AJAX FILE UPLOAD – Uploading local files with AJAX/Javascript to a server

| November 4, 2009 | Ajax, Know more, Tips & Tricks | 3 Comments

Many people say uploading files with AJAX is impossible! Well, they’re wrong 🙂

Granted this solution only works with FireFox/Mozilla. And the user has to change a setting in “about:config” and accept the advanced access privileges.

Anyway, such an AJAX file upload is only suitable for restricted area stuff, such as Content Management Systems, Group-Ware etc., so users can be briefed before actually using it.

Open about:config and check that

signed.applets.codebase_principal_support

is set to “true”

Otherwise Firefox will display something like this

Error: uncaught exception: A script from "http://www.captain.at"
was denied UniversalXPConnect privileges.

Also make sure you check the checkbox “Remember this decision”, when FireFox will display this message

A script from "http://www.captain.at" is requesting enhanced abilities that are
UNSAFE and could be used to compromise your machine or data:
 
Run or install software on your machine
 
Allow these abilities only if you trust this source to be free of viruses or malicious
programs.
[ ] Remember this decision

and click “Allow”, otherwise you have to click “Allow” everytime you upload a file.

The example itself is rather straightforward:

We use some Components.classes and Components.interfaces stuff to open the local file from within FireFox/Mozilla – we read the file, construct our request body for the POST request and send the whole data with an AJAX “multipart/form-data” request.

NOTE about encoding the local files:

Since we also want to upload binary files, we need to encode (javascript “escape”) the file content. This is basically encoding a string for use in an URL. On the server, after uploading the file we need to decode (“urldecode”) the file. “escape” does not encode the plus sign “+”, but on the server PHP’s “urldecode” interprets any “+” and space. So we need an additional preg_replace to replace any “+” to the HEX value “%2B”.

This is a little annoying, since escaping large files (up to 1MB it is still fast) with javascript can hang the browser for a few seconds. The problem here is that the AJAX object XMLHttpRequest doesn’t seem to be able to handle binary data.

ADVANTAGES:

If you upload images and process them on the server, it is common that the server stops the script due too much memory consumption and/or the runtime limit has been exceeded. In such a case PHP is just returning an error message (“Fatal error: memory limit exceeded” or “Fatal error: running too long” or whatever) and the user usually has to back up with the browser back button to repeat the procedure with a smaller image. With AJAX you can check the returned string for errors and if an error has occured, notify the user gracefully.

A possible extension to this example would be:
Let the user select a directory with a custom “directory-browser” or one file in a directory with the regular file-dialog as shown here, then parse the directory automatically for file with a certain extension and upload them in a bulk.

index.html

<html>
<body>
<script>
var url = "post.php";
var binary;
var filename;
var mytext;
 
function upload() {
	filename = document.getElementById('myfile').value;
	mytext = document.getElementById('mytext').value;
	document.getElementById('ajaxbutton').disabled = true;
 
	// request local file read permission
	try {
		netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
	} catch (e) {
		alert("Permission to read file was denied.");
	}
 
	// open the local file
	var file = Components.classes["@mozilla.org/file/local;1"]
		.createInstance(Components.interfaces.nsILocalFile);
	file.initWithPath( filename );
	stream = Components.classes["@mozilla.org/network/file-input-stream;1"]
		.createInstance(Components.interfaces.nsIFileInputStream);
	stream.init(file,	0x01, 00004, null);
	var bstream =  Components.classes["@mozilla.org/network/buffered-input-stream;1"]
		.getService();
	bstream.QueryInterface(Components.interfaces.nsIBufferedInputStream);
	bstream.init(stream, 1000);
	bstream.QueryInterface(Components.interfaces.nsIInputStream);
	binary = Components.classes["@mozilla.org/binaryinputstream;1"]
		.createInstance(Components.interfaces.nsIBinaryInputStream);
	binary.setInputStream (stream);
 
	// start AJAX file upload in 1 second
	window.setTimeout("ajax_upload()", 1000);
}
 
function ajax_upload() {
	// request more permissions
	try {
		netscape.security.PrivilegeManager.enablePrivilege("UniversalXPConnect");
	} catch (e) {
		alert("Permission to read file was denied.");
	}
 
	http_request = false;
	http_request = new XMLHttpRequest();
	if (!http_request) {
		alert('Cannot create XMLHTTP instance');
		return false;
	}
 
	// prepare the MIME POST data
	var boundaryString = 'capitano';
	var boundary = '--' + boundaryString;
	var requestbody = boundary + '\n'
	+ 'Content-Disposition: form-data; name="mytext"' + '\n'
	+ '\n'
	+ mytext + '\n'
	+ '\n'
	+ boundary + '\n'
	+ 'Content-Disposition: form-data; name="myfile"; filename="'
		+ filename + '"' + '\n'
	+ 'Content-Type: application/octet-stream' + '\n'
	+ '\n'
	+ escape(binary.readBytes(binary.available()))
	+ '\n'
	+ boundary;
 
	document.getElementById('sizespan').innerHTML =
		"requestbody.length=" + requestbody.length;
 
	// do the AJAX request
	http_request.onreadystatechange = requestdone;
	http_request.open('POST', url, true);
	http_request.setRequestHeader("Content-type", "multipart/form-data; \
		boundary=\"" + boundaryString + "\"");
	http_request.setRequestHeader("Connection", "close");
	http_request.setRequestHeader("Content-length", requestbody.length);
	http_request.send(requestbody);
 
}
 
function requestdone() {
	if (http_request.readyState == 4) {
		if (http_request.status == 200) {
			result = http_request.responseText;
			document.getElementById('myspan').innerHTML = result;
		} else {
			alert('There was a problem with the request.');
		}
		document.getElementById('ajaxbutton').disabled = false;
	}
}
 
</script>
 
<form>
Text: <input type="text" id="mytext" name="mytext" size="40">
<br />
File: <input type="file" id="myfile" name="datafile" size="40"><br />
<input type="button" id="ajaxbutton" value="AJAX IT" onclick="upload();">
</form>
 
<div id="sizespan"></div>
<hr />
<div id="myspan"></div>
 
</body>
</html>

post.php

<?
print_r($_FILES);
?>
<hr />
<?
print_r($_POST);
 
$fpath = "/tmp/";
 
// move (actually just rename) the temporary file to the real name
move_uploaded_file ( $_FILES{myfile}{tmp_name}, $fpath.$_FILES{myfile}{name} );
 
// convert the uploaded file back to binary
 
// javascript "escape" does not encode the plus sign "+", but "urldecode"
//	in PHP make a space " ". So replace any "+" in the file with %2B first
 
$filename = $fpath.$_FILES{myfile}{name};
$handle = fopen($filename, "r");
$contents = fread($handle, filesize($filename));
fclose($handle);
 
$contents = preg_replace("/\+/", "%2B", $contents);
 
$handle = fopen($filename, "w");
fwrite($handle, urldecode($contents));
fclose($handle);
 
?>
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3 Comments
  1. vador

    Thank you ! I used your method into a xulrunner application for upload a sqlite database.

  2. forex robot

    good article as usual!

  3. sujan

    This is very Firefox Specific..

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