how to select an element id and change the style in js code example

Example 1: add css in javascript

document.getElementById("demo").style.display = "none";

Example 2: how to change an element in a different elements code css

/*If the cube is directly inside the container:*/

#container:hover > #cube { background-color: yellow; }

/*If cube is next to (after containers closing tag) the container:*/

#container:hover + #cube { background-color: yellow; }

/*If the cube is somewhere inside the container:*/

#container:hover #cube { background-color: yellow; }

/*If the cube is a sibling of the container:*/

#container:hover ~ #cube { background-color: yellow; }