function compress(file, limit = 0.5, file_size = 50) {
  return new Promise((resolve, reject) => {
    if (!file) {
      reject(new Error('不存在文件'))
    }
    const type = file.type
    const size = file.size
    const whiteType = ['image/jpeg', 'image/png']
    if (whiteType.indexOf(type) === -1) { // 判断文件类型
      reject(new Error('错误的文件类型'))
    }
    if (Math.ceil(size / 1024) < file_size) { // 判断文件大小
      resolve(file)
    }
    const reader = new FileReader()
    reader.readAsDataURL(file)
    reader.onload = e => {
      const img = new Image()
      img.src = e.target.result
      img.onload = () => {
        const ratio = img.naturalHeight / img.naturalWidth
        const canvas = document.createElement('canvas')
        const ctx = canvas.getContext('2d')
        canvas.width = img.width > 750 ? 750 : img.width // 固定宽度
        canvas.height = img.width * ratio
        ctx.drawImage(img, 0, 0, canvas.width, canvas.height)
        const newImg = canvas.toDataURL('image/jpeg', limit)
        const buffer = atob(newImg.split(',')[1])
        let length = buffer.length
        const bufferArray = new Uint8Array(length)
        while (length--) {
          bufferArray[length] = buffer.charCodeAt(length)
        }
        const newFile = new File([bufferArray], file.name, { type: 'image/jpeg' })
        resolve(newFile)
      }
    }
  })
}

export default compress
