JavaScript DOM coding - 'undefined' error

Question

I am having problem with this error:

'undefined' is null or not an object'

Can you please have a look and let me know. In my coding, I want to have simple DOM JavaScript code

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>Untitled Document</title>
</head>
<script>
    init();
    function init()
    {
        getElementByTabIndex("4", "submit")[0].addEventListener("click", Verify, false);
    }
    function Verify() {
        alert('done');  
        // all verification code will be here...
    }
    function getElementByTabIndex(index, type, node)
    {
        if (!node)
        { 
            node = document.getElementsByTagName('body')[0]; 
        } 

        var a = [];
        els = node.getElementsByTagName('*'); 
        for (var i = 0, j = els.length; i < j; i++) 
        { 
            if (els[i].tabIndex == index && els[i].type == type)
            { 
                a.push(els[i]);
            } 
        } 
    return a; 
}

</script>
<body>
<input type="email" id="email" /><input type="password" id="pass" /> <label class="Login" for="login"><input value="Log In" tabindex="4" type="submit" id="login"></label>
</body>
</html>

When do you get the error? How do you reproduce it? On what line does it occur? — Lightness Races in Orbit
– Lightness Races in Orbit, Commented Aug 12, 2011 at 17:55
LINE 24 : getElementByTabIndex("4", "submit") 0].addEventListener("click", Verify, false); In IE, you can see JavaScript error in status bar or in Safari - the error show as 'TypeError: 'undefined' is not an object (evaluating 'node.getElementsByTagName')' in line 24. — Winthan Aung
– Winthan Aung, Commented Aug 12, 2011 at 17:56
it looks fine to me. I tested it in google Chrome (jsfiddle.net/DavidLaberge/gdgFS/2). What browser are you using? — David Laberge
– David Laberge, Commented Aug 12, 2011 at 17:58
Firefox and Google Chrome works i guess. but in Safari and IE is giving me that errors. — Winthan Aung
– Winthan Aung, Commented Aug 12, 2011 at 18:00

Praveen Prasad · Accepted Answer · 2011-08-12 18:00:34Z

3

You have to move you code at bottom or call init() after body is loaded.

Reason: you are trying to get elements even before they exists.

Eg :

<head>
   <script>
      var elm= document.getElementById('id');

      //this will be always undefied, as trying to read element even before they exist
   </script>
</head>

<body>
  <div id='foo'></div>

   <script>
      var elm= document.getElementById('id');

      //this wont be undefined 
   </script>

</body>

answered Aug 12, 2011 at 18:00

Praveen Prasad

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1 Comment

Winthan Aung Over a year ago

Yes, thanks it works in simple HTML, i am trying to use in my extension now. I will let you know how it comes out in extension.

Lightness Races in Orbit · Accepted Answer · 2011-08-12 17:59:14Z

0

You call:

if (!node) { 
    node = document.getElementsByTagName('body')[0]; 
}

But your script runs before the DOM has finished loading, and so the body tag does not exist.

So node is undefined, and when you attempt the following, you get your error:

node.getElementsByTagName('*');

Run init() on document load, instead of immediately.

^{PS. jsfiddle and Firebug allowed me to debug this very quickly.}

answered Aug 12, 2011 at 17:59

Lightness Races in Orbit

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Comments

mhe · Accepted Answer · 2011-08-12 18:15:11Z

0

'body' isn't available to javascript at the time you are trying to call init().

call your init method when the dom has finished loading, like so:

window.onload = function (){
    init();
}

note that in order to make this work across browsers (if you plan on using it outside your planned Safari extention) you will have to do some extra work. more info: http://www.javascriptkit.com/dhtmltutors/domready.shtml

edited Aug 12, 2011 at 18:15

answered Aug 12, 2011 at 18:06

mhe

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