test(size): add size function tests

src/__tests__/size.spec.ts

@@ -0,0 +1,29 @@
+import size from '../size';
+
+describe('size', () => {
+  test('should return a number', () => {
+    expect(typeof size({})).toBe('number');
+  });
+
+  test('should return the size of the object (number of keys)', () => {
+    const objA = {
+      a: 1,
+      b: 2,
+    };
+    const objB = {
+      a: 1,
+      b: 2,
+      c: 3,
+    };
+
+    expect(size(objA)).toBe(2);
+    expect(size(objB)).toBe(3);
+  });
+
+  test('should throw TypeError for invalid arguments', () => {
+    const invalidObj: unknown = 'testing';
+
+    expect(() => size(invalidObj as OObject))
+      .toThrow(new TypeError('Expected Object, got string testing'));
+  });
+});