为什么这段代码说 Type 'number' 不能分配给 type 'never'

Question

我正在尝试创建一个名为 addToCache 的字符串构建函数，该函数的参数由名为 Payloads 的接口定义

因此，基于从名为 MyStringsTest 的 enum 注册的字符串的键码，Typescript 应该使用该键码执行类型查找。

export enum MyStringsTest {
    DEPART_IN_X_MINUTE='DEPART_IN_X_MINUTE',
    A_DRIVER_IS_ASKING_FOR='A_DRIVER_IS_ASKING_FOR',
}
export interface Payloads {
    [MyStringsTest.DEPART_IN_X_MINUTE]: number,
    [MyStringsTest.A_DRIVER_IS_ASKING_FOR]: string,
}
const cache:{ [K in keyof Payloads]?: (payload: Payloads[K]) => string } = {}
function addToCache<K extends keyof Payloads>(code: K, cb: ((payload: Payloads[K]) => string)): void{
    cache[code] = cb // I have the error here Type 'number' is not assignable to type 'never'
}
addToCache(MyStringsTest.DEPART_IN_X_MINUTE, (data) => `Depart in ${data} minutes!`);
addToCache(MyStringsTest.A_DRIVER_IS_ASKING_FOR, (data) => `a driver is asking for ${data}`);

看起来用法有效，但编译器显示此错误

Type 'number' is not assignable to type 'never'我试图理解为什么但我找不到原因

这是playground

Answer 1

通过为缓存创建单独的类型别名来帮助打字稿工作就好了！

export enum MyStringsTest {
  DEPART_IN_X_MINUTE = 'DEPART_IN_X_MINUTE',
  A_DRIVER_IS_ASKING_FOR = 'A_DRIVER_IS_ASKING_FOR'
}
export interface Payloads {
  [MyStringsTest.DEPART_IN_X_MINUTE]: number
  [MyStringsTest.A_DRIVER_IS_ASKING_FOR]: string
}

type Catch = {
  [K in keyof Payloads]?: (payload: Payloads[K]) => string
}

const cache: Catch = {}
function addToCache<K extends keyof Catch>(code: K, cb: Catch[K]): void {
  cache[code] = cb
}
addToCache(MyStringsTest.DEPART_IN_X_MINUTE, (data) => `Depart in ${data} minutes!`)
addToCache(MyStringsTest.A_DRIVER_IS_ASKING_FOR, (data) => `a driver is asking for ${data}`)

bdw 你真的需要枚举吗，你可以完全摆脱它，如下所示

export interface Payloads {
  DEPART_IN_X_MINUTE: number
  A_DRIVER_IS_ASKING_FOR: string
}

type Catch = {
  [K in keyof Payloads]?: (payload: Payloads[K]) => string
}

const cache: Catch = {}
function addToCache<K extends keyof Catch>(code: K, cb: Catch[K]): void {
  cache[code] = cb
}

addToCache('DEPART_IN_X_MINUTE', (data) => `Depart in ${data} minutes!`)
addToCache('A_DRIVER_IS_ASKING_FOR', (data) => `a driver is asking for ${data}`)