一 Interfaces vs Types
Below is a list of up-to-date differences.
- 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; - Other Types
Unlike an interface, the type alias can also be used for other types such as primitives, unions, and tuples.
// primitivetype Name = string;// objecttype PartialPointX = { x: number; };type PartialPointY = { y: number; };// uniontype PartialPoint = PartialPointX | PartialPointY;// tupletype Data = [number, string]; - 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; }; - 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 typeclass SomePartialPoint implements PartialPoint { x = 1; y = 2;} - 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 };