Access and already created cesium window and add an entity

CesiumJS
1

Hello I have created cesium window with the following code

<div id="cesiumContainer"></div>
function RenderCesium() {
    var cesiumElem = new Cesium.Viewer('cesiumContainer');
}

I am not saving the cesiumElem above. I want to later access this Cesium element through another function. Is there a way to access this element later. I tried the below function. But when I am adding the entity, it gives an error. Please note that I do not want to make cesiumElem as a global variable.

function RenderEntity() {
    var cesiumContainer = document.getElementById("cesiumContainer").;
    cesiumContainer.entities.add({
        position: Cesium.Cartesian3.fromDegrees(-75.1641667, 39.9522222),
        label: {
            text: "Philadelphia",
        },
    });
}

Thanks
Libish