es6代码： 
function formatfilesize(filesize, idx = 0) { const units = ["b", "kb", "mb", "gb"]; if (filesize < 1024 || idx === units.length - 1) { return filesize.tofixed(1) + units[idx]; } return formatfilesize(filesize / 1024, ++idx); }
老版本代码：
function formatfilesize(filesize, idx) { var units = ["b", "kb", "mb", "gb"]; idx = idx || 0; if (filesize < 1024 || idx === units.length - 1) { return filesize.tofixed(1) + units[idx]; } return formatfilesize(filesize / 1024, ++idx); }