protected function getAssetsManifest()
    {
        if ($this->assetsManifest === null) {
            $assetsJsonFile = Mage::getDesign()->getSkinBaseDir().'/rev-manifest.json';

            if (file_exists($assetsJsonFile)) {
                $this->assetsManifest = json_decode(file_get_contents($assetsJsonFile), true);
            } else {
                $this->assetsManifest = [];
            }
        }

        return $this->assetsManifest;
    }