From 4255771f5c68921ebfa9478f7b1236911c8a8841 Mon Sep 17 00:00:00 2001
From: Robert Sesek <rsesek@bluestatic.org>
Date: Sun, 19 Jul 2020 17:58:02 -0400
Subject: [PATCH] Have isFormT match on derived form types by walking the
 prototype chain.

---
 src/core/Form.test.ts | 19 +++++++++++++++++++
 src/core/Form.ts      |  6 +++++-
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/src/core/Form.test.ts b/src/core/Form.test.ts
index 4178d19..5b00536 100644
--- a/src/core/Form.test.ts
+++ b/src/core/Form.test.ts
@@ -92,3 +92,22 @@ test('form types', () => {
   expect(isFormT(new FormA(), FormB)).toBe(false);
   expect(isFormT(new FormB(), FormB)).toBe(true);
 });
+
+test('derived form types', () => {
+  class Base extends Form<any> {
+    readonly name = 'Base';
+    readonly lines = {};
+  };
+  class Derived extends Base {};
+  class SecondDerived extends Derived {};
+
+  expect(isFormT(new Base(), Base)).toBe(true);
+  expect(isFormT(new Derived(), Derived)).toBe(true);
+
+  expect(isFormT(new Derived(), Base)).toBe(true);
+  expect(isFormT(new Base(), Derived)).toBe(false);
+
+  expect(isFormT(new SecondDerived(), SecondDerived)).toBe(true);
+  expect(isFormT(new SecondDerived(), Derived)).toBe(true);
+  expect(isFormT(new SecondDerived(), Base)).toBe(true);
+});
diff --git a/src/core/Form.ts b/src/core/Form.ts
index dc7e70a..6aaece8 100644
--- a/src/core/Form.ts
+++ b/src/core/Form.ts
@@ -66,5 +66,9 @@ export type FormClass<T extends Form> = new (...args: any[]) => T;
 export function isFormT<T extends Form>(form: Form,
                                         formClass: FormClass<T>):
                                             form is T {
-  return form.constructor === formClass;
+  for (let proto = form; proto !== null; proto = Object.getPrototypeOf(proto)) {
+    if (proto.constructor === formClass)
+      return true;
+  }
+  return false;
 }
-- 
2.22.5