UPDATE:

The issue was the div was in a div that was hidden, when divs are hidden length, offset(), position() don't work properly.

// Original Post below.

This should be simple.

I'm trying to test if a div exsists on the page after an ajax .html() output

if($('#'+mydiv).开发者_运维知识库length != 1) {
   // do stuff
}

there is only one problem, if the div your searching for is not there when (document).ready fires then .length doesn't see it.

You could just use the size() method

if($('#'+mydiv).size() != 1) {
   // do stuff
}

This recounts the number of matching elements in the DOM.

If you are performing an async request, which I assume you are (async:true in JQuery) then you should be using size() in the success callback of your function, after the content has been added.

   $.ajax({
        url: requestUrl,
        type: 'POST',
        dataType: 'json',
        data: {},
        success: function(response) {
                $('#container').html(response.data);
                if($('someSearch').size() > 0)
                    alert('exists');
            }
    });

You could use something like:

$('#loader').load('url', {variable1:var1}, function () { 
    if($('#'+mydiv).length != 1) {
       // do stuff
    }
});

That should look for anything loaded after your load call is finished loading.

I run into the exact same problem with the ticker after page content has been loaded using ajax - my solution was to use the livequery plugin : http://github.com/brandonaaron/livequery/downloads. I then use it in the following way:

$('.ticker').livequery(function() {     
    if ($(this).length > 0) {
        var thisObj = $(this);
        if (thisObj.length > 1) {
            jQuery.each(thisObj, function() {                   
                systemObject.tickerExe($(this));
            });
        } else {    
            systemObject.tickerExe(thisObj);
        }
    }
});

I hope this helps.

Ensure your div isn't hidden when checking length. Usually, hidden div's can cause lots of problems.