JSFiddle - React, Tailwind, and code Playground

HTML

<div id="light"><div id="button"></div></div>

CSS

padding: 10px;

#light {
    position: relative;
    width: 40px;
    margin: auto;
    background-color: black;
    z-index: 10;
    margin-top: 420px;
    height: 20px;
    border-radius: 40px;
}
#button {
    width: 16px;
    background-color: cornflowerblue;
    height: 16px;
    border-radius: 40px;
    position: relative;
    top: 2px;
    left: 4px;
    cursor: pointer;
    transition: 0.5s;
}

JavaScript

var button = document.getElementById("button");


            button.onclick = function() {
                if (window.getComputedStyle(button, null).getPropertyValue('left') === "4px") {
                    button.style.left = "20px";
                } else {
                    button.style.left = "4px";
                }  
            }


/*
var stil = window.getComputedStyle(button).getPropertyValue("left"); 
	button.onclick = function() {
  	if (stil === "4px") {
    	stil = "20px";
    } else {
    	stil = "4px";
    }
  }
*/