How to add or replace a query parameter in a URL using Javascript/jQuery?

You could use a jQuery plugin to do the all the heavy lifting for you. It will parse the query string, and also reconstruct the updated query string for you. Much less code to deal with.

Plugin Download Page
Github Repo

// URL: ?a=2&b=3&order_by=old_data

var order_by = $.query.get('order_by');
//=> old_data

// Conditionally modify parameter value
if (order_by) { 
  order_by = “data”;
}

// Inject modified parameter back into query string
var newUrl = $.query.set(“order_by”, order_by).toString();
//=> ?a=2&b=3&order_by=data

For those using Node.js, there is a package for this available in NPM.

NPM Package
Github Repo

var queryString = require('query-string');
var parsed = queryString.parse('?a=2&b=3&order_by=old_data');  // location.search

// Conditionally modify parameter value
if (parsed.order_by) {
  parsed.order_by = 'data';
}

// Inject modified parameter back into query string
const newQueryString = queryString.stringify(parsed);
//=> a=2&b=3&order_by=data

A good solution ought to handle all of the following:

1. A URL that already has an order_by query parameter, optionally with whitespace before the equals sign. This can be further divided into cases where the order_by appears at the start, middle or end of the query string.
2. A URL that doesn't already have and order_by query parameter but does already have a question mark to delimit the query string.
3. A URL that doesn't already have and order_by query parameter and doesn't already have a question mark to delimit the query string.

The following will handle the cases above:

  if (/[?&]order_by\s*=/.test(oldUrl)) {
    newUrl = oldUrl.replace(/(?:([?&])order_by\s*=[^?&]*)/, "$1order_by=" + data_val);
  } else if (/\?/.test(oldUrl)) {
    newUrl = oldUrl + "&order_by=" + data_val;
  } else {
    newUrl = oldUrl + "?order_by=" + data_val;
  }

as demonstrated below:

getNewUrl("?a=1&b=2");
getNewUrl("?a=2&b=3&order_by=old_data");
getNewUrl("?a=2&b=3&order_by = old_data&c=4");
getNewUrl("?order_by=old_data&a=2&b=3");
getNewUrl("http://www.stackoverflow.com");

function getNewUrl(oldUrl) {
  var data_val = "new_data";
  var newUrl;
  if (/[?&]order_by\s*=/.test(oldUrl)) {
    newUrl = oldUrl.replace(/(?:([?&])order_by\s*=[^?&]*)/, "$1order_by=" + data_val);
  } else if (/\?/.test(oldUrl)) {
    newUrl = oldUrl + "&order_by=" + data_val;
  } else {
    newUrl = oldUrl + "?order_by=" + data_val;
  }
  console.log(oldUrl + "\n...becomes...\n" + newUrl);
}  

function addOrReplaceOrderBy(newData) {
  var stringToAdd = "order_by=" + newData;

  if (window.location.search == "")
    return window.location.href + stringToAdd;

  if (window.location.search.indexOf('order_by=') == -1)
    return window.location.href + stringToAdd;

  var newSearchString = "";
  var searchParams = window.location.search.substring(1).split("&");
  for (var i = 0; i < searchParams.length; i++) {
    if (searchParams[i].indexOf('order_by=') > -1) {
      searchParams[i] = "order_by=" + newData;
      break;
    }
  }
  return window.location.href.split("?")[0] + "?" + searchParams.join("&");
}

window.location.href = addOrReplaceOrderBy("new_order_by");

A little long but I think it works as intended.