switch(key) のkey に応じて、value の型を解決させたい

Union

基本的には、複数の型を作ってUnionで定義してあげることになるかと思う。

// こんな感じ
type IncreaseAction = { type: "INCREASE"; payload: { value: number } }
type DecreaseAction = { type: "DECREASE"; payload: { value: number } }
type DebugAction = { type: "DEBUG"; payload: { fn: () => void } }

type Action = IncreaseAction | DecreaseAction | DebugAction;

const runner = (action: Action) => {
  switch(action.type) {
    case "INCREASE": {
      return console.log(action.payload.value + 1);
    }
    case "DECREASE": {
      return console.log(action.payload.value - 1);
    }
    case "DEBUG": {
      return action.payload.fn();
    }
  }
}

`Object.entires`

ただ、１つのオブジェクトにkey,value がまとまってるときはObject.entries を使うことになる。ObjectEntiresを解決できる型を用意してあげればできる。

const attributes: Attributes = {
  k1: "Hello",
  k2: 100,
  k3: () => console.log(":D"),
};

type Attributes = {
  k1: string,
  k2: number,
  k3: () => void,
}

type AttributesPair = ObjectEntries<Attributes>;

const runner = (pair: AttributesPair) => {
  const [key, value] = pair;
  switch(key){
    case "k1": {
      // value の型が stringに解決する
      return console.log(value, " World!")
    }
    case "k2": {
      // value の型が numberに解決する    
      return console.log(value+11, "is 111.")
    }
    case "k3": {
      // value の型が `() => void` に解決する    
      return value();
    }
    default: {
      const _: never = key; // in order to detect ts check
      throw new Error(`invalid data. ${_}`)
    }
  }
}

// ====================
// main.ts
// ====================
Object.entries(attributes).map((_pair: any) => {
  const pair = _pair as AttributesPair;
  runner(pair);
});

// =============
// Helper
// =============
// REF: https://github.com/type-challenges/type-challenges/issues/14052
// type RemoveUndefined<T> = [T] extends [undefined] ? T : Exclude<T, undefined>
// type ObjectEntries<T> = {
//  [K in keyof T]-?: [K, RemoveUndefined<T[K]>]
// }[keyof T]
type ObjectEntries<T, U extends keyof T = keyof T> = U extends any ? [U, Exclude<T[U], undefined>] : never;

REF