Limit size of images in JavaScript

readURL = (input, template, id) ->
  filesToUpload = input.files
  file = filesToUpload[0]
  # Create an image
  img = document.createElement('img')
  # Create a file reader
  reader = new FileReader
  # Set the image once loaded into file reader

  reader.onload = (e) ->
    img.src = e.target.result
    canvas = document.createElement('canvas')
    ctx = canvas.getContext('2d')
    ctx.drawImage img, 0, 0
    MAX_WIDTH = 1024
    MAX_HEIGHT = 1024
    width = img.width
    height = img.height
    if width > height
      if width > MAX_WIDTH
        height *= MAX_WIDTH / width
        width = MAX_WIDTH
    else
      if height > MAX_HEIGHT
        width *= MAX_HEIGHT / height
        height = MAX_HEIGHT
    canvas.width = width
    canvas.height = height
    ctx = canvas.getContext('2d')
    ctx.drawImage img, 0, 0, width, height
    dataurl = canvas.toDataURL('image/png')
    crop(dataurl, template, id)

  # Load files into file reader
  reader.readAsDataURL file