H5图片上传压缩

dataURLtoFile(dataurl, filename) { // 将base64转换为file文件
  let arr = dataurl.split(",")
  let mime = arr[0].match(/:(.*?);/)[1]
  let bstr = atob(arr[1])
  let n = bstr.length
  let u8arr = new Uint8Array(n)
  while (n--) {
    u8arr[n] = bstr.charCodeAt(n)
  }
  return new File([u8arr], filename, {type: mime})
}
afterRead(file) {
  if (file.file.size > 3145728) {
    console.log("压缩了")
    let canvas = document.createElement("canvas") // 创建Canvas对象(画布)
    let context = canvas.getContext("2d")
    let img = new Image()
    img.src = file.content // 指定图片的DataURL(图片的base64编码数据)
    img.onload = () => {
      canvas.width = img.width / 2;//画布宽度
      canvas.height = img.height / 2;//画布高度
      context.drawImage(img, 0, 0, canvas.width, canvas.height)
      file.content = canvas.toDataURL(file.file.type, 0.92) // 0.92为默认压缩质量
      let files = this.dataURLtoFile(file.content, file.file.name)
      const data = new FormData()
      data.append("fileList", files)
      this.imgUploader(data);// 上传方法，此时已经压缩完成
    }
  } else { // 小于3M直接上传
    const data = new FormData()
    data.append("fileList", file.file);
    this.imgUploader(data);
  }
},