Expando - MDN Web Docs Glossary: Definitions of Web-related terms 编辑

Expando properties are properties added to DOM nodes with JavaScript, where those properties are not part of the 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 Array.