TypeScript interface 和 type 比较

使用TypeScript的时候，一定会遇到interface 和 type aliases 的比较。
官方有两句话Because [an ideal property of software is being open to extension](https://en.wikipedia.org/wiki/Open/closed_principle), you should always use an interface over a type alias if possible.On the other hand, if you can’t express some shape with an interface and you need to use a union or tuple type, type aliases are usually the way to go，就是要我们一般情况下使用interface，当interface不能满足我们的需求时，使用type。
下面通过一些例子来比较两者的异同

1. Objects / Functions

两者都可以用来描述对象或函数，但是语法不同。

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

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

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

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

2. Other Types

与interface不同，type也可以用于其他类型，例如原始类型、联合类型、元祖类型。

// 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 继承

两者都可以被继承，但是同样的，语法不同。

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 实现

一个class对于interface和type alias的实现是一样的。但是，class和interface被认为是静态蓝图(static blueprints)，因此，他们不能 Implements / extend 联合类型的 type alias

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; };
// ❌ A class can only implement an object type or intersection of object types with statically known members.t
class SomePartialPoint implements PartialPoint {
    x: 1;
    y: 2;
}

4. Declaration Merging (声明合并)

// 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 };

【参考】typescript-interfaces-vs-types