typescript - 我有一个类型，它是两个接口(interface)的联合，只有一个属性不同，当我省略一个公共(public)属性并将其添加回来时，为什么 TS 无法识别？

Question

Here是带有此问题代码的 Typescript Playground ，也在下方。

本质上

enum Color {
  Red = "RED",
  Orange = "ORANGE"
}

enum Package {
  Single = "SINGLE",
  Multiple = "MULTIPLE"
}
export interface ShipmentSingle {
  id: string;
  color: Color,
  package: Package.Single
}

export interface ShipmentMultiple {
  id: string;
  color: Color,
  package: Package.Multiple,
  quantity: number
}

type Shipment = ShipmentSingle | ShipmentMultiple

const s: Shipment = {
  id: '1', 
  color: Color.Red,
  package: Package.Single 
}


const addIdToOrder = (shipment: Omit<Shipment, 'id'>): Shipment => ({
  ...shipment,
  id: '2'
})

我看到了错误

Type '{ id: string; color: Color; package: Package; }' is not assignable to type 'Shipment'. Property 'quantity' is missing in type '{ id: string; color: Color; package: Package; }' but required in type 'ShipmentMultiple'.

这是为什么？

Answer 1

错误发生是因为 Omit内部使用 keyof T . keyof T当应用于联合类型时(Shipment 是联合类型)仅返回存在于联合组成的两种类型中的键。

type Keys = keyof Shipment; // Keys = 'id' | 'color' | 'package'.

原因是described here .

所以 Omit<Shipment, 'id'>使用 color 生成对象和 package特性。 quantity不存在。

要解决这个问题，我可能会推荐使用 distributive conditional types首先分发Shipment至 ShipmentSingle和 ShipmentMultiple然后申请Omit给他们每个人。所以结果将再次是联合类型。

type OmitWithDistribution<T, K extends keyof T> = T extends any ? Omit<T, K> : never;

const addIdToOrder = (shipment: OmitWithDistribution<Shipment, 'id'>): Shipment => ({
    ...shipment,
    id: '2'
})