Expando

{{GlossarySidebar}} 

Expando properties are properties added to {{glossary("DOM")}}  nodes with {{glossary("JavaScript")}} , where those properties are not part of the {{glossary("object","object's")}}  DOM specification:

window.document.foo = 5; // foo is an expando

The term may also be applied to properties added to objects without respecting the object’s original intent, such as non-numeric named properties added to an {{glossary("Array")}} .