Document.createElement arity

Question

This is related to but not exactly the same as this question. I recently had some code that broke on several browsers because it wrapped document.createElement with a decorator that curried the built-in function.

Opening up the chrome console and typing document.createElement.length yields 1, in FF/Safari, 2.

Why? Is it the optional type extension in mentioned in the other question (which oddly enough is a chrom(ium) thing for webcomponents' custom elements)? What does the standard say about this?

UPDATE

It is indeed (at least in FF) related to the typeExtension, document.createElement.toString() in the console returns

function createElement(tag, typeExtension) {
    if (tag) {
      tag = tag.toLowerCase();
    }
    if (typeExtension) {
      typeExtension = typeExtension.toLowerCase();
    }
    var definition = getRegisteredDefinition(typeExtension || tag);
    if (definition) {
      if (tag == definition.tag && typeExtension == definition.is) {
        return new definition.ctor();
      }
      if (!typeExtension && !definition.is) {
        return new definition.ctor();
      }
    }
    var element;
    if (typeExtension) {
      element = createElement(tag);
      element.setAttribute("is", typeExtension);
      return element;
    }
    element = domCreateElement(tag);
    if (tag.indexOf("-") >= 0) {
      implementPrototype(element, HTMLElement);
    }
    return element;
  }

##UPDATE 2 github issue - see Jeremy's answer.

Answer 1

The source code you've quoted is from the Polymer team's Web Components polyfill , not the native browser implementation. If you try it in a page that doesn't include the polyfill, you'll get different results (Firefox on Ubuntu):

> document.createElement.length
1

> document.createElement.toString()
"function createElement() {
    [native code]
}"

The reason you get 1 in Chrome is because it currently includes a native implementation of Web Components (which still has .length of 1), so the polyfill isn't being used.

The built-in function doesn't yet have .length of 2 in any browsers. However, the specification of createElement is going to change soon, so don't rely on this remaining true.