typescript - 如何避免 TypeScript 区分联合中的双重不可为空检查？

Question

在下面的代码中，TypeScript 不相信 if customMessageis undefined，classNameis NOT undefined：

export default class ClassRequiredInitializationHasNotBeenExecutedError extends Error {

  public static readonly NAME: string = "ClassRequiredInitializationHasNotBeenExecutedError";

  public constructor(
      parametersObject: {
        customMessage: string;
        className?: undefined;
      } | {
        className: string;
        customMessage?: undefined;
      }
  ) {

    super();

    this.name = ClassRequiredInitializationHasNotBeenExecutedError.NAME;

    if (typeof parametersObject.customMessage !== "undefined") {
      this.message = parametersObject.customMessage;
    } else {
      this.message = ClassRequiredInitializationHasNotBeenExecutedError.buildMessage({
          className: parametersObject.className
      })
    }
  }

  private static buildMessage(parametersObject: { className: string }): string {
    return `The class '${parametersObject.className}' has not been executed;`
  }
}

小提琴

错误：

Type 'string | undefined' is not assignable to type 'string'.
  Type 'undefined' is not assignable to type 'string'.(2322)

我知道如果会进行仔细检查，例如：

if (typeof parametersObject.customMessage !== "undefined") {
    this.message = parametersObject.customMessage;
} else if (typeof parametersObject.className !== "undefined") {
    this.message = ClassRequiredInitializationHasNotBeenExecutedError.buildMessage({
        className: parametersObject.className
})

上面的例子可以工作，但是：

1. else if (typeof parametersObject.className !== "undefined")如果可能，我想避免
2. 事件离开它，TypeScript 不会相信其中之一customMessage或className已被初始化。这是关键时的示例：

let message: string;

if (typeof parametersObject.customMessage !== "undefined") {
    message = parametersObject.customMessage;
} else if (typeof parametersObject.className !== "undefined") {
    message = ClassRequiredInitializationHasNotBeenExecutedError.buildMessage({
        className: parametersObject.className
    })
}

console.log(message.length)

Variable 'message' is used before being assigned.(2454)

请注意，这个问题是关于如何让 TypeScript 相信 if customMessageis undefined, classNameis NOT undefined ，反之亦然，而不是如何初始化this.message（ClassRequiredInitializationHasNotBeenExecutedError该类只是示例）

标签： typescript

Answer 1

 parametersObject: {
        customMessage: string;
        className?: undefined;
      } | {
        className: string;
        customMessage?: undefined;
      }

可以简化为：

parametersObject: {
        customMessage: string;
      } | {
        className: string;
      }

因为property?: undefined几乎没有额外的信息。

那么条件可以表示为：

if ('customMessage' in parametersObject) {
  this.message = parametersObject.customMessage;
} else {
  this.message = ClassRequiredInitializationHasNotBeenExecutedError.buildMessage({
      className: parametersObject.className
  })
}

操场

更新

严格强制一个或另一个选项：

type Params<T extends {
        customMessage: string;
      } | {
        className: string;
      }>  = T extends {
        customMessage: any;
        className: any;
      }  ? never : T

export default class ClassRequiredInitializationHasNotBeenExecutedError<P extends {
        customMessage: string;
      } | {
        className: string;
      }> extends Error {

  public static readonly NAME: string = "ClassRequiredInitializationHasNotBeenExecutedError";

  public constructor(
      parametersObject: P extends Params<P> ? P : never
  ) {

    super();

    this.name = ClassRequiredInitializationHasNotBeenExecutedError.NAME;

    if ('customMessage' in parametersObject) {
      this.message = parametersObject.customMessage;
    } else {
      this.message = ClassRequiredInitializationHasNotBeenExecutedError.buildMessage({
          className: parametersObject.className
      })
    }
  }

  private static buildMessage(parametersObject: { className: string }): string {
    return `The class '${parametersObject.className}' has not been executed;`
  }
}

new ClassRequiredInitializationHasNotBeenExecutedError({ customMessage: ''  }) // ok
new ClassRequiredInitializationHasNotBeenExecutedError({ className: ''  }) // ok
new ClassRequiredInitializationHasNotBeenExecutedError({ customMessage: '', className: ''  }) // error as expected

操场