被下载的图片，需要设置 'Access-Control-Allow-Origin` 为安全域

function downloadImg(src){
const canvas = document.createElement('canvas')
const ctx = canvas.getContext('2d')
const image = new Image()
image.setAttribute('crossorigin', 'anonymous')
image.onload = function() {
  ctx.drawImage(image, 0, 0)
  console.log(canvas.toDataURL())
  canvas.toBlob(blob => {
    const link = document.createElement('a')
    link.href = window.URL.createObjectURL(blob)
    link.download = '附件'
    link.click()
  })
}
image.src = src
}

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