Let registry be the current global object's
CustomElementsRegistry object.
If NewTarget is equal to the active function object, then throw a
+ TypeError and abort these steps.
This can occur when a custom element is defined using an element interface as + its constructor:
+ +customElements.define("bad-1", HTMLButtonElement);
+new HTMLButtonElement(); // (1)
+document.createElement("bad-1"); // (2)
+
+ In this case, during the execution of HTMLButtonElement (either explicitly, as
+ in (1), or implicitly, as in (2)), both the active function object and NewTarget
+ are HTMLButtonElement. If this check was not present, it would be possible to
+ create an instance of HTMLButtonElement whose local name was bad-1.
Let definition be the entry in registry with constructor equal to NewTarget. If @@ -9757,10 +9777,10 @@ interface HTMLUnknownElement : HTMLElement { };
This can occur when a custom element is defined to not extend any local names, but
inherits from a non-HTMLElement class:
customElements.define("bad-1", class Bad1 extends HTMLParagraphElement {});
+ customElements.define("bad-2", class Bad2 extends HTMLParagraphElement {});
In this case, during the (implicit) super() call that occurs when
- constructing an instance of Bad1, the active function
+ constructing an instance of Bad2, the active function
object is HTMLParagraphElement, not HTMLElement.
This can occur when a custom element is defined to extend a given local name but inherits from the wrong class:
-customElements.define("bad-2", class Bad2 extends HTMLQuoteElement {}, { extends: "p" });
+ customElements.define("bad-3", class Bad3 extends HTMLQuoteElement {}, { extends: "p" });
In this case, during the (implicit) super() call that occurs when
- constructing an instance of Bad2, valid local names is the
+ constructing an instance of Bad3, valid local names is the
list containing q and blockquote, but definition's local name is p,
which is not in that list.
If IsConstructor(constructor) is false, then throw a
TypeError and abort these steps.
If constructor is either an interface object or named
- constructor, whose corresponding interface either is HTMLElement or has
- HTMLElement in its set of inherited interfaces, throw a
- TypeError and abort these steps.
This prevents passing, for example, HTMLButtonElement as
- constructor, and thus enabling the creation of HTMLButtonElement instances with a local name that is not button.
Author-defined custom element constructors,
- created for example via class extends HTMLElement {} or class extends HTMLButtonElement {}, will pass this step without throwing a
- TypeError, since they are not interface
- objects.
Interface objects that do not inherit from
- HTMLElement (for example TextTrack), or classes
- defined in the JavaScript specification (such as Set), will pass this
- step without throwing a TypeError. However, they will later cause create an
- element to fail.
If name is not a valid custom element name, then throw a
"SyntaxError" DOMException and abort these steps.