Javascript: Warn If Overflow

I recently came across a requirement to provide a warning to the user if the text entered in a text box exceeds the display area in a DIV, and found and old script I cooked up for it. An issue with setting the maximum length for the text box is that the 'W' and 'i' characters don't occupy the same space width variable-width fonts. I though it would be possible to measure the width of the text when the text is placed within a container using the offsetWidth property and I wrote up a little function:

function getSize() {
var divNitinz = document.getElementById('divNitinz');
var txtName = document.getElementById('txtName');
divNitinz.style.display = 'inline';
divNitinz.innerHTML = txtName.value;
var txtWidth = divNitinz.offsetWidth;
divNitinz.style.display = 'none';
return txtWidth;
}

I then created another function to get this value and compare it with the DIV in which I wanted to display the text and that looks like this:

function checkSize() {
var divDisplay = document.getElementById('divDisplay');
var txtName = document.getElementById('txtName');
if (divDisplay.offsetWidth <>
alert('Text overflow');
divDisplay.innerHTML = 'Text width: ' + getSize() + ' Display width: ' + divDisplay.offsetWidth;
} else {
divDisplay.innerHTML = txtName.value;
window.status = 'Text width: ' + getSize() + ' Display width: ' + divDisplay.offsetWidth;
}
}

My HTML is pretty simple:

    <form>

  <input type="text" name="txtName" id="txtName" onkeyup="checkSize();" />
  <input type="text" name="txtName" id="txtName" onkeyup="checkSize();" />

   <span id="divNitinz" style="display: none;"></span>
  <div id="divDisplay" style="width: 120px; background-color: #FFFF99;"></div>
  <input type="button" onclick="checkSize();" value="check" name="btnCheck" />
</form>

You've probably noticed the setTimeout that I've defined on the onkeydown event. I use it to get the text after the key has taken effect. I tried the onkeypress event before using onkeydown and the difference is the keypress event doesn't occur when you press backspace while the keydown event does, which makes it the better of the two.

If you've got any enhancements to this script or a better way to do detect overflow, let me know and I'll post it up here.