Javacript: Set or Update a URL/QueryString Parameter, and update URL using HTML history.replaceState()

gistfile1.js

// Explicitly save/update a url parameter using HTML5's replaceState().
function updateQueryStringParam(param, value) {
  baseUrl = [location.protocol, '//', location.host, location.pathname].join('');
  urlQueryString = document.location.search;
  var newParam = key + '=' + value,
  params = '?' + newParam;

  // If the "search" string exists, then build params from it
  if (urlQueryString) {
    keyRegex = new RegExp('([\?&])' + key + '[^&]*');
    // If param exists already, update it
    if (urlQueryString.match(keyRegex) !== null) {
      params = urlQueryString.replace(keyRegex, "$1" + newParam);
    } else { // Otherwise, add it to end of query string
      params = urlQueryString + '&' + newParam;
    }
  }
  window.history.replaceState({}, "", baseUrl + params);
}

Here is a quick idea based on this stackoverflow answer:
http://stackoverflow.com/questions/1634748/how-can-i-delete-a-query-string-parameter-in-javascript

Not sure if its stable like this ... regex is not one of my strengths

var updateQueryStringParam = function (key, value) {

    var baseUrl = [location.protocol, '//', location.host, location.pathname].join(''),
        urlQueryString = document.location.search,
        newParam = key + '=' + value,
        params = '?' + newParam;

    // If the "search" string exists, then build params from it
    if (urlQueryString) {

        updateRegex = new RegExp('([\?&])' + key + '[^&]*');
        removeRegex = new RegExp('([\?&])' + key + '=[^&;]+[&;]?');

        if( typeof value == 'undefined' || value == null || value == '' ) { // Remove param if value is empty

            params = urlQueryString.replace(removeRegex, "$1");
            params = params.replace( /[&;]$/, "" );

        } else if (urlQueryString.match(updateRegex) !== null) { // If param exists already, update it

            params = urlQueryString.replace(updateRegex, "$1" + newParam);

        } else { // Otherwise, add it to end of query string

            params = urlQueryString + '&' + newParam;

        }

    }
    window.history.replaceState({}, "", baseUrl + params);
};

This is awesome! Gave it a test, and when passing a blank 'value' or just the 'key' and no value, it will remove the query string from the url. Nice!

Not working, for example EventSearch[date] param RegExp return null

@johannesgrandy beautiful, exactly what I was looking for. Now time to back to all my code and refactor haha

added params = params == '?' ? '' : params; otherwise a ? would stay even there are no more params left.

var updateQueryStringParam = function (key, value) {

    var baseUrl = [location.protocol, '//', location.host, location.pathname].join(''),
        urlQueryString = document.location.search,
        newParam = key + '=' + value,
        params = '?' + newParam;

    // If the "search" string exists, then build params from it
    if (urlQueryString) {
        var updateRegex = new RegExp('([\?&])' + key + '[^&]*');
        var removeRegex = new RegExp('([\?&])' + key + '=[^&;]+[&;]?');

        if( typeof value == 'undefined' || value == null || value == '' ) { // Remove param if value is empty
            params = urlQueryString.replace(removeRegex, "$1");
            params = params.replace( /[&;]$/, "" );

        } else if (urlQueryString.match(updateRegex) !== null) { // If param exists already, update it
            params = urlQueryString.replace(updateRegex, "$1" + newParam);

        } else { // Otherwise, add it to end of query string
            params = urlQueryString + '&' + newParam;
        }
    }

    // no parameter was set so we don't need the question mark
    params = params == '?' ? '' : params;

    window.history.replaceState({}, "", baseUrl + params);
};

If you're not using IE/Edge this can be achieved with URLSearchParams

const params = new URLSearchParams(location.search);
params.set('test', 123);
window.history.replaceState({}, '', `${location.pathname}?${params}`);

@johnrees that's great. 👍

Can anyone please show me how do I implement updateQueryStringParam() in a <button> or <a> or <select> [onChange]?