Greasemonkey Undefined JS Errors

I am getting undefined errors on most of the lines in this greasemonkey script. I've tested on Chrome and Firefox, with the same results on both. I can't quite figure out why. I'm somewhat new to JavaScript but I feel that this is pretty basic and I'm just missing something.

After looking at some related pages, I'm not sure that this Greasemonkey related. It's very likely that it's a generic JavaScript problem. Well here's the script in its entirety.

// ==UserScript==
// @name AutoHotkey Forum Line Numberer
// @namespace http://apps.aboutscript.com/gm/
// @description Add line numbers to AutoHotkey forum code boxes
// @ico开发者_如何学Pythonn http://apps.aboutscript.com/gm/linenumbers/autohotkey.png
// @include http://www.autohotkey.com/forum/viewtopic.php*
// @include http://www.autohotkey.com/forum/posting.php
// @version 0.5
// ==/UserScript==

var linenumbers = {

    count_lines: function(text) {
        var lines = text.split('\n');
        return (lines.length);
    },

    make_div: function(max) {
        var contents = "";
        for (i=1; i<=max; i++) {
            contents += '<span style="margin:0 auto 0 auto; ';
            if (i%2) {
                contents += 'color:#FFAAAA; '
            } else {
                contents += 'color:#FFCCCC; '
            }
            contents += '">' + i + '</span><br>';
        }
        if (max <= 15) {
            contents = '<div style="min-width:25px; height:200px; overflow-    y:auto;">' + contents + '</div>';
        } else {
            contents = '<div>' + contents + '</div>';
        }
        return contents;
    },

    add_numbers: function() {
        var code_tables = document.getElementsByClassName('code');
        for (i in code_tables) {
            var td = code_tables[i];
            var div = td.firstChild;
            var codetext = div.innerText;
            var total_count = this.count_lines(codetext);
            var to_inject = make_div(total_count);
            var newtd = document.createElement('td');
            newtd.innerHTML = to_inject;
            document.body.insertBefore(newtd, td);
        }
    }
};

linenumbers.add_numbers();

So my questions are: What is the problem? How can I fix it? How can I prevent it in the future?

Thanks,

Frankie (parse)

By the way, I think the 'greasemonkey' tag applies and I'm surprised it doesn't exist. If you have 1500+ rep, please add it.

var to_inject = make_div(total_count); You forgot this. var to_inject = this.make_div(total_count);

And you also didn't declare i :

for (i in code_tables) { => for (var i in code_tables) {

for (i=1; i<=max; i++) { => for (var i=1; i<=max; i++) {

so when add_numbers will call make_div, it'll fait because they will both loop on the same i variable.