calcBase64FileSize

var setContextOption = (newCtx, {
  smoothEnabled = true,
  smoothQuality = 'high'
} = {}) => {
  newCtx.imageSmoothingEnabled = smoothEnabled // 图片平滑, 默认为 true
  newCtx.msImageSmoothingEnabled = smoothEnabled
  newCtx.webkitImageSmoothingEnabled = smoothEnabled
  newCtx.mozImageSmoothingEnabled = smoothEnabled
  newCtx.imageSmoothingQuality = smoothQuality // 调整平滑质量
  newCtx.msImageSmoothingQuality = smoothQuality
  newCtx.webkitImageSmoothingQuality = smoothQuality
  newCtx.mozImageSmoothingQuality = smoothQuality
}
// 原图url: https://www.baidu.com/img/bd_logo1.png
// 使用此方法获取base64后，体积为 15kb，原图为7.9kb
var getBaseInfoByImageInfo = (imageURL, {
    targetWidth,
    targetHeight,
} = {}) => {
    return new Promise((resolve, reject) => {
        let image = new Image(targetWidth, targetHeight)
        image.crossOrigin = 'anonymous'
        if (targetWidth && targetHeight) {
          image.width = targetWidth
          image.height = targetHeight  
        }
        
        image.onload = async function() {
            try {
                const newCanvas = document.createElement('canvas')
                const newCtx = newCanvas.getContext('2d')
                newCanvas.width = targetWidth || image.naturalWidth
                newCanvas.height = targetHeight || image.naturalHeight

                setContextOption(newCtx)
                console.log('getBaseInfoByImageInfo() image: ', image.naturalWidth, image.naturalHeight)

                newCtx.drawImage(image, 0, 0, image.naturalWidth, image.naturalHeight, 0, 0, newCanvas.width, newCanvas.height)
                const base64 = newCanvas.toDataURL('image/png')
                resolve(base64)
            }
            catch (e) {
                console.error(e)
                reject(e)
            }
        }
        image.onerror = function(e) {
            reject(e)
        }
        image.src = imageURL
    })
}

var getFileSizeByBase64 = baseStr => {
  // 去除 base64 前缀，例如: data:image/png;base64,
  var tag = 'base64,'
  var tagIndex = baseStr.indexOf(tag)
  baseStr = baseStr.substring(tagIndex + tag.length)
  // 去掉base64编码中的 =
  var eqTagIndex = baseStr.indexOf('=')
  baseStr = eqTagIndex != -1
    ? baseStr.substring(0, eqTagIndex)
    : baseStr
  // 计算文件流大小
  var strLen = baseStr.length
  var fileSize = strLen - (strLen/8)*2
  return fileSize
}

// doc: https://my.oschina.net/peachyy/blog/1809638
getBaseInfoByImageInfo('https://www.baidu.com/img/bd_logo1.png').then(base64 => {
  var fileSize = getFileSizeByBase64(base64)
  window.base64Info = base64
  console.log('fileSize: ', fileSize)
})