<div id='container'>50</div>

var a = document.getElementById('container').value;
var b = document.getElementById('container1').value;

var element = document.getElementById('container1');
if (element != undefined) //note: this can be shortened to just "if (element)"
{
    alert(element.value);
}

var a=null;
var b=null;
var o = document.getElementById('container');
if (o) a=o.value;
o = document.getElementById('container1');
if (o) b=o.value;

var a = document.getElementById('container').value;
var b = document.getElementById('container1');
if ( b > 0  ) {
    b.value;
}