Fix instanceof when subclassing Error, closes #159

The `[Symbol.hasInstance]` property is inherited by subclasses.
Previously each class had a fixed target checks were performed
against. This caused issues where subclasses of `Error` were
treated as `Error`s themselves in `instanceof` checks leading to
incorrect results. This change uses the bound `this` as the target.

packages/runner-vm/src/instanceof.ts

@@ -77,7 +77,7 @@ import vm from "vm";
 import { ValueOf } from "@miniflare/shared";
 
 // https://tc39.es/ecma262/multipage/abstract-operations.html#sec-ordinaryhasinstance
-function ordinaryHasInstance(C: any, O: any): boolean {
+function ordinaryHasInstance(C: unknown, O: unknown): boolean {
   // 1. If IsCallable(C) is false, return false.
   //    - https://tc39.es/ecma262/multipage/abstract-operations.html#sec-iscallable
   //    - https://tc39.es/ecma262/multipage/ecmascript-language-expressions.html#table-typeof-operator-results
@@ -133,11 +133,11 @@ const outsideTargets = {
 };
 
 function defineHasInstance(insideTarget: ValueOf<typeof outsideTargets>) {
-  const outsideTarget =
-    outsideTargets[insideTarget.name as keyof typeof outsideTargets];
   Object.defineProperty(insideTarget, Symbol.hasInstance, {
     value(value: any) {
-      return instanceOf(value, insideTarget, outsideTarget);
+      const outsideTarget =
+        outsideTargets[this.name as keyof typeof outsideTargets];
+      return instanceOf(value, this, outsideTarget);
     },
   });
 }
@@ -152,12 +152,6 @@ const defineHasInstancesScript = new vm.Script(
   defineHasInstance(Promise);
   defineHasInstance(RegExp);
   defineHasInstance(Error);
-  defineHasInstance(EvalError);
-  defineHasInstance(RangeError);
-  defineHasInstance(ReferenceError);
-  defineHasInstance(SyntaxError);
-  defineHasInstance(TypeError);
-  defineHasInstance(URIError);
 })`,
   { filename: "<defineHasInstancesScript>" }
 );

packages/runner-vm/test/instanceof.spec.ts

@@ -73,3 +73,41 @@ test("calling defineHasInstances on same context multiple times doesn't throw",
   defineHasInstances(ctx);
   t.pass();
 });
+
+test("Error subclasses have correct instanceof behaviour", (t) => {
+  // https://github.com/cloudflare/miniflare/issues/159
+  const ctx = vm.createContext({});
+  defineHasInstances(ctx);
+  const result = vm.runInContext(
+    `
+    class CustomError extends Error {}
+    ({
+      errorInstanceOfError: new Error() instanceof Error,
+      errorInstanceOfTypeError: new Error() instanceof TypeError,
+      errorInstanceOfCustomError: new Error() instanceof CustomError,
+      
+      typeErrorInstanceOfError: new TypeError() instanceof Error,
+      typeErrorInstanceOfTypeError: new TypeError() instanceof TypeError,
+      typeErrorInstanceOfCustomError: new TypeError() instanceof CustomError,
+      
+      customErrorInstanceOfError: new CustomError() instanceof Error,
+      customErrorInstanceOfTypeError: new CustomError() instanceof TypeError,
+      customErrorInstanceOfCustomError: new CustomError() instanceof CustomError,
+    })
+    `,
+    ctx
+  );
+  t.deepEqual(result, {
+    errorInstanceOfError: true,
+    errorInstanceOfTypeError: false,
+    errorInstanceOfCustomError: false,
+
+    typeErrorInstanceOfError: true,
+    typeErrorInstanceOfTypeError: true,
+    typeErrorInstanceOfCustomError: false,
+
+    customErrorInstanceOfError: true,
+    customErrorInstanceOfTypeError: false,
+    customErrorInstanceOfCustomError: true,
+  });
+});