

Key Differences

Extensibility

One of the most significant differences between `type` and `interface` is how they handle extensibility:

• Interface: Interfaces can be extended using the `extends` keyword. This allows you to create a new interface that inherits properties from one or more existing interfaces.
• Type: Types can also be combined using intersections (`&`), but they cannot be extended in the same way as interfaces.

interface Person {
    name: string;
    age: number;
}

interface Employee extends Person {
    employeeId: number;
}

type Person = {
    name: string;
    age: number;
};

type Employee = Person & {
    employeeId: number;
};

Declaration Merging

Another important aspect is declaration merging:

• Interface: Interfaces support declaration merging, which means you can define the same interface multiple times, and TypeScript will merge them into a single interface.
• Type: Types do not support declaration merging. If you try to declare the same type more than once, you will receive an error.

interface Car {
    make: string;
}

interface Car {
    model: string;
}

// Merged interface
const myCar: Car = {
    make: 'Toyota',
    model: 'Camry'
};

Use Cases

Choosing between `type` and `interface` often depends on the use case:

• Use `interface` when:
• You need to define the shape of an object or class.
• You want to take advantage of declaration merging.
• You are working in an object-oriented context.
• Use `type` when:
• You need to define union types or intersection types.
• You want to create a type alias for a primitive type or a complex type.
• You are working with tuples or mapped types.

Common Mistakes

Some common mistakes developers make when using `type` and `interface` include:

• Choosing `type` when they need the extensibility of an `interface`.
• Assuming that `type` and `interface` are interchangeable without understanding their differences.
• Neglecting to use declaration merging when it could simplify their code.

In conclusion, while both `type` and `interface` serve similar purposes in TypeScript, understanding their differences will allow you to make informed decisions that enhance your code's readability and maintainability.