Parse and store UTM params from query string with minimal dependencies

utmParser.js

var utmParser = function() {
  var query = window.location.search

  function decode(value) {
    try {
      return decodeURIComponent(value);
    } catch (e) {
      debug('error `decode(%o)` - %o', value, e)
    }
  }

  function trim(str){
    return str.replace(/^\s*|\s*$/g, '');
  }

  function parse(str){
    var pattern = /(\w+)\[(\d+)\]/;
    if ('string' != typeof str) return {};

    str = trim(str);
    if ('' == str) return {};
    if ('?' == str.charAt(0)) str = str.slice(1);

    var obj = {};
    var pairs = str.split('&');
    for (var i = 0; i < pairs.length; i++) {
      var parts = pairs[i].split('=');
      var key = decode(parts[0]);
      var m;

      if (m = pattern.exec(key)) {
        obj[m[1]] = obj[m[1]] || [];
        obj[m[1]][m[2]] = decode(parts[1]);
        continue;
      }

      obj[parts[0]] = null == parts[1]
        ? ''
        : decode(parts[1]);
    }

    return obj;
  };
  
  function utm(query) {
    // Remove leading ? if present
    if (query.charAt(0) === '?') {
      query = query.substring(1);
    }

    query = query.replace(/\?/g, '&');

    var param;
    var params = parse(query);
    var results = {};

    for (var key in params) {
      if (Object.prototype.hasOwnProperty.call(params, key)) {
        if (key.substr(0, 4) === 'utm_') {
          param = key.substr(4);
          if (param === 'campaign') param = 'name';
          results[param] = params[key];
        }
      }
    }
    console.log(results)
    return results;
  }
  
  return utm(query)
}