求助：如何实现Feature内部Geometry类型的通用Typeguard

现有类型定义 #

工作中使用的GeoJSON相关TypeScript类型如下：

export type GeoJSON = Geometry | Feature;

export type GeoJsonTypes = GeoJSON['type'];

export interface GeoJsonObject { type: GeoJsonTypes; }

export interface Point extends GeoJsonObject {
   type: 'Point';
   coordinates: number[];
 }

export interface MultiPoint extends GeoJsonObject {
   type: "MultiPoint",
   coordinates: number[][];
 }

export type Geometry =
 | Point
 | MultiPoint

export interface Feature<G extends Geometry = Geometry>
  extends GeoJsonObject {
  type: 'Feature';
  geometry: G;
}

问题场景 #

当前收窄Feature<Geometry>类型时，必须为每种Geometry对应的Feature编写单独的类型守卫，例如：

export function isPolygon(f: Feature): f is Feature<Polygon> {
  return f && f.geometry && f.geometry.type === 'Polygon';
}

尝试编写通用类型守卫时遇到类型错误：将泛型T约束为Feature会触发A type Predicate's type must be assignable to it's parameter type；约束为Geometry则会出现T could be instantiated with an arbitrary type which could be unrelated错误。

解决方案 #

调整泛型约束逻辑，直接针对Geometry的类型值做限定，即可实现合法的通用类型守卫：

export function isFeature<U extends Geometry['type']>(
  feature: Feature<Geometry>,
  type: U
): feature is Feature<Extract<Geometry, { type: U }>> {
  return !!feature && !!feature.geometry && feature.geometry.type === type;
}

关键说明 #

• 泛型U直接绑定到Geometry['type']，确保传入的type参数只能是合法的Geometry类型标识（如'Point'、'MultiPoint'）
• 参数feature直接使用最宽泛的Feature<Geometry>类型，规避泛型参数T带来的类型兼容性冲突
• 通过Extract工具类型从Geometry联合类型中筛选出对应type的具体类型，再包装为对应的Feature类型，保证类型断言的合法性

使用示例 #

const feature: Feature<Geometry> = {/* 任意Feature数据 */};

if (isFeature(feature, 'Point')) {
  // 此处feature类型自动收窄为Feature<Point>
  console.log(feature.geometry.coordinates); // 类型为number[]
}

if (isFeature(feature, 'MultiPoint')) {
  // 此处feature类型自动收窄为Feature<MultiPoint>
  console.log(feature.geometry.coordinates); // 类型为number[][]
}

内容的提问来源于stack exchange，提问作者Martin Hansson