Javascript DOM: Setting custom DOM element properties

Question

Is it ok to set custom properties on DOM elements, and rely on them persisting?

For example, given

 
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Answer 1

No.

Let's say 5 years ago you thought the property .contains was a good use-case for saying whether an element contained some special data.

So you wrote document.getElementById("someId").contains = true

Then you had checks like if (document.getElementByID("someId").contains)

Today that breaks because Node.prototype.contains is a method.

basically your code is not future safe.