Cross‑Window Real‑Time Rendering with Canvas, window.screenX/Y and localStorage

The author describes a technique for building a cross‑window, real‑time interactive animation entirely with JavaScript, without third‑party libraries. Each browser window draws a circle at its center on a canvas , stores its screen coordinates ( window.screenX , window.screenY ) and a color in localStorage , and reads other windows' data to render connecting lines.

Key APIs used :

window.requestAnimationFrame – for the animation loop.

const { screenX, screenY } = window – to obtain the window's position relative to the screen.

const barHeight = window.outerHeight - window.innerHeight – to calculate the browser UI height.

canvas – the drawing surface.

localStorage – shared storage for all pages under the same origin.

Core drawing logic (simplified) :

const alert1 = document.getElementById('alert1');
const alert2 = document.getElementById('alert2');
const canvas1 = document.getElementById('canvas1');
const ctx = canvas1.getContext('2d');

function draw() {
  const { clientWidth, clientHeight } = document.body;
  const { screenX, screenY } = window;
  const barHeight = window.outerHeight - window.innerHeight;

  // log current window info
  alert1.textContent = JSON.stringify({ clientWidth, clientHeight, screenX, screenY, barHeight }, null, 2);

  canvas1.width = clientWidth;
  canvas1.height = clientHeight;

  const x = clientWidth / 2;
  const y = clientHeight / 2;

  // draw own circle
  ctx.fillStyle = color;
  ctx.beginPath();
  ctx.arc(x, y, 15, 0, Math.PI * 2);
  ctx.fill();

  const position = { top: y + barHeight + screenY, left: x + screenX, color };

  // draw other windows' circles and connecting lines
  getOtherKeys().forEach(k => {
    const position2 = JSON.parse(localStorage.getItem(k));
    const w = position2.left - position.left;
    const h = position2.top - position.top;

    ctx.fillStyle = position2.color;
    ctx.beginPath();
    ctx.arc(x + w, y + h, 15, 0, Math.PI * 2);
    ctx.fill();

    ctx.strokeStyle = 'black';
    ctx.beginPath();
    ctx.moveTo(x, y);
    ctx.lineTo(x + w, y + h);
    ctx.stroke();
  });

  // update own position in storage
  localStorage.setItem(key, JSON.stringify(position));
  alert2.textContent = JSON.stringify(localStorage, null, 2);

  window.requestAnimationFrame(draw);
}
window.requestAnimationFrame(draw);

Utility functions manage storage keys, assign a unique key to each window, and clean up on window.onunload :

function getOtherKeys() {
  const keys = [];
  for (let i = 0; i < localStorage.length; i++) {
    const k = Number(localStorage.key(i));
    if (!isNaN(k)) keys.push(k);
  }
  return keys.sort((a, b) => a - b);
}

window.onunload = function () {
  localStorage.removeItem(key);
};

The article concludes with the full source code, a summary of current limitations (e.g., missing connection lines when windows overlap), and links to the GitHub repository and live demo for further exploration.