Updates a URL query parameter (or adds it if it doesn't already exist). Useful with history.pushState() / history.replaceState().

updateQueryParam.js

/**
 * Create or update the value of a URL query parameter.
 *
 * Given an input URL, a query parameter, and a value, if the query parameter
 * already exists in the URL, this function replaces that paramter's value with
 * the given value; otherwise, this function adds the parameter to the URL and
 * sets it to the given value. Returns the modified URL.
 */
function updateQueryParam(url, param, value) {
    var search = new RegExp('([?&])(' + param + ')=[^&]*|$', 'i');
    return url.replace(search, function(m, p1, p2, o, s) {
        return (p1 ? p1 : (s.indexOf('?') === -1 ? '?' : '&')) + (p2 ? p2 : param) + '=' + value;
    });
}

// Tests ======================================================================

// Test cases: original => replacement
var urls = {
    'index.html':                     'index.html?param=ABC',
    'index.html?param=XYZ':           'index.html?param=ABC',
    'index.html?PARAM=XYZ':           'index.html?PARAM=ABC',
    'index.html?param=XYZ&a=1':       'index.html?param=ABC&a=1',
    'index.html?a=1&param=XYZ':       'index.html?a=1&param=ABC',
    'index.html?a=1&param=XYZ&b=2':   'index.html?a=1&param=ABC&b=2',
    'index.html?param=':              'index.html?param=ABC',
    'index.html?param=&a=1':          'index.html?param=ABC&a=1',
    'index.html?a=1&param=':          'index.html?a=1&param=ABC',
    'index.html?a=1&param=&b=2':      'index.html?a=1&param=ABC&b=2',
    'index.html?a=1':                 'index.html?a=1&param=ABC',
    'index.html?a=1&b=2':             'index.html?a=1&b=2&param=ABC',
    'index.html?params=XYZ':          'index.html?params=XYZ&param=ABC',
    'index.html?a=1&params=XYZ&b=2':  'index.html?a=1&params=XYZ&b=2&param=ABC',
    'index.html?aparam=XYZ':          'index.html?aparam=XYZ&param=ABC',
    'index.html?a=1&aparam=XYZ&b=2':  'index.html?a=1&aparam=XYZ&b=2&param=ABC',
    'index.html?a=1&aparams=XYZ&b=2': 'index.html?a=1&aparams=XYZ&b=2&param=ABC',
};

// Run tests
var s = 0, // success counter
    f = 0; // failure counter
for (var prop in urls) {
    if (urls.hasOwnProperty(prop)) {
        var u = updateQueryParam(prop, 'param', 'ABC');
        if (u !== urls[prop]) {
            console.log(prop, urls[prop], u);
            f++;
        }
        else {
            s++;
        }
    }
}
console.log(s + ' test' + (s === 1 ? '' : 's') + ' ran successfully\n' + f + ' test' + (f === 1 ? '' : 's') + ' failed');