Using files from web applications

Using the File API, which was added to the DOM in HTML5, it's now possible for web content to ask the user to select local files, then read the contents of those files. This selection can be done by either using an HTML <input> element, or by drag and drop.

If you want to use the DOM File API from extensions or other browser chrome code, you can; in fact, there are some additional features to be aware of. See Using the DOM File API in chrome code for details.

Accessing selected file(s)

Consider this HTML:

<input type="file" id="input">

The File API makes it possible to access a FileList containing File objects representing the files selected by the user.

If the user selects just one file, it is then only necessary to consider the first file of the list.

Accessing one selected file using a classical DOM selector:

var selected_file = document.getElementById('input').files[0];

Accessing one selected file using a jQuery selector:

var selected_file = $('#input').get(0).files[0];

If you get a "files is undefined" error: you didn't select the right HTML element, forgetting that a jQuery selector returns a list of matched DOM elements. So get the DOM element to call the "files" method on it.

Accessing selected file(s) on a change event

It is also possible (but not mandatory) to access the FileList through the change event:

<input type="file" id="input" onchange="handleFiles(this.files)">

When the user selects a file, the handleFiles() function gets called with a FileList object containing File objects representing the files selected by the user.

If you want to let the user select multiple files, simply use the multiple attribute on the input element:

<input type="file" id="input" multiple onchange="handleFiles(this.files)">

In this case, the file list passed to the handleFiles() function contains one File object for each file the user selected.

Dynamically adding a change listener

If your input field was created using a JavaScript library such as jQuery, you'll need to use element.addEventListener() to add the change event listener, like this:

var inputElement = document.getElementById("input");
inputElement.addEventListener("change", handleFiles, false);
function handleFiles() {
  var fileList = this.files; /* now you can work with the file list */
}

Note that in this case, the handleFiles() function itself is the event handler, unlike previous examples where it was called by an event handler which passed it a parameter.

Getting information about selected file(s)

The FileList object provided by the DOM lists all the files selected by the user, each specified as a File object. You can determine how many files the user selected by checking the value of the file list's length attribute:

var numFiles = files.length;

Individual File objects can be retrieved by simply accessing the list as an array:

for (var i = 0, numFiles = files.length; i < numFiles; i++) {
  var file = files[i];
  ..
}

This loop iterates over all the files in the file list.

There are three attributes provided by the File object that contain useful information about the file.

name
The file's name as a read-only string. This is just the file name, and does not include any path information.
size
The size of the file in bytes as a read-only 64-bit integer.
type
The MIME type of the file as a read-only string, or "" if the type couldn't be determined.

Example: Showing file(s) size

The following example shows a possible use of the size property:

<!DOCTYPE html>
<html>
<head>
<meta charset="UTF-8">
<title>File(s) size</title>
<script>
function updateSize() {
  var nBytes = 0,
      oFiles = document.getElementById("uploadInput").files,
      nFiles = oFiles.length;
  for (var nFileId = 0; nFileId < nFiles; nFileId++) {
    nBytes += oFiles[nFileId].size;
  }
  var sOutput = nBytes + " bytes";
  // optional code for multiples approximation
  for (var aMultiples = ["KiB", "MiB", "GiB", "TiB", "PiB", "EiB", "ZiB", "YiB"], nMultiple = 0, nApprox = nBytes / 1024; nApprox > 1; nApprox /= 1024, nMultiple++) {
    sOutput = nApprox.toFixed(3) + " " + aMultiples[nMultiple] + " (" + nBytes + " bytes)";
  }
  // end of optional code
  document.getElementById("fileNum").innerHTML = nFiles;
  document.getElementById("fileSize").innerHTML = sOutput;
}
</script>
</head>

<body onload="updateSize();">
<form name="uploadForm">
<p><input id="uploadInput" type="file" name="myFiles" onchange="updateSize();" multiple> selected files: <span id="fileNum">0</span>; total size: <span id="fileSize">0</span></p>
<p><input type="submit" value="Send file"></p>
</form>
</body>
</html>

Using hidden file input elements using the click() method

Starting in Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1), you can hide the admittedly ugly file <input> element and present your own interface for opening the file picker and displaying which file or files the user has selected. You can do this by styling the input element with display:none and calling the click() method on the <input> element.

