Fix undefined target element does not throw

When `assert.dom(undefined)` is called, it
incorrectly defaults to the root element instead
of throwing `TypeError: Unexpected Parameter:
undefined`. This means code like
`assert.dom(findAll('.foo')[0]).exists()` silently
passes even when `.foo` doesn't exist (because
`findAll` returns an empty array, and `array[0]`
is `undefined`).

PR#2107 changed the argument detection logic from
`arguments.length === 0` to `target !== undefined`,
which cannot distinguish between "no argument" and
passing "undefined".

This behavior was not caught by tests because
tests only used mock implementation of assert.dom.

This commit restores the original behavior that
correctly distinguishes passing no arguments and
passing undefined and adds a test to ensure
passing undefined throws an error as expected.

Addresses:
#2186

Author: eliasdawson

packages/qunit-dom/lib/assertions.ts

@@ -54,7 +54,7 @@ export default class DOMAssertions {
    * @hideconstructor
    */
   constructor(
-    target: string | Element | null | IDOMElementDescriptor,
+    target: string | Element | null | IDOMElementDescriptor | undefined,
     rootElement: RootElement,
     private testContext: Assert
   ) {

packages/qunit-dom/lib/descriptor.ts

@@ -47,7 +47,7 @@ class ElementData {
  * Create an {@link IDOMElementDescriptor} from a target and a root element
  */
 export default function createDescriptor(
-  target: string | Element | null | IDOMElementDescriptor,
+  target: string | Element | null | IDOMElementDescriptor | undefined,
   rootElement: RootElement
 ): IDOMElementDescriptor {
   if (typeof target === 'string') {

packages/qunit-dom/lib/install.ts

@@ -25,11 +25,13 @@ export default function (assert: Assert) {
 
     rootElement = rootElement || this.dom.rootElement || getRootElement();
 
-    return new DOMAssertions(
-      target !== undefined ? target : rootElement instanceof Element ? rootElement : null,
-      rootElement,
-      this
-    );
+    // Only default to rootElement when NO arguments provided.
+    // This allows assert.dom(undefined) to throw while assert.dom() still works.
+    if (arguments.length === 0) {
+      target = rootElement instanceof Element ? rootElement : null;
+    }
+
+    return new DOMAssertions(target, rootElement, this);
   };
 
   function isValidRootElement(element: any): element is Element {

packages/test-app/tests/acceptance/qunit-dom-test.js

@@ -51,5 +51,11 @@ module('Acceptance | qunit-dom', function (hooks) {
       () => assert.dom('foo', 'bar'),
       /bar is not a valid root element/,
     );
+
+    assert.throws(
+      () => assert.dom(undefined),
+      /Unexpected Parameter: undefined/,
+      'assert.dom(undefined) throws',
+    );
   });
 });