Item 42: Avoid Types Based on Anecdotal Data

Things to Remember

• Avoid writing types by hand based on data that you've seen. It's easy to misunderstand a schema or get nullability wrong.
• Prefer types sourced from official clients or the community. If these don't exist, generate TypeScript types from schemas.## Code Samples

function calculateBoundingBox(f: GeoJSONFeature): BoundingBox | null {
  let box: BoundingBox | null = null;

  const helper = (coords: any[]) => {
    // ...
  };

  const {geometry} = f;
  if (geometry) {
    helper(geometry.coordinates);
  }

  return box;
}

💻 playground

---

interface GeoJSONFeature {
  type: 'Feature';
  geometry: GeoJSONGeometry | null;
  properties: unknown;
}
interface GeoJSONGeometry {
  type: 'Point' | 'LineString' | 'Polygon' | 'MultiPolygon';
  coordinates: number[] | number[][] | number[][][] | number[][][][];
}

💻 playground

---

import {Feature} from 'geojson';

function calculateBoundingBox(f: Feature): BoundingBox | null {
  let box: BoundingBox | null = null;

  const helper = (coords: any[]) => {
    // ...
  };

  const {geometry} = f;
  if (geometry) {
    helper(geometry.coordinates);
    //              ~~~~~~~~~~~
    //   Property 'coordinates' does not exist on type 'Geometry'
    //     Property 'coordinates' does not exist on type 'GeometryCollection'
  }

  return box;
}

💻 playground

---

const {geometry} = f;
if (geometry) {
  if (geometry.type === 'GeometryCollection') {
    throw new Error('GeometryCollections are not supported.');
  }
  helper(geometry.coordinates);  // OK
}

💻 playground

---

const geometryHelper = (g: Geometry) => {
  if (g.type === 'GeometryCollection') {
    g.geometries.forEach(geometryHelper);
  } else {
    helper(g.coordinates);  // OK
  }
}

const {geometry} = f;
if (geometry) {
  geometryHelper(geometry);
}

💻 playground