Add enhance behavior tests

packages/enhance/src/stub.test.ts

@@ -1,3 +1,43 @@
-import { describe } from "node:test";
+import assert from "node:assert/strict";
+import { describe, it } from "node:test";
+import { enhance } from "./enhance.lib.ts";
 
-describe("Stub", () => {});
+describe("enhance", () => {
+  it("enhances objects with custom methods", () => {
+    const base = { value: 1 };
+    const enhanced = enhance("numbers", base, {
+      double: function () {
+        return this.value * 2;
+      },
+      add: function (n: number) {
+        return this.value + n;
+      },
+    });
+
+    assert.equal(enhanced.double(), 2);
+    assert.equal(enhanced.add(3), 4);
+    assert.strictEqual(enhanced.$(), base);
+  });
+
+  it("does not mutate the original object when enhanced again", () => {
+    const base = { value: 1 };
+    const first = enhance("numbers", base, {
+      inc: function () {
+        return this.value + 1;
+      },
+    });
+
+    const second = enhance("numbers", first, {
+      double: function () {
+        return this.value * 2;
+      },
+    });
+
+    assert.equal(typeof (first as any).double, "undefined");
+    assert.deepEqual(base, { value: 1 });
+    assert.equal(first.inc(), 2);
+    assert.equal(second.double(), 2);
+    assert.strictEqual(first.$(), base);
+    assert.strictEqual(second.$().$(), base);
+  });
+});