unable to get property 'getEnumerator' of undefined or null reference error

Question

When i try to retrieve list items in sharepoint list using javascript i get the error my code

<script src="/_layouts/MicrosoftAjax.js" type="text/javascript"></script>
<script type="text/javascript" src="//code.jquery.com/jquery-1.11.0.min.js"></script>
<script type='text/javascript' src='https://www.google.com/jsapi'></script>
<script type="text/javascript">
    google.load('visualization', '1', {packages:['orgchart']});
    google.setOnLoadCallback(onQuerySucceeded);
    var hostweburl = "http://zadbns004/";
    $(document).ready(function () {
        //hostweburl = "http://spdev:52001";
        // The js files are in a URL in the form:
        // web_url/_layouts/15/resource_file
        var scriptbase = hostweburl + "/_layouts/15/";
        // Load the js files and continue to
        // the execOperation function.
        $.getScript(scriptbase + "SP.Runtime.js",
            function () {
                $.getScript(scriptbase + "SP.js", execOperation);
            }
        );
    });
    // Function to execute basic operations.
    function execOperation() {
        retrieveListItems(hostweburl);
    }    

    function retrieveListItems(siteUrl) {
        var input = document.getItemById('siteDrop').value;
        var clientContext = new SP.ClientContext(siteUrl);
        var oList = clientContext.get_web().get_lists().getByTitle('Employees');            
        var camlQuery = new SP.CamlQuery();
        camlQuery.set_viewXml(
            '<View><Query><Where><Geq><FieldRef Name=\'Site\'><Value Type=\'Text\'>'+input+'</Value></Geq></Where></Query></View>'
        );
        this.collListItem = oList.getItems(camlQuery);            
        clientContext.load(collListItem);
        clientContext.executeQueryAsync(
            Function.createDelegate(this, this.onQuerySucceeded), 
            Function.createDelegate(this, this.onQueryFailed)
        ); 
    }

    function onQuerySucceeded(sender, args) { 

        var data = new google.visualization.DataTable();
        data.addColumn('string', 'Name');
        data.addColumn('string', 'Manager');
        data.addColumn('string', 'ToolTip');
        //data.addRows([['Bob', '', 'CEO'],['Carol', 'Bob', 'MD']]);


        var listItemInfo = '';
        var listItemEnumerator = collListItem.getEnumerator();

        var _name = '';
        var _manager = '';
        var _jobtitle = '';
        while (listItemEnumerator.moveNext()) {
            var oListItem = listItemEnumerator.get_current();            
            _name = oListItem.get_item('EmpFullName');
            _manager = oListItem.get_item('EmpManager')
            _jobtitle = oListItem.get_item('EmpJobTitle');
            if(_manager){
            data.addRow([_name, _manager,_jobtitle]);}
            listItemInfo += _name + " " + _manager +'\n';
        }
        //alert(listItemInfo.toString());
        var chart = new google.visualization.OrgChart(document.getElementById('chart_div'));
        chart.draw(data, {allowHtml:true});        
    }
    function onQueryFailed(sender, args) {
        //alert('Request failed. ' + args.get_message() + 
        //    '\n' + args.get_stackTrace());
    }     
</script>
<h1>Organisational Chart</h1>
<div id='chart_div'></div>
<select id='siteDrop>
<option Value='Item 1'>Item 1</option>
<option Value='Item 2'>Item 2</option>
<option Value='Item 3'>Item 3</option>
<option Value='Item 4'>Item 4</option>
</select>
<input type="button" value="Select" onclick="execOperation()">

Answer 1

Try declaring collListItem outside the scope of the function and removing the 'this' keyword from the line:

this.collListItem = oList.getItems(camlQuery);

For example:

var collListItem;
function retrieveListItems(siteUrl) {
        var input = document.getItemById('siteDrop').value;
        var clientContext = new SP.ClientContext(siteUrl);
        var oList = clientContext.get_web().get_lists().getByTitle('Employees');            
        var camlQuery = new SP.CamlQuery();
        camlQuery.set_viewXml(
            '<View><Query><Where><Geq><FieldRef Name=\'Site\'><Value Type=\'Text\'>'+input+'</Value></Geq></Where></Query></View>'
        );
        collListItem = oList.getItems(camlQuery);            
        clientContext.load(collListItem);
        clientContext.executeQueryAsync(
            Function.createDelegate(this, this.onQuerySucceeded), 
            Function.createDelegate(this, this.onQueryFailed)
        ); 
    }