Is it possible to intercept the file from a <input type=file> in javascript?

Question

Is it possible to read the content of a file that has been selected with a file input control? I wish to do something like:

<input type="file" id="fileInput" onblur="readFile(this)"/>

<script language="javascript">
   function readFile(file) {
     document.write(file);
   }
</script>

Is anything like this possible? or does the file upload just send the file to the server.

Answer 1

It is possible in Firefox, but it is not standardized, so it is not possible portably across browsers (for instance, WebKit does not support it). Your best bet would probably be to upload the file to the server, and then download it again using an XMLHTTPRequest.

Answer 2

You can if you use HTA (Hypertext Terminal Application, see http://msdn.microsoft.com/en-us/library/ms536496(VS.85).aspx). If you do, you're bound to Internet Explorer, but free to access files, the registry etc. There are (of course) security issues.

Answer 3

It is probably not possible in many browsers. What would happen if we gave arbitraty javascript the ability to read an arbitrary file in the filesystem, using the user's credentials? BAD THINGS. Malicious javascript could easily take the file data and post it back to the server, quietly snooping all your files in the background.

I doubt this is possible, and I strongly recommend against it.

If it needs to be exclusively client-side, why are you using a web application at all? The only files this could display are plain-text, for which there are many easier ways of viewing content.