improve object/has types

Author: temoncher

src/object/__tests__/has.ts

@@ -1,3 +1,4 @@
+import { expectType } from 'ts-expect';
 import has from '../has';
 
 describe('utils/object/has', () => {
@@ -14,4 +15,56 @@ describe('utils/object/has', () => {
         C.prototype = { a: 5 };
         expect(has('a', new C())).toBe(false);
     });
+
+    it('should have `false` type for `undefined` and `null` inputs', () => {
+        expectType<false>(has('a', null));
+        expectType<false>(has('a')(null));
+        expectType<false>(has('a', undefined));
+        expectType<false>(has('a')(undefined));
+    });
+
+    it('should exclude `null` and `undefined` from union', () => {
+        const something = { id: 12 } as { id: number } | null | undefined;
+        
+        if (has('name', something)) {
+            expectType<{ id: number; name: unknown }>(something);
+        }
+
+        const hasId = has('id');
+        if(hasId(something)) {
+            expectType<{ id: number }>(something);
+        }
+    });
+
+    it('should narrow union', () => {
+        const something = { id: 12 } as { id: number } | { name: string } | { name: string; somethingElse: string };
+        
+        if (has('name', something)) {
+            expectType<{ name: string } | { name: string; somethingElse: string }>(something);
+        }
+        const hasName = has('name');
+        if (hasName(something)) {
+            expectType<{ name: string } | { name: string; somethingElse: string }>(something);
+        }
+        
+        if (has('id', something)) {
+            expectType<{ id: number }>(something);
+        }
+        const hasId = has('id');
+        if (hasId(something)) {
+            expectType<{ id: number }>(something);
+        }
+    });
+
+    it('should add missing property as `unknown`', () => {
+        const something = { id: 12 } as { id: number } | { name: string };
+        
+        if (has('missing', something)) {
+            expectType<({ name: string } | { id: number }) & { missing: unknown }>(something);
+        }
+        const hasMissing = has('missing');
+        if (hasMissing(something)) {
+            expectType<({ name: string } | { id: number }) & { missing: unknown }>(something);
+        }
+    });
 });

src/object/has.ts

@@ -1,9 +1,17 @@
 import curryN from '../function/curryN';
 import { Prop } from '../typings/types';
 
+interface HasPredicate<P extends Prop> {
+    (obj: undefined | null): false;
+    <T extends { [K in P]: unknown }, U>(obj: U | T): obj is T;
+    <T extends object>(obj: T): obj is T & Record<P, unknown>;
+}
+
 interface Has {
-    <K extends Prop>(s: K, obj): boolean;
-    <K extends Prop>(s: K): (obj) => boolean;
+    <P extends Prop>(prop: P): HasPredicate<P>
+    (prop: Prop, obj: undefined | null): false;
+    <P extends Prop, T extends { [K in P]: unknown }, U>(prop: P, obj: U | T): obj is T;
+    <P extends Prop, T extends object>(prop: P, obj: T): obj is T & Record<P, unknown>;
 }
 
 /**