带有枚举的 TypeScript 条件类型答案

【问题标题】：TypeScript conditional types with enums带有枚举的 TypeScript 条件类型
【发布时间】：2021-09-07 09:08:36
【问题描述】：

我有以下 TypeScript 枚举：

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

还有这个条件类型：

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，它应该为泛型文字类型 UIConfigurationType.SearchAttributes 静态返回文字类型 "search-attributes"（实际上它是正确的）。

然后我有下面的函数，它动态实现了前面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}`);
  }
}

此函数定义导致 TypeScript 在每个 case 分支中引发错误：

TypeScript errors on getUIConfigurationEntitySuffix function

我错过了什么？谢谢！

【问题讨论】：

标签： typescript enums conditional-types

【解决方案1】：

我明白了！无论出于何种原因，TypeScript 都没有推断出返回的类型是 UIConfigurationEntitySuffix<T> 所期望的正确类型，所以我必须明确地转换它们：

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}`);
  }
}

【讨论】：