search-replace-param.js

        function replaceUrlParam(url, paramName, paramValue){
    if(paramValue == null)
        paramValue = '';
    var finalurl = null;
    var pattern = new RegExp('\\b('+paramName+'=).*?(&|$)');
    if(url.search(pattern)>=0){
        
        if(paramValue == ''){
            finalurl = url.replace(pattern,'$2');
            if(finalurl.substr(finalurl.length - 1) == '?'){
                finalurl = finalurl.substring(0, finalurl.length - 1);
            }
            
        }else{
            finalurl = url.replace(pattern,'$1' + paramValue + '$2');
        }
        return finalurl;
        /*
        if(paramValue != '')
            return url.replace(pattern,'$1' + paramValue + '$2');
        else
            return url.replace(pattern,'$1' + paramValue + '$2');
        */
    }

    return url + (url.indexOf('?')>0 ? '&' : '?') + paramName + '=' + paramValue 
}

useage:
//Sample url: http://example.com/abc/?ref=xxx
var laturl = replaceUrlParam(window.location.href,'lat',position.coords.latitude); //url:http://example.com/abc/?ref=xxx&lat=yyy
var lonurl = replaceUrlParam(laturl,'lon',position.coords.longitude); //url:http://example.com/abc/?ref=xxx&lat=yyy&lon=zzz
window.location =lonurl;