[Javascript] Image detection

[Andrew Clover]

Roland Neilands <rneilands at pulsemining.com.au> wrote:

> Does anyone know of a simple way of detecting in Javascript
> if a given file is an image or not?

Try setting the src of an <img> to point to the file in question. If you 
get an onload event, it was an image in a format the browser recognises. 
If you get an onerror event, it wasn't (or the resource was unfetchable).

> Is checking a list of files extensions the only way?

Pedantically speaking, it isn't a way at all - a URI's file extension 
may not have anything to do with the type of the resource it returns. A 
common example is a .cgi script, which could spit out a file with any 
MIME type. I don't know if there are cases like this in your intranet 
though.

If you *do* go for guessing-from-extension, a regexp might not be the 
clearest solution. The shortest solution, perhaps, but traditionally not 
necessarily the most correct. For example the version you posted:

 > new RegExp("jpg|gif|bmp|tif","i");

would match anywhere in the string, so 'stiff_gift.exe' would be 
considered an image. One way to do it without regexps might be:

   var path= uri.split('?', 1)[0];  // remove any query string
   var parts= path.split('.');
   var ext= path[path.length-1];    // get the last .xxx part
   var isim= ext=='jpg'||ext=='jpeg'||ext=='gif'||ext=='png'||ext=='bmp';

If you want TIFF in there too you should probably test for .tiff as well 
as .tif, but typically web browsers won't display this format anyway.

-- 
Andrew Clover
mailto:and at doxdesk.com
http://www.doxdesk.com/