改写成JavaScript代码：if $operation == DECODE $mystr = base64_decodesubstr$mystr $ckey_length; else $expiry_time = $expiry $expiry + time 0; $md5_string = substrmd5$mystr$keyb 0

if (operation === 'DECODE') { mystr = atob(mystr.substring(ckey_length)); } else { const expiry_time = expiry ? expiry + Date.now() : 0; const md5_string = md5(mystr + keyb).substring(0, 16); mystr = ${String(expiry_time).padStart(10, '0')}${md5_string}${mystr}; }