code: function getSizes(validReq) {
    const sizes = validReq.sizes;
    if (!sizes || sizes.length < 1) return null;
    let sizesStr = '';
    for (const i in sizes) {
      const size = sizes[i];
      if (size.length !== 2) return null;
      sizesStr += `${size[0]}x${size[1]},`;
    }
    if (sizesStr || sizesStr.lastIndexOf(',') === sizesStr.length - 1) {
      sizesStr = sizesStr.substring(0, sizesStr.length - 1);
    }
    return sizesStr;
  }
docs: request
  @param validReq request
  @returns {?string} 300x250,320x50...