Question

There is a HTML5 conformance test suite with a test for prototype for HTMLCanvasElement.

This test fails for Safari, Firefox but passes for Opera on Windows 7.

The test has a script which tries to delete the getContext property of the HTMLCanvasElement, and further trying to read getContext should give undefined.

delete window.HTMLCanvasElement.prototype.getContext;
_assertSame(window.HTMLCanvasElement.prototype.getContext, undefined, "window.HTMLCanvasElement.prototype.getContext", "undefined");

This test fails for WebKit (Safari) because the getContext property has DontDelete attribute and so it does not allow the script to delete this property.

Is there any description in the HTML5 specification which says deletion of getContext property by script is valid?

Answer 1

From what I understand, the configurability ([[DontDelete]] in ES3, [[Configurable]] in ES5) of getContext method is described in WebIDL — as any other CanvasRenderingContext2D methods.

Take a look at "Interface Prototype Object" section, which says:

There must exist an interface prototype object for every interface defined, regardless of whether the interface was declared with the [NoInterfaceObject] extended attribute. The interface prototype object for a particular interface has properties that correspond to the attributes and operations defined on that interface. These properties are described in more detail in sections 4.5.5 and 4.5.6 below.

And in 4.5.6, you can see:

For each unique identifier of an operation defined on the interface, there must be a corresponding property on the interface prototype object (if it is a regular operation) or the interface object (if it is a static operation), unless the effective overload set for that identifier and operation and with an argument count of 0 (for the ECMAScript language binding) has no entries.

The characteristics of such a corresponding property are as follows:

The name of the property is the identifier.

The property has attributes { [[Writable]]: true, [[Enumerable]]: true, [[Configurable]]: true }.

Note the "[[Configurable]]: true" bit (emphasis mine).