Как указать на выбранный элемент

document.body.hideobj = function () { 
    this.style.display = "none"; 
} 
document.body.hideobj();

BODY CONTENT

HTMLElement.prototype.hideobj = function () { 
    this.style.display = "none"; 
} 
document.forms[0].hideobj();

BODY CONTENT 
<form>NE BODY CONTENT</form>

function hideobj() { 
    this.style.display = "none"; 
} 
hideobj.bind(document.forms[0])(); 
hideobj.call(document.forms[1]); 
hideobj.apply(document.forms[2]);

<form>Bind</form> 
<form>Call</form> 
<form>Apply</form>