CallBack Function Usage in Javascript in Hosted App

Main Function to call for CallBack

function checkDuplicates(projectCode, callback) {
    var url = appWebUrl + "/_api/SP.AppContextSite(@target)/web/lists/getbytitle('" + projectListName + "')/items?$select=ProjectCode&$filter=ProjectCode eq '" + projectCode + "'&@target='" + hostWebUrl + "'";
    var IsDuplicate = false;
    var executor = new SP.RequestExecutor(appWebUrl);
    executor.executeAsync({
        url: url,
        method: "GET",
        headers:
            {
            "accept": "application/json;odata=verbose"
        }, contentType: 'application/json',

        success: function (data)
        {
            var jsonObject = JSON.parse(data.body);
            var object = jsonObject.d.results
            if (object.length > 0)
            {
                IsDuplicate = true;
            }
            callback(IsDuplicate);
        },
        error: function (errorData) { console.log(errorData.message); }
    });
}

Way of calling the CallBack function 
 
checkDuplicates(enteredProjectCode, function (data)