如何在 function 道具中声明可选的泛型类型？

Question

我想从 redux-toolkit 创建类似于 createAction 的东西。 我需要将有效负载类型传递给 createAction，但我不明白如果传递了可选属性，如何强制它。

这是我的实现。

type ActionFormat<P = undefined> = {type: string} & (P extends undefined ? {} : {payload: P})

export function createAction<P = undefined>(type: string): (payload?: P) => ActionFormat {
  const actionCreator = (payload?: P) => ({ type, payload: payload });
  return actionCreator;
}

创建动作

 const startFetch = createAction('START_FETCH');
 const successFetch = createAction<number>('SUCCESS_FETCH')

运行动作

startFetch(); //there are no errors. No payload required 
startFetch('test'); //must be an error Payload has been submitted
successFetch(); //must be an error Payload was not submitted
successFetch(123); // there are no errors.

Answer 1

您可以使用条件类型将元组类型传播到undeinfed参数，根据P是否扩展undefined （即未定义或包含未定义的联合）使参数成为必需或不需要

export function createAction<P = undefined>(type: string): (...payload: 
  P extends undefined 
      ? [payload?: P] // P === undefined, no parameters required, but pay be passed in.
      : [payload: P] //P does not contain undefined parameter is required
  ) => ActionFormat {
  const actionCreator = (payload?: P) => ({ type, payload: payload });
  return actionCreator;
}

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