Add and/or replace URL parameters in links

affiliate.js

(function() {
  this.Affiliate = function(options) {
    this.settings = {
      affiliates: [],
    };

    function extendDefaults(source, properties) {
      for(var property in properties) {
        if(properties.hasOwnProperty(property)) {
          source[property] = properties[property];
        }
      }

      return source;
    }
    
    this.settings = extendDefaults(this.settings, options);
  }
  
  Affiliate.prototype.replace = function() {
    var links = document.getElementsByTagName("a");
    for(var i = 0; i < links.length; i++) {
      var link = links[i];
      var href = link.href;

      for(var j = 0; j < this.settings.domains.length; j++) {
        var affiliate = this.settings.domains[j];
        var params = affiliate.params;

        var re = new RegExp("^(https?\:\/\/)(www\\.)?" + affiliate.domain + ".*");
        var matches = href.match(re);
        if(matches) {
          var keys = Object.keys(params);
          var usedParams = [];

          if(href.indexOf("?") > -1) {
            var currentUrl = href.split("?")[0];
            var currentParams = href.split("?")[1].split("&");            
            href = currentUrl + "?";

            for(var k = 0; k < currentParams.length; k++) {
              var paramKey = currentParams[k].split("=")[0];
              var paramValue = currentParams[k].split("=")[1];
              
              if(keys.indexOf(paramKey) > -1) {
                paramValue = params[paramKey];
              }

              href += paramKey + "=" + paramValue + "&";
              usedParams.push(paramKey);
            }
          } else {
            href += "?";
          }

          for(var l = 0; l < keys.length; l++) {
            var key = keys[l];
            if(usedParams.indexOf(key) < 0) {
              var param = params[key];
              href += keys[l] + "=" + param + "&";
            }
          }

          href = href.slice(0, -1);
          link.href = encodeURI(href);
        }
      }
    }
  }
}());

var affiliate = new Affiliate({
  'domains': [
    {
      'domain': 'banggood.com',
      'params': {
        'p': 'p-param',
        't': 't param'
      },
    },
    {
      'domain': 'gearbest.com',
      'params': {
        'p': 'p-param',
        't': 't param'
      },
    },
  ],
});
affiliate.replace();