how to add javascript in css file code example

Example 1: javascript css link append

document.getElementsByTagName("head")[0].insertAdjacentHTML(
    "beforeend",
    "<link rel=\"stylesheet\" href=\"path/to/style.css\" />");

Example 2: js add css to document

document.querySelector('head').innerHTML += '<link rel="stylesheet" href="styles.css" type="text/css"/>';