I hope you can help me with a js-php Tutorial.

A js question. I hope you can help me find a layer called & lt; divclass & quot; demo & quot; & gt;. he wants to use js to put class & quot; demo & quot; replace it with id & quot; demo & quot; But varstr & quot; how can I write it here? ------ Solution -------------------- first find the corresponding js problem and hope you can help
I have a layer called

He wants to use js to replace class = "demo" with id = "demo"

But var str = "" How can I write it here?

------ Solution --------------------
Find the object, obj. id = 'demo'; obj. className = '';
------ Solution --------------------
You already have an id, right?

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To change