Consider this HTML:

<input type="file" id="fileElem" multiple accept="image/*" style="display:none" onchange="handleFiles(this.files)">
<a href="#" id="fileSelect">Select some files</a>

The code that handles the click event can look like this:

var fileSelect = document.getElementById("fileSelect"),
  fileElem = document.getElementById("fileElem");

fileSelect.addEventListener("click", function (e) {
  if (fileElem) {
    fileElem.click();
  }
  e.preventDefault(); // prevent navigation to "#"
}, false);

Obviously you can style the new button for opening the file picker as you wish.

Selecting files using drag and drop

You can also let the user drag and drop files into your web application.

The first step is to establish a drop zone. Exactly what part of your content will accept drops may vary depending on the design of your application, but making an element receive drop events is easy:

var dropbox;

dropbox = document.getElementById("dropbox");
dropbox.addEventListener("dragenter", dragenter, false);
dropbox.addEventListener("dragover", dragover, false);
dropbox.addEventListener("drop", drop, false);

In this example, we're turning the element with the ID dropbox into our drop zone. This is done by adding listeners for the dragenter, dragover, and drop events.

We don't actually need to do anything with the dragenter and dragover events in our case, so these functions are both simple. They just stop propagation of the event and prevent the default action from occurring:

function dragenter(e) {
  e.stopPropagation();
  e.preventDefault();
}

function dragover(e) {
  e.stopPropagation();
  e.preventDefault();
} 

The real magic happens in the drop() function:

function drop(e) {
  e.stopPropagation();
  e.preventDefault();

  var dt = e.dataTransfer;
  var files = dt.files;

  handleFiles(files);
}

Here, we retrieve the dataTransfer field from the event, then pull the file list out of it, passing that to handleFiles(). From this point on, handling the files is the same whether the user used the input element or drag and drop.

Example: Showing thumbnails of user-selected images

Let's say you're developing the next great photo-sharing web site, and want to use HTML5 to display thumbnail previews of images before the user actually uploads them. You can establish your input element or drop zone as discussed previously, and have them call a function such as the handleFiles() function below.

function handleFiles(files) {
  for (var i = 0; i < files.length; i++) {
    var file = files[i];
    var imageType = /image.*/;
    
    if (!file.type.match(imageType)) {
      continue;
    }
    
    var img = document.createElement("img");
    img.classList.add("obj");
    img.file = file;
    preview.appendChild(img);
    
    var reader = new FileReader();
    reader.onload = (function(aImg) { return function(e) { aImg.src = e.target.result; }; })(img);
    reader.readAsDataURL(file);
  }
}

Here our loop handling the user-selected files looks at each file's type attribute to see if it's an image file (by doing a regular expression match on the string "image.*"). For each file that is an image, we create a new img element. CSS can be used to establish any pretty borders, shadows, and to specify the size of the image, so that doesn't even need to be done here.

Each image has the CSS class obj added to it, to make them easy to find in the DOM tree. We also add a file attribute to each image specifying the File for the image; this will let us fetch the images for actually uploading later. Finally, we use Node.appendChild() to add the new thumbnail to the preview area of our document.

Then we establish the FileReader to handle actually asynchronously loading the image and attaching it to the img element. After creating the new FileReader object, we set up its onload function, then call readAsDataURL() to start the read operation in the background. When the entire contents of the image file are loaded, they are converted into a data: URL, which is passed to the onload callback. Our implementation of this routine simply sets the img element's src attribute to the loaded image, which results in the image appearing in the thumbnail on the user's screen.

Using object URLs

Gecko 2.0 (Firefox 4 / Thunderbird 3.3 / SeaMonkey 2.1) introduces support for the DOM window.URL.createObjectURL() and window.URL.revokeObjectURL() methods. These let you create simple URL strings that can be used to reference any data that can be referred to using a DOM File object, including local files on the user's computer.

