So I am wanting to replace GET variable values in a url and if the variable does not exist, then add it to the url.

EDIT: I am doing this to a elements href not the pages current location..

I am not good with javascript but I do know how to use jQuery quite well and the basics of javascript. I do know how to write regex but not how to use the javascript syntax of regex and what functions to use it with.

Here is what I have so far and it does have an error on line 3: See it on jsfiddle(or below): /

function addParameter(url, param, value) {
    var pattern = new RegExp(param + '=(.*?);', 'gi');
    return url.replace(pattern, param + '=' + value + ';');

    alert(url);
}

So I am wanting to replace GET variable values in a url and if the variable does not exist, then add it to the url.

EDIT: I am doing this to a elements href not the pages current location..

I am not good with javascript but I do know how to use jQuery quite well and the basics of javascript. I do know how to write regex but not how to use the javascript syntax of regex and what functions to use it with.

Here is what I have so far and it does have an error on line 3: See it on jsfiddle(or below): http://jsfiddle.net/MadLittleMods/C93mD/

function addParameter(url, param, value) {
    var pattern = new RegExp(param + '=(.*?);', 'gi');
    return url.replace(pattern, param + '=' + value + ';');

    alert(url);
}

• javascript
• variables
• replace
• get
• query-string

• 1 Note: As soon as you change the page's URL the browser will navigate to that location, so you can only change it once. – Diodeus - James MacFarlane Commented Oct 3, 2011 at 20:32
• 1 I can change it as many times as I want. I am editing a elements href not the page. – MLM Commented Oct 4, 2011 at 0:39

4 Answers 4

No need to use jQuery on this one. Regular Expressions and string functions are sufficient. See my commented code below:

function addParameter(url, param, value) {
    // Using a positive lookahead (?=\=) to find the
    // given parameter, preceded by a ? or &, and followed
    // by a = with a value after than (using a non-greedy selector)
    // and then followed by a & or the end of the string
    var val = new RegExp('(\\?|\\&)' + param + '=.*?(?=(&|$))'),
        parts = url.toString().split('#'),
        url = parts[0],
        hash = parts[1]
        qstring = /\?.+$/,
        newURL = url;

    // Check if the parameter exists
    if (val.test(url))
    {
        // if it does, replace it, using the captured group
        // to determine & or ? at the beginning
        newURL = url.replace(val, '$1' + param + '=' + value);
    }
    else if (qstring.test(url))
    {
        // otherwise, if there is a query string at all
        // add the param to the end of it
        newURL = url + '&' + param + '=' + value;
    }
    else
    {
        // if there's no query string, add one
        newURL = url + '?' + param + '=' + value;
    }

    if (hash)
    {
        newURL += '#' + hash;
    }

    return newURL;
}

And here is the Fiddle

Update:

The code now handles the case where there is a hash on the URL.

Edit

Missed a case! The code now checks to see if there is a query string at all.

I would go with this small but complete library to handle urls in js:

https://github.com/Mikhus/jsurl

See Change URL parameters. It answers your question in a more general manner (changing any url parameter). There are solutions for both jQuery and regular js in the answers section.

It also looks like url.replace should be location.replace but I may be wrong (that statement's based on a quick google search for 'url.replace javascript').

<script type="text/javascript">
    $(document).ready(function () {
        $('input.letter').click(function () {
            //0- prepare values
            var qsTargeted = 'letter=' + this.value; //"letter=A";
            var windowUrl = '';
            var qskey = qsTargeted.split('=')[0];
            var qsvalue = qsTargeted.split('=')[1];
            //1- get row url
            var originalURL = window.location.href;
            //2- get query string part, and url
            if (originalURL.split('?').length > 1) //qs is exists
            {
                windowUrl = originalURL.split('?')[0];
                var qs = originalURL.split('?')[1];
                //3- get list of query strings
                var qsArray = qs.split('&');
                var flag = false;
                //4- try to find query string key
                for (var i = 0; i < qsArray.length; i++) {
                    if (qsArray[i].split('=').length > 0) {
                        if (qskey == qsArray[i].split('=')[0]) {
                            //exists key
                            qsArray[i] = qskey + '=' + qsvalue;
                            flag = true;
                            break;
                        }
                    }
                }
                if (!flag)//   //5- if exists modify,else add
                {
                    qsArray.push(qsTargeted);
                }
                var finalQs = qsArray.join('&');
                //6- prepare final url
                window.location = windowUrl + '?' + finalQs;
            }
            else {
                //6- prepare final url
                //add query string
                window.location = originalURL + '?' + qsTargeted;
            }
        })
    });
</script>