Bind method this to its parent object

extractMethods pushed each function into methodsById directly, so the
host invoked it with `this` undefined. Methods like { n: 0, inc() {
return ++this.n } } failed at runtime. Bind the function to its source
container on extraction so `this` points at the original object.

Author: aduros

src/host.ts

@@ -144,7 +144,7 @@ export async function createSandbox(opts?: CreateSandboxOptions): Promise<Sandbo
       switch (typeof value) {
         case "function":
           methods[key] = methodsById.length;
-          methodsById.push(value);
+          methodsById.push(value.bind(source));
           break;
         case "object": {
           if (value === null) {

tests/basics.test.ts

@@ -209,6 +209,53 @@ describe("globals", () => {
   test("rejects unserializable globals", async () => {
     await expect(createSandbox({ globals: { sym: Symbol("nope") } })).rejects.toThrow();
   });
+
+  test("method `this` binds to its parent object", async () => {
+    const counter = {
+      n: 0,
+      inc() {
+        return ++this.n;
+      },
+    };
+    sandbox = await createSandbox({ globals: { counter } });
+    expect(await sandbox.evaluate("counter.inc()")).toBe(1);
+    expect(await sandbox.evaluate("counter.inc()")).toBe(2);
+    expect(counter.n).toBe(2);
+  });
+
+  test("nested method `this` binds to its immediate parent", async () => {
+    const inner = {
+      label: "hi",
+      greet() {
+        return this.label;
+      },
+    };
+    sandbox = await createSandbox({ globals: { outer: { inner } } });
+    expect(await expression(sandbox, "await outer.inner.greet()")).toBe("hi");
+  });
+
+  test("sibling methods share the same `this`", async () => {
+    const store = {
+      value: 10,
+      get() {
+        return this.value;
+      },
+      set(v: number) {
+        this.value = v;
+      },
+    };
+    sandbox = await createSandbox({ globals: { store } });
+    expect(await sandbox.evaluate("store.get()")).toBe(10);
+    await sandbox.run("await store.set(99)");
+    expect(await sandbox.evaluate("store.get()")).toBe(99);
+  });
+
+  test("top-level functions still work without `this`", async () => {
+    sandbox = await createSandbox({
+      globals: { add: (a: number, b: number) => a + b },
+    });
+    expect(await expression(sandbox, "await add(2, 3)")).toBe(5);
+  });
 });
 
 describe("global value types", () => {