[typescript] TypeScript: Interfaces vs Types

2019 Update

The current answers and the official documentation are outdated. And for those new to TypeScript, the terminology used isn't clear without examples. Below is a list of up-to-date differences.

1. Objects / Functions

Both can be used to describe the shape of an object or a function signature. But the syntax differs.

Interface

interface Point {
  x: number;
  y: number;
}

interface SetPoint {
  (x: number, y: number): void;
}

Type alias

type Point = {
  x: number;
  y: number;
};

type SetPoint = (x: number, y: number) => void;

2. Other Types

Unlike an interface, the type alias can also be used for other types such as primitives, unions, and tuples.

// primitive
type Name = string;

// object
type PartialPointX = { x: number; };
type PartialPointY = { y: number; };

// union
type PartialPoint = PartialPointX | PartialPointY;

// tuple
type Data = [number, string];

3. Extend

Both can be extended, but again, the syntax differs. Additionally, note that an interface and type alias are not mutually exclusive. An interface can extend a type alias, and vice versa.

Interface extends interface

interface PartialPointX { x: number; }
interface Point extends PartialPointX { y: number; }

Type alias extends type alias

type PartialPointX = { x: number; };
type Point = PartialPointX & { y: number; };

Interface extends type alias

type PartialPointX = { x: number; };
interface Point extends PartialPointX { y: number; }

Type alias extends interface

interface PartialPointX { x: number; }
type Point = PartialPointX & { y: number; };

4. Implements

A class can implement an interface or type alias, both in the same exact way. Note however that a class and interface are considered static blueprints. Therefore, they can not implement / extend a type alias that names a union type.

interface Point {
  x: number;
  y: number;
}

class SomePoint implements Point {
  x = 1;
  y = 2;
}

type Point2 = {
  x: number;
  y: number;
};

class SomePoint2 implements Point2 {
  x = 1;
  y = 2;
}

type PartialPoint = { x: number; } | { y: number; };

// FIXME: can not implement a union type
class SomePartialPoint implements PartialPoint {
  x = 1;
  y = 2;
}

5. Declaration merging

Unlike a type alias, an interface can be defined multiple times, and will be treated as a single interface (with members of all declarations being merged).

// These two declarations become:
// interface Point { x: number; y: number; }
interface Point { x: number; }
interface Point { y: number; }

const point: Point = { x: 1, y: 2 };