test(filter): add filter function tests

src/__tests__/filter.spec.ts

@@ -0,0 +1,72 @@
+import filter from '../filter';
+
+describe('filter', () => {
+  test('should only return key/values which the callback evaluates as true', () => {
+    const obj = {
+      a: 1,
+      b: 2,
+      c: 'testing',
+    };
+
+    const resultA = filter(obj, (k, v) => typeof v === 'number');
+    const resultB = filter(obj, (k, v) => typeof v === 'string');
+
+    expect(Object.keys(resultA)).toHaveLength(2);
+    expect(Object.keys(resultB)).toHaveLength(1);
+  });
+
+  test('should evaluate all key/values', () => {
+    const obj = {
+      a: 1,
+      b: 2,
+    };
+    const cb = jest.fn(() => true);
+
+    filter(obj, cb);
+
+    expect(cb.mock.calls).toHaveLength(2);
+  });
+
+  test('should follow deep objects if follow is true', () => {
+    const obj = {
+      a: 1,
+      b: 2,
+      c: {
+        d: 3,
+        e: 'testing',
+      },
+    };
+
+    const cbA: FilterCallback = (k, v) => typeof v === 'number';
+    const cbB: FilterCallback = (k, v) => typeof v === 'string';
+
+    const resultA = filter(obj, cbA, {
+      follow: false,
+    });
+    const resultB = filter(obj, cbA, {
+      follow: true,
+    });
+    const resultC = filter(obj, cbB, {
+      follow: true,
+    });
+
+    expect(resultA.a + resultA.b).toBe(3);
+    expect(resultB.a + resultB.b + resultB.c.d).toBe(6);
+    expect(resultC.c.e).toBe('testing');
+  });
+
+  test('should throw TypeError for invalid arguments', () => {
+    const invalidObj: unknown = 'testing';
+    const invalidCallback: unknown = 'testing';
+    const invalidFollow: unknown = 'testing';
+
+    expect(() => filter(invalidObj as OObject, jest.fn()))
+      .toThrow(new TypeError('Expected Object, got string testing'));
+    expect(() => filter({}, invalidCallback as EveryCallback))
+      .toThrow(new TypeError('Expected Function, got string testing'));
+    expect(() => filter({}, jest.fn(), {
+      follow: invalidFollow as boolean,
+    }))
+      .toThrow(new TypeError('Expected Boolean, got string testing'));
+  });
+});