base64图片大小的计算与压缩

通常我们需要限制上传图片的大小，下面的函数可以帮助我们计算出base64图片的大小，然后根据大小按需压缩

// 获取base64图片大小，返回MB数字
function calSize(base64url: string) {
  let str = base64url.replace('data:image/png;base64,', '');
  const equalIndex = str.indexOf('=');
  if(str.indexOf('=') > 0) {
    str = str.substring(0, equalIndex);
  }
  const strLength = str.length;
  const fileLength = strLength - (strLength / 8) * 2;
  // 返回单位为MB的大小
  return  (fileLength / (1024 * 1024)).toFixed(2);
}

// 通过canvas压缩base64图片
function dealImage(base64: any, callback: any, w: number = 1000) {
    const newImage = new Image();
    const quality = 0.8;    // 压缩系数0-1之间
    newImage.src = base64;
    newImage.setAttribute("crossOrigin", 'Anonymous');    // url为外域时需要
    let imgWidth;
    let imgHeight;
    newImage.onload = function() {
      // @ts-ignore
      imgWidth = this.width;
      // @ts-ignore
      imgHeight = this.height;
      const canvas = document.createElement("canvas");
      const ctx = canvas.getContext("2d") as any;
      if (Math.max(imgWidth, imgHeight) > w) {
        if (imgWidth > imgHeight) {
          canvas.width = w;
          canvas.height = w * imgHeight / imgWidth;
        } else {
          canvas.height = w;
          canvas.width = w * imgWidth / imgHeight;
        }
      } else {
        canvas.width = imgWidth;
        canvas.height = imgHeight;
      }
      ctx.clearRect(0, 0, canvas.width, canvas.height);
      // @ts-ignore
      ctx.drawImage(this, 0, 0, canvas.width, canvas.height);
      const newBase64 = canvas.toDataURL("image/jpeg", quality);
      callback(newBase64);
    }
  }
}