JSFiddle - React, Tailwind, and code Playground

HTML

<svg width=100% height=100%>
    <rect id=rect rx=0 ry=0 x=0 y=0 width=100% height=50px fill=HoneyDew />
</svg>

JavaScript

var el = document.getElementById('rect');
var width_plan = el.getBoundingClientRect().width
alert(width_plan);