magicDragon.js

const canvas = document.querySelector('#draw');
const ctx = canvas.getContext('2d');
canvas.width = window.innerWidth;
canvas.height = window.innerHeight;

ctx.strokeStyle = '#BADA55';
ctx.lineJoin = 'round';
ctx.lineCap = 'round';

let isDraw = false;
let lastX = 0;
let lastY = 0;
let hue = 0;
let direction = true;

function draw(e) {
  if (!isDrawing) return;
  console.log('draw');
  ctx.strokeStyle = `hsl(${hue}, 100%, 50%)`;
  ctx.beginPath();
  ctx.moveTo(lastX, lastY);
  ctx.lineTo(e.offsetX, e.offsetY);
  ctx.stroke();
  ctx.globalCompositeOperation="source-over";

  //es6 is supercool - destructuring an array:
  [lastX, lastY] = [e.offsetX, e.offsetY];
  hue++;
  if (hue >= 360) {
    hue = 0;
  }
}

if (ctx.lineWidth >= 100 || ctx.lineWidth <=1){
  direction = !direction;
}
if(direction) {
  ctx.lineWidth++;
} else {
  ctx.lineWidth--;
}

// touchi touchi
canvas.addEventListener("touchmove", draw);
canvas.addEventListener("touchstart",  (e) => {
  isDrawing = true;
  [lastX, lastY] = [e.offsetX, e.offsetY];
});
canvas.addEventListener("touchend", , () => isDrawing = false);
canvas.addEventListener("touchcancel", () => isDrawing = false);

// mousi mousi
canvas.addEventListener('mousemove', draw);
canvas.addEventListener('mousedown', (e) => {
  isDrawing = true;
  [lastX, lastY] = [e.offsetX, e.offsetY];
});
canvas.addEventListener('mouseup', () => isDrawing = false);
canvas.addEventListener('mouseout', () => isDrawing = false);