When you have a File object you'd like to reference by URL from HTML, you can create an object URL for it like this:

var objectURL = window.URL.createObjectURL(fileObj);

The object URL is a string identifying the File object. Each time you call window.URL.createObjectURL(), a unique object URL is created, even if you've created an object URL for that file already. Each of these must be released. While they are released automatically when the document is unloaded, if your page uses them dynamically, you should release them explicitly by calling window.URL.revokeObjectURL():

window.URL.revokeObjectURL(objectURL);

Example: Using object URLs to display images

This example uses object URLs to display image thumbnails. In addition, it displays other file information including their names and sizes. You can view the example live.

The HTML that presents the interface looks like this:

<input type="file" id="fileElem" multiple accept="image/*" style="display:none" onchange="handleFiles(this.files)">
<a href="#" id="fileSelect">Select some files</a> 
<div id="fileList">
  <p>No files selected!</p>
</div>

This establishes our file <input> element, as well as a link that invokes the file picker, since we keep the file input hidden to prevent that less-than-attractive UI from being displayed. This is explained in the section Using hidden file input elements using the click() method, as is the method that invokes the file picker.

The handleFiles() method follows:

window.URL = window.URL || window.webkitURL;

var fileSelect = document.getElementById("fileSelect"),
    fileElem = document.getElementById("fileElem"),
    fileList = document.getElementById("fileList");

fileSelect.addEventListener("click", function (e) {
  if (fileElem) {
    fileElem.click();
  }
  e.preventDefault(); // prevent navigation to "#"
}, false);

function handleFiles(files) {
  if (!files.length) {
    fileList.innerHTML = "<p>No files selected!</p>";
  } else {
    var list = document.createElement("ul");
    for (var i = 0; i < files.length; i++) {
      var li = document.createElement("li");
      list.appendChild(li);
      
      var img = document.createElement("img");
      img.src = window.URL.createObjectURL(files[i]);
      img.height = 60;
      img.onload = function(e) {
        window.URL.revokeObjectURL(this.src);
      }
      li.appendChild(img);
      
      var info = document.createElement("span");
      info.innerHTML = files[i].name + ": " + files[i].size + " bytes";
      li.appendChild(info);
    }
    fileList.appendChild(list);
  }
}

This starts by fetching the URL of the <div> with the ID fileList. This is the block into which we'll insert out file list, including thumbmails.

If the FileList object passed to handleFiles() is null, we simply set the inner HTML of the block to display "No files selected!". Otherwise, we start building our file list, as follows:

  1. A new unordered list (<ul>) element is created.
  2. The new list element is inserted into the <div> block by calling its element.appendChild() method.
  3. For each File in the FileList represented by files:
    1. Create a new list item (<li>) element and insert it into the list.
    2. Create a new image (<img>) element.
    3. Set the image's source to a new object URL representing the file, using window.URL.createObjectURL() to create the blob URL.
    4. Set the image's height to 60 pixels.
    5. Set up the image's load event handler to release the object URL, since it's no longer needed once the image has been loaded. This is done by calling the window.URL.revokeObjectURL() method, passing in the object URL string as specified by img.src.
    6. Append the new list item to the list.

Example: Uploading a user-selected file

Another thing you might want to do is let the user upload the selected file or files (such as the images selected using the previous example) to a server. This can be done asynchronously very easily.

Creating the upload tasks

Continuing with the code that builds the thumbnails in the previous example, recall that every thumbnail image is in the CSS class obj, with the corresponding File attached in a file attribute. This lets us very easily select all the images the user has chosen for uploading using Document.querySelectorAll(), like this:

function sendFiles() {
  var imgs = document.querySelectorAll(".obj");
  
  for (var i = 0; i < imgs.length; i++) {
    new FileUpload(imgs[i], imgs[i].file);
  }
}

Line 2 fetches a NodeList, called imgs, of all the elements in the document with the CSS class obj. In our case, these will be all the image thumbnails. Once we have that list, it's trivial to go through the list, creating a new FileUpload instance for each. Each of these handles uploading the corresponding file.

