How to recover deleted JavaScript files using the cache in Chrome or Firefox

So you’ve been writing some JavaScript files (or HTML files or images) and testing them in Google Chrome or Mozilla Firefox. Unfortunately, you’ve deleted the JavaScript files, forgot to check them into the repository, didn’t make a backup, and they’re gone. Don’t fear! There is still hope!

Your missing file is stored in your browser’s cache, which can be accessed by typing about:cache into the URL box in Chrome or about:cache?device=disk in Firefox. This will list links to every file in your cache (and thus may take a moment to open). Individual files can be accessed by going to chrome://net-internals/view-cache/[[URL]] in Chrome or about:cache-entry?client=HTTP&sb=1&key=[[URL]] in Firefox.

The cached version will look like the following in Chrome:

[[URL]]
HTTP/1.1 200 OK
Date: Wed, 19 May 2010 15:17:36 GMT
Server: Apache
Last-Modified: Wed, 19 May 2010 15:13:53 GMT
ETag: "18aa1e6-1761-486f3e7ebae40"
Accept-Ranges: bytes
Content-Length: 5985
Content-Type: application/javascript
00000000:  fb  00  00  00  01  00  00  00  34  62  bc  3d  8a  e5  2d  00  ........4b.=..-.
00000010:  ef  1a  be  3d  8a  e5  2d  00  e3  00  00  00  48  54  54  50  ...=..-.....HTTP
00000020:  2f  31  2e  31  20  32  30  30  20  4f  4b  00  44  61  74  65  /1.1 200 OK.Date
00000030:  3a  20  57  65  64  2c  20  31  39  20  4d  61  79  20  32  30  : Wed, 19 May 20
00000040:  31  30  20  31  35  3a  31  37  3a  33  36  20  47  4d  54  00  10 15:17:36 GMT.
00000050:  53  65  72  76  65  72  3a  20  41  70  61  63  68  65  00  4c  Server: Apache.L
00000060:  61  73  74  2d  4d  6f  64  69  66  69  65  64  3a  20  57  65  ast-Modified: We
00000070:  64  2c  20  31  39  20  4d  61  79  20  32  30  31  30  20  31  d, 19 May 2010 1
00000080:  35  3a  31  33  3a  35  33  20  47  4d  54  00  45  54  61  67  5:13:53 GMT.ETag
00000090:  3a  20  22  31  38  61  61  31  65  36  2d  31  37  36  31  2d  : "18aa1e6-1761-
000000a0:  34  38  36  66  33  65  37  65  62  61  65  34  30  22  00  41  486f3e7ebae40".A
000000b0:  63  63  65  70  74  2d  52  61  6e  67  65  73  3a  20  62  79  ccept-Ranges: by
000000c0:  74  65  73  00  43  6f  6e  74  65  6e  74  2d  4c  65  6e  67  tes.Content-Leng
000000d0:  74  68  3a  20  35  39  38  35  00  43  6f  6e  74  65  6e  74  th: 5985.Content
000000e0:  2d  54  79  70  65  3a  20  61  70  70  6c  69  63  61  74  69  -Type: applicati
000000f0:  6f  6e  2f  6a  61  76  61  73  63  72  69  70  74  00  00      on/javascript..
00000000:  2f  2a  2a  0a  20  2a  20  40  61  75  74  68  6f  72  20  41  /**. * @author A
00000010:  6c  65  78  20  4b  6f  72  6e  20  3c  61  6c  65  78  6b  6f  lex Korn <alexko
...

The last section (in both browsers) displays the hexadecimal representation and the text of your file. The last column of this section displays newline characters as periods in many text editors, so we can’t use it to recover the file. We’ll have to use the hexadecimal. I wrote the following PHP script, which will extract and convert the hexadecimal representation into the text version:

<?php
$cacheString = ''; // COPY THE LAST SECTION IN HERE (Use /'/\\\'/g to escape quotation marks)

$matches = array();
preg_match_all('/\s[0-9a-f]{2}\s/', $cacheString, $matches);
foreach ($matches[0] as $match)
{
    echo chr(hexdec($match));
}
?>

Voilà! Your once-lost code is now found!

A couple notes:

• If your server has compression enabled (such as gzip), you’ll have to go through the extra step of unzipping the content
• Chrome version: 5.0.375.38 beta
• Firefox version: 3.6.3

Update 2011/05/14: I changed the \b’s in the RegEx to \s’s as recommended in this post on recovering gzipped files.