feat(testing/asserts): add assertNotInstanceOf (#2530)

testing/README.md

@@ -23,6 +23,8 @@ pretty-printed diff of failing assertion.
   `expected`, according to a given `epsilon` _(defaults to `1e-7`)_
 - `assertInstanceOf()` - Make an assertion that `actual` is an instance of
   `expectedType`.
+- `assertNotInstanceOf()` - Make an assertion that `actual` is not an instance
+  of `expectedType`.
 - `assertStringIncludes()` - Make an assertion that `actual` includes
   `expected`.
 - `assertMatch()` - Make an assertion that `actual` match RegExp `expected`.

testing/asserts.ts

@@ -374,6 +374,18 @@ export function assertInstanceOf<T extends AnyConstructor>(
   assert(actual instanceof expectedType, msg);
 }
 
+/**
+ * Make an assertion that `obj` is not an instance of `type`.
+ * If so, then throw.
+ */
+export function assertNotInstanceOf<T extends AnyConstructor>(
+  actual: unknown,
+  unexpectedType: T,
+  msg = `Expected object to not be an instance of "${typeof unexpectedType}"`,
+) {
+  assertFalse(actual instanceof unexpectedType, msg);
+}
+
 /**
  * Make an assertion that actual is not null or undefined.
  * If not then throw.

testing/asserts_test.ts

@@ -11,6 +11,7 @@ import {
   assertIsError,
   assertMatch,
   assertNotEquals,
+  assertNotInstanceOf,
   assertNotMatch,
   assertNotStrictEquals,
   assertObjectMatch,
@@ -1424,6 +1425,15 @@ Deno.test({
   },
 });
 
+Deno.test({
+  name: "assertNotInstanceOf",
+  fn() {
+    assertNotInstanceOf("not a number", Number);
+    assertNotInstanceOf(42, String);
+    assertNotInstanceOf(new URL("http://example.com"), Boolean);
+  },
+});
+
 Deno.test({
   name: "assert* functions with specified type parameter",
   fn(): void {