canvas壓縮圖片

    // 圖片壓縮，默認同比例壓縮
    compressImg (URL, callback) {
      // 生成canvas
      let canvas = document.createElement('canvas')
      let ctx = canvas.getContext('2d')
      let image = new Image()
      image.setAttribute('crossOrigin', 'anonymous')
      image.src = URL
      let quality = 0.3
      image.onload = () => {
        canvas.width = image.width
        canvas.height = image.height
        ctx.drawImage(image, 0, 0, canvas.width, canvas.height)
        let newImageData = canvas.toDataURL('image/jpeg', quality)
        callback(newImageData)
      }
    }