public function evaluateJavascript($expression, $limit = null)
    {
        $key = '___lastValue___' . uniqid();
        $this->executeStep(
            sprintf("_sahi.setServerVarPlain(%s, JSON.stringify(%s))", json_encode($key), $expression),
            $limit
        );

        $resp = $this->executeCommand('getVariable', array('key' => $key));

        return json_decode($resp, true);
    }