How to pass Apex controller variable value in Javascript after ActionFunction Call?

Below is my VF page (only imp content given)

<apex:page showheader="false" sidebar="false" standardStylesheets="false" id="page" controller="BAR_ABNCheck_CC" action="{!initDetailPage}">

        <apex:outputPanel id="tempPanel1">                      
            <script>
            function saveMyDetails(){
                //getFileMap();        
                var myJson = $('form').serializeObjectCustom();
                for (var key in myJson) {
                    if (myJson.hasOwnProperty(key)) {                               
                        if (typeof myJson[key] == 'object') {
                            for (var arrayKey in myJson[key]) {
                                if (myJson[key].hasOwnProperty(arrayKey)) {
                                    myJson[key][arrayKey] = replaceBlankValues(myJson[key][arrayKey]);  
                                }
                            }    
                        } else {
                            myJson[key] = replaceBlankValues(myJson[key]);                                    
                        }                
                    }
                }

                var barJson = JSON.stringify(myJson);
                showSpinner();
                //alert(newBarId);
                saveDetailsfunction(barJson);
                var attachId = **'{!saveDetailId}'**;
                **alert(attachId);**
                attachFilesToBAR(attachId,false); <!-- javascript method to upload files from page
            }
            </script>
        </apex:outputPanel>

Below is my Apex controller

public class BAR_ABNCheck_CC {
       public String barJson {get; set;}
       public String saveDetailId {get; set;}

public PageReference saveMyDetails(){
Id recTypeId = BAR_Utilities.getRecordTypeId('Opportunity', 'Enterprise_Business_BAR_EPR');
String accId = ApexPages.currentPage().getParameters().get('accountId') ;
String oppId = ApexPages.currentPage().getParameters().get('oppId');
List<Opportunity> incompleteOpps = [Select Id, BAR_isDetailsPageCompleted__c, BAR_ApplicationStatus__c 
                                    From Opportunity 
                                    Where Account.Id = :accId AND RecordTypeId =: recTypeId 
                                                              AND OwnerId =: UserInfo.getUserId() 
                                                              AND (BAR_ApplicationStatus__c != 'Completed' OR BAR_ApplicationStatus__c != '')];
System.debug('incompleteOpps '+incompleteOpps);                                                              
String OpportunityId;      
    //try {            
        if(String.isBlank(accId)) {
            Opportunity opp = [Select Id, Account.Id From Opportunity Where Id = :oppId];
            accId = opp.Account.Id;
            //throw new BAR_DetailsFormException('Account Id should not be null.');
        }         
        System.debug('barJson: ' +barJson);  
        BAR_FormWrapper form = BAR_FormWrapper.parse(barJson);

        Account acc = [Select id, Name, Dealer_Code__c, BAR_Account_PIN_code__c, BillingState, BillingStreet, BillingPostalCode, BillingCity, BillingCountry, BAR_Address_for_Email_Bill__c, TradingStartDate__c, BAR_Registered_Date__c
                                                                        From Account Where Id = :accId];
        if(incompleteOpps.isEmpty() && String.isBlank(oppId)){
            OpportunityId = updateOpportunityDetails(form, acc, null, false, recTypeId) ;
        } 
        else{
            OpportunityId = updateOpportunityDetails(form, acc, oppId, false, recTypeId) ;
        }                                                               

        // update billing details
        updateAccountDetails(form, acc);
    //} catch (Exception e){
        //return BAR_GlobalConstants.errorMsgPrefix + e.getMessage() +' '+ e.getStackTraceString();
    //}
    **saveDetailId = OpportunityId;
    System.debug('saveDetailId'+saveDetailId);**
    //PageReference pr = new PageReference('/'+OpportunityId);
    //pr.setRedirect(false); 
    //return pr;  
    return null;  
} 
}

I am getting value of 'saveDetailId ' in debug log but alert in javascript is blank