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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.

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 Contributors to this page: wbamberg, klez, MartinRinehart, Andrew_Pfeiffer, Biraj
 Last updated by: wbamberg,