Changing data content on an Object Tag in HTML

Answer 1

You can do it with setAttribute

document.getElementById("contentarea").setAttribute('data', 'newPage.html');

EDIT: It is also recommended that you use the window.onload to ensure that the DOM has loaded, otherwise you will not be able to access objects within it.

It could be something like this:

function changeData(newURL) {
    if(!document.getElementById("contentarea")) 
        return false;
    document.getElementById("contentarea").setAttribute('data', newURL);
}

window.onload = changeData;

You can read more about window.onload here