Add a query parameter and value to a given URL string

withParam.js

// Usage:
//
// ('http://www.example.com').withParam('foo', 'bar');
// -> http://www.example.com?foo=bar
//
// ('http://www.example.com').withParam('foo');
// -> http://www.example.com?foo
//
// Credit: Lessan Vaezi
//         (See: http://stackoverflow.com/a/487084/23341)

String.prototype.withParam = function (name, value){
	var url = this,
		parameterName = name,
		parameterValue = value || '',
		atStart,
		cl,
		urlParts,
		urlhash,
		sourceUrl,
		newQueryString,
		parameters,
		parameterParts,
		i,
		replaceDuplicates = true;

	if (url.indexOf('#') > 0) {
		cl = url.indexOf('#');
		urlhash = url.substring(url.indexOf('#'),url.length);
	}
	else {
		urlhash = '';
		cl = url.length;
	}

	sourceUrl = url.substring(0, cl);

	urlParts = sourceUrl.split('?');
	newQueryString = '';

	if (urlParts.length > 1) {
		parameters = urlParts[1].split('&');
		for (i = 0; (i < parameters.length); i++) {
			parameterParts = parameters[i].split('=');
			if (!(replaceDuplicates && parameterParts[0] === parameterName)) {
				if (newQueryString === '')
					newQueryString = '?';
				else
					newQueryString += '&';
				newQueryString += parameterParts[0] + '=' + (parameterParts[1] ? parameterParts[1] : '');
			}
		}
	}

	if (newQueryString === '')
		newQueryString = '?';

	if (newQueryString !== '' && newQueryString != '?')
		newQueryString += '&';

	newQueryString += parameterName + '=' + (parameterValue ? parameterValue : '');

	return urlParts[0] + newQueryString + urlhash;
};