How to Asynchronously Upload Files Using HTML5 and Ajax

Key Takeaways

• Asynchronous file uploads using HTML5 and Ajax can occur in the background, allowing users to complete other tasks on the page while the upload is in process. The process involves creating an HTML form to upload files to a PHP page, with JavaScript ensuring only JPG images under 300,000 bytes are uploaded.
• The JavaScript upload function requires the XMLHttpRequest2 object, available in Firefox and Chrome. The XMLHttpRequest .open() method is set to POST data to the PHP page, setting an HTTP header to the file’s name and passing the File object to the .send() method.
• The PHP file checks for the X_FILENAME HTTP header to differentiate between Ajax requests and standard form POSTs. If a filename is set, PHP retrieves the posted data and outputs it to a new file in an ‘uploads’ folder. Standard HTML multipart/form-data posts are handled using the usual PHP $_FILE functions.

In my previous posts, we discovered How to Use HTML5 File Drag & Drop, and Open Files Using HTML5 and JavaScript. Now we have a valid set of files, it possible to upload each one to the server. The process occurs asynchronously in the background so the user can complete other on-page tasks while it occurs.

The HTML

Let’s examine our HTML form again:

<form id="upload" action="upload.php" method="POST" enctype="multipart/form-data">

<fieldset>
<legend>HTML File Upload</legend>

<input type="hidden" id="MAX_FILE_SIZE" name="MAX_FILE_SIZE" value="300000" />

<div>
	<label for="fileselect">Files to upload:</label>
	<input type="file" id="fileselect" name="fileselect[]" multiple="multiple" />
	<div id="filedrag">or drop files here</div>
</div>

<div id="submitbutton">
	<button type="submit">Upload Files</button>
</div>

</fieldset>

</form>

We’ll be uploading files to a PHP page, upload.php. The page will handle both the Ajax upload requests and standard form POSTs when the user clicks “Upload Files”.

Our JavaScript will ensure that only JPG images are uploaded which are smaller than 300,000 bytes — the value specified in MAX_FILE_SIZE.

The JavaScript

First, we require an additional line within our FileSelectHandler() function which is called when one or more files is chosen or dropped. Within our File loop, we’ll call an additional function — UploadFile():

// file selection
function FileSelectHandler(e) {

	// cancel event and hover styling
	FileDragHover(e);

	// fetch FileList object
	var files = e.target.files || e.dataTransfer.files;

	// process all File objects
	for (var i = 0, f; f = files[i]; i++) {
		ParseFile(f);
		UploadFile(f);
	}

}

File uploading requires the XMLHttpRequest2 object which is currently available in Firefox and Chrome. Before we make the Ajax call, we ensure an .upload() method is available and that we have a JPG with a file size less than the MAX_FILE_SIZE form value:

// upload JPEG files
function UploadFile(file) {

	var xhr = new XMLHttpRequest();
	if (xhr.upload && file.type == "image/jpeg" && file.size <= $id("MAX_FILE_SIZE").value) {

The XMLHttpRequest .open() method is set to POST data to upload.php, the action attribute of our upload form. In addition, we set an HTTP header to the file’s name and pass the File object to the .send() method:

		// start upload
		xhr.open("POST", $id("upload").action, true);
		xhr.setRequestHeader("X_FILENAME", file.name);
		xhr.send(file);

	}

}

The PHP

Our PHP file, upload.php, now checks for the X_FILENAME HTTP header to differentiate between Ajax requests and standard form POSTs:

<?php
$fn = (isset($_SERVER['HTTP_X_FILENAME']) ? $_SERVER['HTTP_X_FILENAME'] : false);

If a filename has been set, PHP can retrieve the posted data and output it to a new file in an ‘uploads’ folder. Amazingly, this can be achieved in a single line of code:

if ($fn) {

	// AJAX call
	file_put_contents(
		'uploads/' . $fn,
		file_get_contents('php://input')
	);
	echo "$fn uploaded";
	exit();
	
}

Standard HTML multipart/form-data posts can be handled using the usual PHP $_FILE functions:

else {

	// form submit
	$files = $_FILES['fileselect'];

	foreach ($files['error'] as $id => $err) {
		if ($err == UPLOAD_ERR_OK) {
			$fn = $files['name'][$id];
			move_uploaded_file(
				$files['tmp_name'][$id],
				'uploads/' . $fn
			);
			echo "<p>File $fn uploaded.</p>";
		}
	}

}

You can view the demonstration page, however, please note it is hosted on a server without PHP support and the upload will not occur. Therefore, please download the files to examine the code and install it on your own PHP server.

The code above will work, but the user won’t know whether a file upload has started, finished or failed. You need to read the final instalment in this series: How to Create File Upload Progress Bars in HTML5 and JavaScript…
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Frequently Asked Questions (FAQs) about HTML5 AJAX File Upload