What is the difference between using TypeScript interface and type for defining types?

General Discussions
1

What is the difference between using TypeScript interface and type for defining types?

For example, consider the following statements:

interface X {
    a: number;
    b: string;
}

type X = {
    a: number;
    b: string;
};

Could anyone explain the distinctions between these two approaches?

2

Interface supports declaration merging, allowing you to define multiple interfaces with the same name and automatically combine their members. This is useful for extending or augmenting existing interfaces.

For example:

interface X {
    a: number;
}

interface X {
    b: string;
}

// Resulting type: { a: number; b: string; }

In contrast, type does not support declaration merging. If you try to declare a type with the same name more than once, TypeScript will throw an error.

3

Hi,

type can represent union and intersection types, which allows for more complex type compositions.

For example:

type A = { a: number; };
type B = { b: string; };

type X = A & B;  // Intersection type: { a: number; b: string; }

type Y = A | B;  // Union type: { a: number; } | { b: string; }

Interface does not support union types directly, though you can achieve similar effects through interface extension and merging.

4

Interface is generally used for defining object shapes and can be extended by other interfaces or classes. It is specifically designed for object-oriented programming patterns.

On the other hand, type is more flexible and can represent not just object shapes but also primitives, tuples, and more complex types:

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

type Animal = {
    species: string;
    age: number;
};

type UnionType = Person | Animal;  // Union of Person and Animal types