diff --git a/test/pollution.test.js b/test/pollution.test.js
new file mode 100644
index 0000000..882400f
--- /dev/null
+++ b/test/pollution.test.js
@@ -0,0 +1,85 @@
+import addedDiff from "../src/added";
+
+describe("Prototype pollution", () => {
+  test("Demonstrate prototype pollution globally across all objects", () => {
+    const a = {};
+    const b = new Object();
+
+    expect(a.hello).toBeUndefined();
+    expect(b.hello).toBeUndefined();
+    expect({}.hello).toBeUndefined();
+
+    b.__proto__.hello = "world";
+
+    expect(a.hello).toBe("world");
+    expect(b.hello).toBe("world");
+    expect({}.hello).toBe("world");
+  });
+
+  test("addedDiff does not pollute global prototype when running diff with added `__proto__` key", () => {
+    const a = { role: "user" };
+    const b = JSON.parse('{ "__proto__": { "role": "admin" } }');
+
+    expect(a.role).toBe("user");
+    expect(a.__proto__.role).toBeUndefined();
+    expect(b.role).toBeUndefined();
+    expect(b.__proto__.role).toBe("admin");
+    expect({}.role).toBeUndefined();
+    expect({}.__proto__role).toBeUndefined();
+
+    const difference = addedDiff(a, b);
+
+    expect(a.role).toBe("user");
+    expect(a.__proto__.role).toBeUndefined();
+    expect(b.__proto__.role).toBe("admin");
+    expect(b.role).toBeUndefined();
+    expect({}.role).toBeUndefined();
+    expect({}.__proto__role).toBeUndefined();
+
+    expect(difference).toEqual({ __proto__: { role: "admin" } });
+  });
+
+  test("addedDiff does not pollute global prototype when running diff with added `__proto__` key generated from JSON.parse and mutating original left hand object", () => {
+    let a = { role: "user" };
+    // Note: Don't trust `JSON.parse`!!!
+    const b = JSON.parse('{ "__proto__": { "role": "admin" } }');
+
+    expect(a.role).toBe("user");
+    expect(a.__proto__.role).toBeUndefined();
+    expect(b.role).toBeUndefined();
+    expect(b.__proto__.role).toBe("admin");
+    expect({}.role).toBeUndefined();
+    expect({}.__proto__role).toBeUndefined();
+
+    // Note: although this does not pollute the global proto, it does pollute the original object. (Don't mutate kids!)
+    a = addedDiff(a, b);
+
+    expect(a.role).toBe("admin");
+    expect(a.__proto__.role).toBe("admin");
+    expect(b.__proto__.role).toBe("admin");
+    expect(b.role).toBeUndefined();
+    expect({}.role).toBeUndefined();
+    expect({}.__proto__role).toBeUndefined();
+  });
+
+  test("addedDiff does not pollute global prototype or original object when running diff with added `__proto__` key", () => {
+    let a = { role: "user" };
+    const b = { __proto__: { role: "admin" } };
+
+    expect(a.role).toBe("user");
+    expect(a.__proto__.role).toBeUndefined();
+    expect(b.role).toBe("admin");
+    expect(b.__proto__.role).toBe("admin");
+    expect({}.role).toBeUndefined();
+    expect({}.__proto__role).toBeUndefined();
+
+    a = addedDiff(a, b);
+
+    expect(a.role).toBeUndefined();
+    expect(a.__proto__.role).toBeUndefined();
+    expect(b.role).toBe("admin");
+    expect(b.__proto__.role).toBe("admin");
+    expect({}.role).toBeUndefined();
+    expect({}.__proto__role).toBeUndefined();
+  });
+});