test: 💍 add test for math.diff

Author: jasoniangreen

src/__tests__/math.spec.js

@@ -4,4 +4,10 @@ describe("math", () => {
     it("should define length function", () => {
         expect(math.len({x: 3, y: 4})).toBe(5);
     });
+    it("should define diff function", () => {
+        expect(math.diff({x: 3, y: 4}, {x: 3, y: 4})).toEqual({x: 0, y: 0});
+        expect(math.diff({x: 1, y: 4}, {x: 3, y: 4})).toEqual({x: 2, y: 0});
+        expect(math.diff({x: 3, y: 2}, {x: 3, y: 4})).toEqual({x: 0, y: 2});
+        expect(math.diff({x: 10, y: 10}, {x: 3, y: 4})).toEqual({x: -7, y: -6});
+    });
 });