<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Random canvas circles</title>
    <style>
      html {
        width: 100%;
        height: inherit;
      }

      canvas {
        display: block;
      }

      body {
        margin: 0;
      }

      button {
        position: absolute;
        top: 5px;
        left: 5px;
      }
    </style>
  </head>
  <body>

  <button>Update</button>

  <canvas></canvas>


    <script>
    const btn = document.querySelector('button');
    const 