https://adventofcode.com/2018/day/3

day3a.js

async function day3() {
  const canvas = document.querySelector('canvas');
  const ctx = canvas.getContext('2d');
  ctx.fillStyle = 'rgb(128, 0, 0, 0.5)';

  const response = await fetch('./day3.txt', { encoding: 'UTF-8'});
  const text = await response.text();
  const lines = text.split('\n').filter(Boolean);
  for (const line of lines) {
    const [id, left, top, width, height] = line.split(/[@,:x ]+/);
    ctx.fillRect(left, top, width, height);
  }

  const imageData = ctx.getImageData(0, 0, 2000, 2000);
  let result = 0;
  for (const px of imageData.data) {
    if (px > 128) { result++; }
  }
  console.log(result);
}

document.addEventListener('DOMContentLoaded', day3);