JSFiddle - React, Tailwind, and code Playground

HTML

<svg class="triangulo" height="210" width="500">
   <polygon points="200,10 250,190 160,210" style="fill:lime;stroke:purple;stroke-width:1" />
</svg>
<div id="distancia"></div>

JavaScript

var triangulo = document.querySelector('.triangulo polygon');

// Os dois primeiros pontos do triângulo
var p1 = triangulo.points[0];
var p2 = triangulo.points[1];
var d = Math.sqrt(Math.pow(p2.x - p1.x, 2) + Math.pow(p2.y - p1.y, 2));

document.getElementById('distancia').innerHTML = d;