Expando Properties

From GreaseSpot

Expando Properties are attributes (of a DOM element in this case) which are not part of the standard definition.

Example: <div id="d1" test="true" onclick="myFunc();">...</div>

• Here the id attribute is an officially defined attribute, settable and readable from the DOM at any time. XPCNativeWrappers transparently give access to this property.

• The test attribute is a pure private invention. It is stored on the DOM object and can be read end changed through get/setAttribute calls. When using XPCNativeWrappers, the property is set on the wrapped object, not the underlying native DOM element.

• The onclick attribute is not part of the official DOM specification. However, it is mapped to an internal event handler setter for backward compatibility with the so called DOM level 0 model. XPCNativeWrapper does not support this mapping and ignores the onclick property for the underlying DOM element.

The term Expando Properties is refers to a global expando property (MSDN) on the document model of MS Internet Explorer since version 4.0.

The term seems to have a double meaning, as both the ad-hoc invented attribute name, as well as the wrapper functionality which some expando's have (e.g. onclick, which adds an event handler for the click event).

See also Examining innerHTML and custom attribute implementations.