Question

I was trying to delete .insertBefore(); of HTMLElement.prototype but when I check for it, instead of an undefined value, it returns the actual function? Why?

console.log(HTMLElement.prototype.insertBefore); // function insertBefore() { [native code] }

delete HTMLElement.prototype.insertBefore;

console.log(HTMLElement.prototype.insertBefore); // function insertBefore() { [native code] }

Can you explain why this isn’t working. I tried this on my own constructor and it worked, but for the built in ones, it didn’t.

Answer 1

You cannot use DELETE like that:

http://perfectionkills.com/understanding-delete/

Property attributes

Every property can have zero or more attributes from the following set — ReadOnly, DontEnum, DontDelete and Internal. You can think of them as flags — an attribute can either exist on a property or not. For the purposes of today’s discussion, we are only interested in DontDelete.

Built-ins and DontDelete

So this is what it’s all about: a special attribute on a property that controls whether this property can be deleted or not. Note that some of the properties of built-in objects are specified to have DontDelete, and so can not be deleted. Special arguments variable (or, as we know now, a property of Activation object) has DontDelete. length property of any function instance has DontDelete as well.

Seriously man, read that article. It has a ludicrously clear and concise explanation of all this!

---

Note: even though they have the dontDelete attribute set, you can still overwrite it:

HTMLElement.prototype.insertBefore = '';
// or
HTMLElement.prototype.insertBefore = null;
// or
HTMLElement.prototype.insertBefore = undefined;