JSFiddle - React, Tailwind, and code Playground

HTML

<div class="wrap">
    <div class="slider" id="slider"></div>
</div>

CSS

html {
  font-size: 24px;
}
.wrap {
padding: 1rem;
border: 1px solid red;
height: 100px;
width: 390px;
}

JavaScript

class Slider {
    constructor( options ) {

        let slider = document.querySelector( options.id );
        let parent = slider.parentNode;
        let width = parent.offsetWidth;
        alert( width );
    }
}
new Slider({
        id: '.slider',
});

alert(document.getElementById("slider").offsetWidth);