fix: isElement works on elements in an iframe

fixes #667

src/utilities-iselement.ts

@@ -2,5 +2,9 @@
  * Guard function that checks if provided `input` is an Element.
  */
 export function isElement(input: unknown): input is Element {
-  return input && input instanceof Element;
+  return (
+    typeof input === "object" &&
+    input !== null &&
+    (input as Element).nodeType === Node.ELEMENT_NODE
+  );
 }

test/utilities-is-element.spec.ts

@@ -0,0 +1,25 @@
+import { assert } from "chai";
+import { isElement } from "../src/utilities-iselement";
+
+describe("utilities - isElement", () => {
+  it("should identify non-element", () => {
+    assert.isFalse(isElement(null));
+    assert.isFalse(isElement(undefined));
+    assert.isFalse(isElement(""));
+    assert.isFalse(isElement(0));
+    assert.isFalse(isElement({}));
+    assert.isFalse(isElement([]));
+    assert.isFalse(isElement(() => {}));
+  });
+  it("should identify a valid element", () => {
+    const element = document.createElement("div");
+    assert.isTrue(isElement(element));
+  });
+  it("should identify a valid element inside an iframe", () => {
+    const iframe = document.createElement("iframe");
+    document.body.appendChild(iframe);
+    const element = iframe.contentDocument.createElement("div");
+    assert.isTrue(isElement(element));
+    document.body.removeChild(iframe);
+  });
+});