JavaScript: How can I get / change an attribute within the Placeholder="{" id ":" 10 "," selected ": false}"

Hi People .

First of all, I did not even know what to put these attributes in. of the Placeholder, hehehe, had never even seen it.

<div class="cellContent" placeholder="{"id":"103","selected":false}"></div>  

If possible any of you can send me some link where they have some explanation about this feature ... because I did not find anything about these specific attributes, but most importantly, and if you guys know how to change them, that doubt has arisen in me.

Something like this: change the property (selected) or check which the id it contains.

Tamo together.