feat: better typing on filterMap for use with predicates

expo · Jan 8, 2021 · 5e0f697 · 5e0f697
1 parent 8354741
commit 5e0f697
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Showing 2 changed files with 25 additions and 3 deletions.
diff --git a/packages/entity/src/utils/collections/__tests__/maps-test.ts b/packages/entity/src/utils/collections/__tests__/maps-test.ts
@@ -117,4 +117,18 @@ describe(filterMap, () => {
     expect(filteredMap.get('a')).toBeUndefined();
     expect(filteredMap.get('b')).toBe(true);
   });
+
+  it('can use predicates', () => {
+    function truthy<TValue>(value: TValue | null | undefined): value is TValue {
+      return !!value;
+    }
+
+    const map = new Map<string, string | null>([
+      ['a', 'yes'],
+      ['b', null],
+    ]);
+
+    const filteredMap: Map<string, string> = filterMap(map, truthy);
+    expect(filteredMap.size).toBe(1);
+  });
 });
diff --git a/packages/entity/src/utils/collections/maps.ts b/packages/entity/src/utils/collections/maps.ts
@@ -83,15 +83,23 @@ export const reduceMapAsync = async <K, V, A>(
   return newAccumulator;
 };
 
-export const filterMap = <K, V>(
+export function filterMap<K, V, S extends V>(
+  map: ReadonlyMap<K, V>,
+  predicate: (value: V, key: K) => value is S
+): Map<K, S>;
+export function filterMap<K, V>(
   map: ReadonlyMap<K, V>,
   predicate: (value: V, key: K) => boolean
-): Map<K, V> => {
+): Map<K, V>;
+export function filterMap<K, V>(
+  map: ReadonlyMap<K, V>,
+  predicate: (value: V, key: K) => unknown
+): Map<K, V> {
   const resultingMap = new Map();
   map.forEach((v, k) => {
     if (predicate(v, k)) {
       resultingMap.set(k, v);
     }
   });
   return resultingMap;
-};
+}