How can I add or update a query string parameter?

Update (2020): URLSearchParams is now supported by all modern browsers.

The URLSearchParams utility can be useful for this in combination with window.location.search. For example:

if ('URLSearchParams' in window) {
    var searchParams = new URLSearchParams(window.location.search);
    searchParams.set("foo", "bar");
    window.location.search = searchParams.toString();
}

Now foo has been set to bar regardless of whether or not it already existed.

However, the above assignment to window.location.search will cause a page load, so if that's not desirable use the History API as follows:

if ('URLSearchParams' in window) {
    var searchParams = new URLSearchParams(window.location.search)
    searchParams.set("foo", "bar");
    var newRelativePathQuery = window.location.pathname + '?' + searchParams.toString();
    history.pushState(null, '', newRelativePathQuery);
}

Now you don't need to write your own regex or logic to handle the possible existence of query strings.

However, browser support is poor as it's currently experimental and only in use in recent versions of Chrome, Firefox, Safari, iOS Safari, Android Browser, Android Chrome and Opera. Use with a polyfill if you do decide to use it.

I wrote the following function which accomplishes what I want to achieve:

function updateQueryStringParameter(uri, key, value) {
  var re = new RegExp("([?&])" + key + "=.*?(&|$)", "i");
  var separator = uri.indexOf('?') !== -1 ? "&" : "?";
  if (uri.match(re)) {
    return uri.replace(re, '$1' + key + "=" + value + '$2');
  }
  else {
    return uri + separator + key + "=" + value;
  }
}