[SalesForce] Get selected value of picklist in javascript

There is a hidden field rendered in the page which corresponds to the lookup field. This hidden field contains the ID of the lookup value after the selection is made in the lookup dialog.

It is named the same as the lookup field's (visible) textbox and is suffixed with _lkid.

var SalesOrgText = document.getElementById('{!$Component.lookupfield}').value;
var SalesOrgId = document.getElementById('{!$Component.lookupfield}_lkid').value;

console.log('SelectedSalesText: ' + SalesOrgText);
console.log('SelectedSalesOrgId: ' + SalesOrgId);

Given a page like this:

<apex:page standardcontroller="Contact" id="thePage">
    <apex:form id="theForm">
        <apex:inputField id="theInput" value="{!Contact.AccountId}"/>
    </apex:form>
</apex:page>

An input:text field will be rendered with the id: thePage:theForm:theInput which will contain the name of the record which was selected.

As well as an input:hidden field with the id: thePage:theForm:theInput_lkid which will store the selected record's id value.

You could wrap a script in an outputPanel and rerender it on every save. Then you'd have the most up-to-date value of the variable. eg:

<apex:outputPanel id="saveResultScript">
    <script>
    function confirmProceed (){
        var dataSaved = "{!JSENCODE(dataSaved)}";
        if(dataSaved == 'false' && !isDirty) {
            if(confirm('Are you sure you wish to continue - any unsaved changes will be lost.')) {
            }
        }else {
            return false;
        }
    }
    </script>
    </apex:outputText>
</apex:outputPanel>

Outside this block we have another script tag which we'll put the isDirty variable (so it doesn't get rerendered on save)

<script>
var isDirty = false;
</script>

As long as this entire panel is rerendered on every save, we can check the newly updated dataSaved variable and the locally updated isDirty variable (you said you had a variable that performs this function on any onenter event)

This way, we don't need to talk to the controller for confirmProceed to work. Hopefully this helps a little. I feel a little unsure of your requirements, but this is something that I have done several times myself and it works pretty well.

Oh and using a RemoteAction as suggested in the comments would probably work too, as long as you keep in mind that RemoteAction methods need to be static. They definitely return a value immediately for you to act on.

Update One last thought - you could use a RemoteAction to save your data and then (depending on the result) call an actionFunction to update the dataSaved var on the controller non-statically - while you can do what you want without waiting for a rerender - this should be very performant.