Handling the upload process for a file

The FileUpload function accepts two inputs: an image element and a file from which to read the image data.

function FileUpload(img, file) {
  var reader = new FileReader();  
  this.ctrl = createThrobber(img);
  var xhr = new XMLHttpRequest();
  this.xhr = xhr;
  
  var self = this;
  this.xhr.upload.addEventListener("progress", function(e) {
        if (e.lengthComputable) {
          var percentage = Math.round((e.loaded * 100) / e.total);
          self.ctrl.update(percentage);
        }
      }, false);
  
  xhr.upload.addEventListener("load", function(e){
          self.ctrl.update(100);
          var canvas = self.ctrl.ctx.canvas;
          canvas.parentNode.removeChild(canvas);
      }, false);
  xhr.open("POST", "http://demos.hacks.mozilla.org/paul/demos/resources/webservices/devnull.php");
  xhr.overrideMimeType('text/plain; charset=x-user-defined-binary');
  reader.onload = function(evt) {
    xhr.sendAsBinary(evt.target.result);
  };
  reader.readAsBinaryString(file);
}

The FileUpload() function shown above creates a throbber, which is used to display progress information, then creates an XMLHttpRequest to handle uploading the data.

Before actually transferring the data, several preparatory steps are taken:

  1. The XMLHttpRequest's upload progress listener is set to update the throbber with new percentage information, so that as the upload progresses, the throbber will be updated based on the latest information.
  2. The XMLHttpRequest's upload load event handler is set to update the throbber with 100% as the progress information (to ensure the progress indicator actually reaches 100%, in case of granularity quirks during the process). It then removes the throbber, since it's no longer needed. This causes the throbber to disappear once the upload is complete.
  3. The request to upload the image file is opened by calling XMLHttpRequest's open() method to start generating a POST request.
  4. The MIME type for the upload is set by calling the XMLHttpRequest function overrideMimeType(). In this case, we're using a generic MIME type; you may or may not need to set the MIME type at all, depending on your use case.
  5. The FileReader object is used to convert the file to a binary string.
  6. Finally, when the content is loaded the XMLHttpRequest function sendAsBinary() is called to upload the file's content.

Handling the upload process for a file, asynchronously

<?php
if (isset($_FILES['myFile'])) {
    // Example:
    move_uploaded_file($_FILES['myFile']['tmp_name'], "uploads/" . $_FILES['myFile']['name']);
    exit;
}
?><!DOCTYPE html>
<html>
<head>
    <title>dnd binary upload</title>
    <meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
    <script type="text/javascript">
        function sendFile(file) {
            var uri = "/index.php";
            var xhr = new XMLHttpRequest();
            var fd = new FormData();
            
            xhr.open("POST", uri, true);
            xhr.onreadystatechange = function() {
                if (xhr.readyState == 4 && xhr.status == 200) {
                    // Handle response.
                    alert(xhr.responseText); // handle response.
                }
            };
            fd.append('myFile', file);
            // Initiate a multipart/form-data upload
            xhr.send(fd);
        }

        window.onload = function() {
            var dropzone = document.getElementById("dropzone");
            dropzone.ondragover = dropzone.ondragenter = function(event) {
                event.stopPropagation();
                event.preventDefault();
            }
    
            dropzone.ondrop = function(event) {
                event.stopPropagation();
                event.preventDefault();

                var filesArray = event.dataTransfer.files;
                for (var i=0; i<filesArray.length; i++) {
                    sendFile(filesArray[i]);
                }
            }
        }
    </script>
</head>
<body>
    <div>
        <div id="dropzone" style="margin:30px; width:500px; height:300px; border:1px dotted grey;">Drag & drop your file here...</div>
    </div>
</body>
</html>

Example: Using object URLs to display PDF

Object URLs can be used for other things than just images! They can be used to display embedded PDF files, or any other resources that can be displayed by the browser.

In Firefox, to have the PDF appear embedded in the iframe, and not proposed as a downloaded file, the preference pdfjs.disabled must be set to false .

<iframe id="viewer">

And here is the change of the src attribute:

var obj_url = window.URL.createObjectURL(blob);
var iframe = document.getElementById('viewer');
iframe.setAttribute('src', obj_url);
window.URL.revokeObjectURL(obj_url);

Example: Using object URLs with other file types

You can manipulate files of other formats the same way. Here is how to preview uploaded video:

var video = document.getElementById('video');
var obj_url = window.URL.createObjectURL(blob);
video.src = obj_url;
video.play()
window.URL.revokeObjectURL(obj_url);

Specifications

See also