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The current answers and the official documentation are outdated.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 };