Parse and manipulate query args in JavaScript

args.js

/*global _:true */

/**
 * Args builds an object with a property for each query argument in the query
 * portion of a url.  Properties on the object can then be modified and an
 * updated url with query string can be produced from the Args instance.
 */
function Args(url) {
  var self = this;

  //|
  //| Get url query parameters and set object properties
  //|

  var urlFragments = url.split("?");
  var queryString = ( urlFragments.length > 1 ) ? urlFragments[1] : "";
  var queryFragments = queryString.split("&");

  // Get key value pairs
  _.each(queryFragments, function(queryFragment) {
    var argFragments = queryFragment.split("=");

    var argName = decodeURIComponent(argFragments[0]);
    var argValue = ( argFragments.length > 1 ) ? decodeURIComponent(argFragments[1]) : undefined;

    self[argName] = argValue;
  });

  //|
  //| Private properties, getters, and setters
  //|

  // Path of url (without query string)
  var path = urlFragments[0];
  // In case a specific order for parameters is needed, it is stored here.
  // Contains an array that looks like: ["prop_name1", "prop_name2", etc.]
  var order;

  this.getPath = function() { return path; };
  this.getOrder = function() { return order; };
  this.setOrder = function(o) { order = o; };
}

Args.propertyToString = function(name, value) {
  if ( _.isFunction(value) )
    return undefined;
  else if ( value === undefined )
    return encodeURIComponent(name);
  else
    return encodeURIComponent(name) + "=" + encodeURIComponent(value);
};

/**
 * Converts an Args object into a string with the original url path and all
 * get parameters encoded as a query string.  Must be set directly on the
 * prototype because IE doesn't like overriding Object methods via
 * Underscore.extend().
 */
Args.prototype.toString = function() {
  return this.getPath() + "?" + this.getQueryString();
};

_.extend(Args.prototype, {

  /**
   * Converts query data in an Args object into a query string.
   */
  getQueryString: function() {
    var self = this;
    var order = this.getOrder();

    // Build query fragments (["name1=value1", "name2", undefined, etc.])
    var queryFragments;

    if ( order !== undefined ) {
      var orderedProperties = _.map(order, function(propName) {
        if ( _.has(self, propName) ) {
          return {
            name: propName,
            value: self[propName]
          };
        }
        // Return undefined if property does not exist so it can be filtered
        return undefined;
      });

      // Filter out undefined values
      orderedProperties = _.compact(orderedProperties);

      queryFragments = _.map(orderedProperties, function(prop) {
        return Args.propertyToString(prop.name, prop.value);
      });
    } else {
      queryFragments = _.map(this, Args.propertyToString);
    }

    queryFragments = _.reject(queryFragments, function(i) {
      return (i === undefined);
    });

    return queryFragments.join("&");
  }

});