带有枚举的TypeScript条件类型

Question

我有以下TypeScript枚举：

enum UIConfigurationType {
  DisplayTableFields = "display-table-field",
  FormFields = "form-field",
  MainAttributes = "main-attribute",
  SearchAttributes = "search-attribute",
}

和这个条件类型：

type UIConfigurationEntitySuffix<T extends UIConfigurationType> =
  T extends UIConfigurationType.DisplayTableFields
    ? "display-table-fields"
    : T extends UIConfigurationType.FormFields
    ? "form-fields"
    : T extends UIConfigurationType.MainAttributes
    ? "main-attributes"
    : T extends UIConfigurationType.SearchAttributes
    ? "search-attributes"
    : never;

例如，它应该静态地返回泛型文字类型UIConfigurationType.SearchAttributes的文字类型"search-attributes" (它实际上做得很正确)。

然后我有了下面的函数，它动态地实现了由前面的条件类型定义的映射：

function getUIConfigurationEntitySuffix<T extends UIConfigurationType>(
  uiConfigurationType: T
): UIConfigurationEntitySuffix<T> {
  switch (uiConfigurationType) {
    case UIConfigurationType.DisplayTableFields:
      return "display-table-fields";
    case UIConfigurationType.FormFields:
      return "form-fields";
    case UIConfigurationType.MainAttributes:
      return "main-attributes";
    case UIConfigurationType.SearchAttributes:
      return "search-attributes";
    default:
      throw new Error(`Unknown uiConfigurationType: ${uiConfigurationType}`);
  }
}

此函数定义会导致TypeScript在每个case分支中引发错误：

TypeScript errors on getUIConfigurationEntitySuffix function

我遗漏了什么？谢谢!

Answer 1

这样啊，原来是这么回事!无论出于什么原因，TypeScript都没有推断出返回的类型就是UIConfigurationEntitySuffix<T>期望的类型，所以我必须显式地转换它们：

function getUIConfigurationEntitySuffix<T extends UIConfigurationType>(
  uiConfigurationType: T
): UIConfigurationEntitySuffix<T> {
  switch (uiConfigurationType) {
    case UIConfigurationType.DisplayTableFields:
      return "display-table-fields" as UIConfigurationEntitySuffix<T>;
    case UIConfigurationType.FormFields:
      return "form-fields" as UIConfigurationEntitySuffix<T>;
    case UIConfigurationType.MainAttributes:
      return "main-attributes" as UIConfigurationEntitySuffix<T>;
    case UIConfigurationType.SearchAttributes:
      return "search-attributes" as UIConfigurationEntitySuffix<T>;
    default:
      throw new Error(`Unknown uiConfigurationType: ${uiConfigurationType}`);
  }
}