index.js

const $force = document.querySelectorAll('#force')[0]
const $touches = document.querySelectorAll('#touches')[0]
const canvas = document.querySelectorAll('canvas')[0]
const context = canvas.getContext('2d')
let lineWidth = 0
let isMousedown = false
let points = []

canvas.width = window.innerWidth * 2
canvas.height = window.innerHeight * 2

const strokeHistory = []

const requestIdleCallback = window.requestIdleCallback || function (fn) { setTimeout(fn, 1) };

/**
 * This function takes in an array of points and draws them onto the canvas.
 * @param {array} stroke array of points to draw on the canvas
 * @return {void}
 */
function drawOnCanvas (stroke) {
  context.strokeStyle = 'black'
  context.lineCap = 'round'
  context.lineJoin = 'round'

  const l = stroke.length - 1
  if (stroke.length >= 3) {
    const xc = (stroke[l].x + stroke[l - 1].x) / 2
    const yc = (stroke[l].y + stroke[l - 1].y) / 2
    context.lineWidth = stroke[l - 1].lineWidth
    context.quadraticCurveTo(stroke[l - 1].x, stroke[l - 1].y, xc, yc)
    context.stroke()
    context.beginPath()
    context.moveTo(xc, yc)
  } else {
    const point = stroke[l];
    context.lineWidth = point.lineWidth
    context.strokeStyle = point.color
    context.beginPath()
    context.moveTo(point.x, point.y)
    context.stroke()
  }
}

/**
 * Remove the previous stroke from history and repaint the entire canvas based on history
 * @return {void}
 */
function undoDraw () {
  strokeHistory.pop()
  context.clearRect(0, 0, canvas.width, canvas.height)

  strokeHistory.map(function (stroke) {
    if (strokeHistory.length === 0) return

    context.beginPath()

    let strokePath = [];
    stroke.map(function (point) {
      strokePath.push(point)
      drawOnCanvas(strokePath)
    })
  })
}

for (const ev of ["touchstart", "mousedown"]) {
  canvas.addEventListener(ev, function (e) {
    let pressure = 0.1;
    let x, y;
    if (e.touches && e.touches[0] && typeof e.touches[0]["force"] !== "undefined") {
      if (e.touches[0]["force"] > 0) {
        pressure = e.touches[0]["force"]
      }
      x = e.touches[0].pageX * 2
      y = e.touches[0].pageY * 2
    } else {
      pressure = 1.0
      x = e.pageX * 2
      y = e.pageY * 2
    }

    isMousedown = true

    lineWidth = Math.log(pressure + 1) * 40
    context.lineWidth = lineWidth// pressure * 50;

    points.push({ x, y, lineWidth })
    drawOnCanvas(points)
  })
}

for (const ev of ['touchmove', 'mousemove']) {
  canvas.addEventListener(ev, function (e) {
    if (!isMousedown) return
    e.preventDefault()

    let pressure = 0.1
    let x, y
    if (e.touches && e.touches[0] && typeof e.touches[0]["force"] !== "undefined") {
      if (e.touches[0]["force"] > 0) {
        pressure = e.touches[0]["force"]
      }
      x = e.touches[0].pageX * 2
      y = e.touches[0].pageY * 2
    } else {
      pressure = 1.0
      x = e.pageX * 2
      y = e.pageY * 2
    }

    // smoothen line width
    lineWidth = (Math.log(pressure + 1) * 40 * 0.2 + lineWidth * 0.8)
    points.push({ x, y, lineWidth })

    drawOnCanvas(points);

    requestIdleCallback(() => {
      $force.textContent = 'force = ' + pressure

      const touch = e.touches ? e.touches[0] : null
      if (touch) {
        $touches.innerHTML = `
          touchType = ${touch.touchType} ${touch.touchType === 'direct' ? '👆' : '✍️'} <br/>
          radiusX = ${touch.radiusX} <br/>
          radiusY = ${touch.radiusY} <br/>
          rotationAngle = ${touch.rotationAngle} <br/>
          altitudeAngle = ${touch.altitudeAngle} <br/>
          azimuthAngle = ${touch.azimuthAngle} <br/>
        `
      }
    })
  })
}

for (const ev of ['touchend', 'touchleave', 'mouseup']) {
  canvas.addEventListener(ev, function (e) {
    let pressure = 0.1;
    let x, y;

    if (e.touches && e.touches[0] && typeof e.touches[0]["force"] !== "undefined") {
      if (e.touches[0]["force"] > 0) {
        pressure = e.touches[0]["force"]
      }
      x = e.touches[0].pageX * 2
      y = e.touches[0].pageY * 2
    } else {
      pressure = 1.0
      x = e.pageX * 2
      y = e.pageY * 2
    }

    isMousedown = false

    requestIdleCallback(function () { strokeHistory.push([...points]); points = []})

    lineWidth = 0
  })
};