window.location.search will return everything from the ? on. This will remove the ?, use split to separate into key/value arrays, then assign named properties to the params object:

function getSearchParameters() {
      var prmstr = window.location.search.substr(1);
      return prmstr != null && prmstr != "" ? transformToAssocArray(prmstr) : {};
}

function transformToAssocArray( prmstr ) {
    var params = {};
    var prmarr = prmstr.split("&");
    for ( var i = 0; i < prmarr.length; i++) {
        var tmparr = prmarr[i].split("=");
        params[tmparr[0]] = tmparr[1];
    }
    return params;
}

var params = getSearchParameters();
You can then get the test parameter from http://myurl.com/?test=1 by calling params.test.