Insert Substring Function

I'm working on a function that will insert tags into my textbox. I've modified a script I found online to get it to work almost perfectly. Here's the function:

function insertAtCursor(obj, text1, text2) { 
    if(obj.selectionStart) 
    { 
        var start = obj.selectionStart; 
        var end   = obj.selectionEnd; 
        if (start == end)
        {
           obj.value = obj.value.substr(0, start) 
           + text1 + text2
           + obj.value.substr(end, obj.value.length);    
        }
        else 
        {
           obj.focus();
           obj.value = obj.value.substr(0, end) + text2 + obj.value.substr(end, obj.value.length);
           obj.value = obj.value.substr(0,start) + text1 + obj.value.substr(start, obj.value.length);
        } 
    }
} 

Here's an example of the parameters:

<a href="#" onClick="insertAtCursor(document.postform.inputfield, '<div id=h1>','</div>')">Header</a>

The function works so that it either inserts "<div id=h1></div>" or those tags around a highlighted substring. The problem is, it won't insert at the 0th position. Meaning to get it to work, I have to insert one character or a space before I click the link to insert the tag. This happens with either of the two methods I described. Is this some sort of off by one error or a kind of bug?

why not try this:

obj.value += text1 + text2 + obj.value.substr(end, obj.value.length);

also note, I really don't see a problem with the code above, if anything the substr is giving you a pain in the balls because it has no data to manipulate

oh sorry.. edit #2.. scratch the above.. just add in a clause..

  if (obj.value.length) {
    obj.value += text1+text2;
  } else {
    //do ur code
  }

That'll be because the .selectionStart property equals 0. Your IF expression evaluates to false because 0 == false. An easy way in your situation to get around the problem would be to just check if the property is defined, as opposed to true:

if (obj.selectionStart != undefined)

Awesome, that was it. I love when a solution comes down to just one simple error. Thanks a lot for the help guys. This one has been bugging me for half a week.