vue将图片链接转为二进制文件流

应用场景：用户修改图片后会覆盖以前上传的图片，需将以前的的图片转为文件流与修改的图片一起上传

1.需先将图片链接转为Base64

imageUrlToBase64(imageUrl) {
  let image = new Image() // 一定要设置为let，不然图片不显示
  image.setAttribute('crossOrigin', 'anonymous') // 解决跨域问题
  image.src = imageUrl +"&v=" + Math.random()
  image.onload = () => {
    var canvas = document.createElement("canvas")
	canvas.width = image.width
	canvas.height = image.height
	var context = canvas.getContext('2d')
	context.drawImage(image, 0, 0, image.width, image.height)
	var quality = 0.8
	var dataURL = canvas.toDataURL("image/jpeg", quality) // 使用toDataUrl将图片转换成jpeg的格式,不要把图片压缩成png，因为压缩成png后base64的字符串可能比不转换前的长！
    this.base64toFile(dataURL)
  }
}

2.再将base64转为文件流

base64toFile (dataurl, filename = 'file') {
  let arr = dataurl.split(',')
  let mime = arr[0].match(/:(.*?);/)[1]
  let suffix = mime.split('/')[1]
  let bstr = atob(arr[1])
  let n = bstr.length
  let u8arr = new Uint8Array(n)

  while (n--) {
    u8arr[n] = bstr.charCodeAt(n)
  }

  let file = new File([u8arr], `${filename}.${suffix}`, {
    type: mime
  })
  console.log(file)
}