JavaScript – Modify URL Parameters

Recently I had to develop a solution which involved changing the HREF attributes of links in the navigation bar of a site. This had to be done on the JavaScript side because at my company we are the 3rd-party JavaScript solution for modifying other companies’ sites. That being said, I came up with one solution because the URL parameter was always the same but I figured I would publicly release the code for doing this in such a way that you can modify the value of any URL parameter:

/**
 * @license Copyright 2013 - Chris West - MIT Licensed
 */
(function(expCharsToEscape, expEscapedSpace, expNoStart, undefined) {
  /**
   * Modifies the given URL, returning it with the given parameter
   * changed to the given value.  The parameter is added if it didn't
   * already exist.  The parameter is removed if null or undefined is
   * specified as the value.
   * @param {string} url  The URL to be modified.
   * @param {string} paramName  The URL parameter whose value will be
   *     modified.
   * @param {string} paramValue  The value to assign.  This will be
   *     escaped using encodeURIComponent.
   * @return {string}  The updated URL.
   */
  modURLParam = function(url, paramName, paramValue) {
    paramValue = paramValue != undefined
      ? encodeURIComponent(paramValue).replace(expEscapedSpace, '+')
      : paramValue;
    var pattern = new RegExp(
      '([?&]'
      + paramName.replace(expCharsToEscape, '\\$1')
      + '=)[^&]*'
    );
    if(pattern.test(url)) {
      return url.replace(
        pattern,
        function($0, $1) {
          return paramValue != undefined ? $1 + paramValue : ''; 
        }
      ).replace(expNoStart, '$1?');
    }
    else if (paramValue != undefined) {
      return url + (url.indexOf('?') + 1 ? '&' : '?')
        + paramName + '=' + paramValue;
    }
    else {
      return url;
    }
  };
})(/([\\\/\[\]{}().*+?|^$])/g, /%20/g, /^([^?]+)&/);

The following are some example calls to this function:

// Initial URL
var url = 'http://example.com/';
alert(url);

// http://example.com/?q=search+term
url = modURLParam(url, 'q', 'search term');
alert(url);

// http://example.com/?q=search+term&name=Guillermo
url = modURLParam(url, 'name', 'Guillermo');
alert(url);

// http://example.com/?q=search+term+2&name=Guillermo
url = modURLParam(url, 'q', 'search term 2');
alert(url);

// http://example.com/?name=Guillermo
url = modURLParam(url, 'q');
alert(url);

// http://example.com/?name=Guillermo&q=termino
url = modURLParam(url, 'q', 'termino');
alert(url);

// http://example.com/?name=Guillermo
url = modURLParam(url, 'q', null);
alert(url);

Feel free to re-use the code! 8)