JavaScript -  Get URL QueryString Parameters - ES5 / Best Browser Compatibility

getUrlParams.es5.js

/**
 * Accepts either a URL or querystring and returns an object associating 
 * each querystring parameter to its value. 
 *
 * Returns an empty object if no querystring parameters found.
 */
function getUrlParams(urlOrQueryString) {
  if ((i = urlOrQueryString.indexOf('?')) >= 0) {
    const queryString = urlOrQueryString.substring(i+1);
    if (queryString) {
      return _mapUrlParams(queryString);
    } 
  }
  
  return {};
}

/**
 * Helper function for `getUrlParams()`
 * Builds the querystring parameter to value object map.
 *
 * @param queryString {string} - The full querystring, without the leading '?'.
 */
function _mapUrlParams(queryString) {
  return queryString    
    .split('&') 
    .reduce(function(urlParams, urlParam) {
      urlParam = urlParam.split('=');
      if (Number.isInteger(parseInt(urlParam[1])) && parseInt(urlParam[1]) == urlParam[1]) {
        urlParams[urlParam[0]] = parseInt(urlParam[1]);
      } else {
        urlParams[urlParam[0]] = decodeURI(urlParam[1]);
      }
      return urlParams;
    }, {});
}

function _mapUrlParams(queryString) {
return queryString
.split('&')
.reduce((urlParams, urlParam) => {
urlParam = urlParam.split('=');
const test = Number.isInteger(parseInt(urlParam[1])) && parseInt(urlParam[1]) == urlParam[1];
urlParams[urlParam[0]] = test ? 'parseInt' : 'decodeURI'
return urlParams;
}, {});
}