Javascript 'undefined error' in asp.net webform

I'm not a javascript expert, but I've used it before, and this problem is perplexing me. Here's the javascript:

function formatNumThousands(myNumber) {
    if (myNumber.length > 0) {
        var index = myNumber.indexOf(',');
        while (index != -1) {
             myNumber = myNumber.replace(',', '');
             index = myNumber.indexOf(',');
        }
        var myResult = '';
        for (var i = myNumber.length - 1; i >= 0; i--) {
            myResult = myNumber[开发者_Go百科i] + myResult;
            if ((myNumber.length - i) % 3 == 0 & i > 0) myResult = ',' + myResult;
        }
        document.getElementById('<%=txtMyNumber.ClientID%>').value = myResult;
     }
  }

In the code behind, I set the attribute for this text box's OnBlur event:

this.txtMyNumber.Attributes.Add("onblur", "formatNumThousands(this.value);");

I build and publish the app and it runs fine on my machine. Two other users get 'undefined' in the text box, regardless of what they do.

---

Try this trimmed version (see in jsfiddle). See that I replaced the [] operator for the charAt() method, as it's a more conservative approach for extracting individual characters from a string:

function formatNumThousands(editElement) {
    if (!editElement || editElement.value.length == 0)
        return;

    var myNumber = editElement.value.replace(/,/g, '');

    var myResult = '';
    for (var i = myNumber.length - 1; i >= 0; i--) {
        myResult = myNumber.charAt(i) + myResult;
        if ((myNumber.length - i) % 3 == 0 & i > 0)
            myResult = ',' + myResult;
    }

    editElement.value = myResult;
}

Don't forget to change the code behind to

// Notice that `.value´ is gone:
this.txtMyNumber.Attributes.Add("onblur", "formatNumThousands(this)");