Item 63: Use Optional Never Properties to Model Exclusive Or

要点

• In TypeScript, "or" is "inclusive or": A | B means either A, B, or both.
• Consider the "both" possibility in your code, and either handle it or disallow it.
• Use tagged unions to model exclusive or where it's convenient. Consider using optional never properties where it isn't.
• 在 TypeScript 中，“或”是“包括性或”：A | B 意味着可以是 A、B，或者两者都有。
• 在代码中考虑“同时”的情况，并对其进行处理或禁止。
• 在方便的情况下，使用标记联合（tagged unions）来建模互斥或（exclusive or）。在不方便使用时，可以考虑使用可选的 never 属性。

正文

interface ThingOne {
  shirtColor: string
}
interface ThingTwo {
  hairColor: string
}
type Thing = ThingOne | ThingTwo

💻 playground

---

const bothThings = {
  shirtColor: 'red',
  hairColor: 'blue',
}
const thing1: ThingOne = bothThings // ok
const thing2: ThingTwo = bothThings // ok

💻 playground

---

interface OnlyThingOne {
  shirtColor: string
  hairColor?: never
}
interface OnlyThingTwo {
  hairColor: string
  shirtColor?: never
}
type ExclusiveThing = OnlyThingOne | OnlyThingTwo

💻 playground

---

const thing1: OnlyThingOne = bothThings
//    ~~~~~~ Types of property 'hairColor' are incompatible.
const thing2: OnlyThingTwo = bothThings
//    ~~~~~~ Types of property 'shirtColor' are incompatible.
const allThings: ExclusiveThing = {
  //  ~~~~~~~~~ Types of property 'hairColor' are incompatible.
  shirtColor: 'red',
  hairColor: 'blue',
}

💻 playground

---

interface Vector2D {
  x: number
  y: number
  z?: never
}

💻 playground

---

function norm(v: Vector2D) {
  return Math.sqrt(v.x ** 2 + v.y ** 2)
}
const v = { x: 3, y: 4, z: 5 }
const d = norm(v)
//             ~ Types of property 'z' are incompatible.

💻 playground

---

interface ThingOneTag {
  type: 'one'
  shirtColor: string
}
interface ThingTwoTag {
  type: 'two'
  hairColor: string
}
type Thing = ThingOneTag | ThingTwoTag

💻 playground

---

type XOR<T1, T2> =
  | (T1 & { [k in Exclude<keyof T2, keyof T1>]?: never })
  | (T2 & { [k in Exclude<keyof T1, keyof T2>]?: never })

💻 playground

---

type ExclusiveThing = XOR<ThingOne, ThingTwo>
const allThings: ExclusiveThing = {
  //  ~~~~~~~~~ Types of property 'hairColor' are incompatible.
  shirtColor: 'red',
  hairColor: 'blue',
}

💻 playground