Using files from web applications

  • Revision slug: Using_files_from_web_applications
  • Revision title: Using files from web applications
  • Revision id: 36072
  • Created:
  • Creator: NickLothian
  • Is current revision? No
  • Comment fixing https://bugzilla.mozilla.org/show_bug.cgi?id=546179; 8 words added, 8 words removed

Revision Content

{{ gecko_minversion_header("1.9.2") }}

Using the File API 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.

Selecting files using HTML

Selecting a single file for use with the File API is simple:

<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 the File object representing the file 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="true" 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 {{ domxref("element.addEventListener") }} to add the change event handler, like this:

var inputElement = document.getElementById("inputField");
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 looks up the file list instead of accepting a parameter, since event listeners added in this way can't accept an input parameter.

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.

Getting information about selected files

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; i < files.length; 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 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. Simply 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.onloadend = (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 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 onloadend 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 onloadend 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.

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 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 creates an array, 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) {
  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');
  xhr.sendAsBinary(file.getAsBinary());
}

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. Finally, the XMLHttpRequest function sendAsBinary() is called to upload the file's content. This needs to be revised; currently using the deprecated synchronous getAsBinary() routine to pull the data from the file.

See also

Revision Source

<p>{{ gecko_minversion_header("1.9.2") }}</p>
<p>Using the File API 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 <a href="/en/DOM/Input" title="en/DOM/Input"><code>input</code></a> element, or by drag and drop.</p>
<h2>Selecting files using HTML</h2>
<p>Selecting a single file for use with the File API is simple:</p>
<pre><code>&lt;input type="file" id="input" onchange="handleFiles(this.files)" /&gt;</code>
</pre>
<p>When the user selects a file, the <code>handleFiles()</code> function gets called with a <a href="/en/DOM/FileList" title="en/DOM/FileList"><code>FileList</code></a> object containing the <a href="/en/DOM/File" title="en/DOM/File"><code>File</code></a> object representing the file selected by the user.</p>
<p>If you want to let the user select multiple files, simply use the <code>multiple</code> attribute on the <code>input</code> element:</p>
<pre><code>&lt;input type="file" id="input" multiple="true" onchange="handleFiles(this.files)" /&gt;</code>
</pre>
<p>In this case, the file list passed to the <code>handleFiles()</code> function contains one <a href="/en/DOM/File" title="en/DOM/File"><code>File</code></a> object for each file the user selected.</p>
<h3>Dynamically adding a change listener</h3>
<p>If your input field was created using a JavaScript library such as <a class=" external" href="http://www.jquery.com/" title="http://www.jquery.com/">jQuery</a>, you'll need to use {{ domxref("element.addEventListener") }} to add the change event handler, like this:</p>
<pre class="brush: js">var inputElement = document.getElementById("inputField");
inputElement.addEventListener("change", handleFiles, false);

function handleFiles() {
  var fileList = this.files;

  /* now you can work with the file list */
}
</pre>
<p>Note that in this case, the <code>handleFiles()</code> function looks up the file list instead of accepting a parameter, since event listeners added in this way can't accept an input parameter.</p>
<h2>Selecting files using drag and drop</h2>
<p>You can also let the user drag and drop files into your web application.</p>
<p>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:</p>
<pre class="brush: js">var dropbox;

dropbox = document.getElementById("dropbox");
dropbox.addEventListener("dragenter", dragenter, false);
dropbox.addEventListener("dragover", dragover, false);
dropbox.addEventListener("drop", drop, false);
</pre>
<p>In this example, we're turning the element with the ID "dropbox" into our drop zone. This is done by adding listeners for the <code>dragenter</code>, <code>dragover</code>, and <code>drop</code> events.</p>
<p>We don't actually need to do anything with the <code>dragenter</code> and <code>dragover</code> events in our case, so these functions are both simple. They just stop propagation of the event and prevent the default action from occurring:</p>
<pre class="brush: js">function dragenter(e) {
  e.stopPropagation();
  e.preventDefault();
}

function dragover(e) {
  e.stopPropagation();
  e.preventDefault();
} 
</pre>
<p>The real magic happens in the <code>drop()</code> function:</p>
<pre class="brush: js">function drop(e) {
  e.stopPropagation();
  e.preventDefault();

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

  handleFiles(files);
}
</pre>
<p>Here, we retrieve the <code>dataTransfer</code> field from the event, then pull the file list out of it, passing that to <code>handleFiles()</code>. From this point on, handling the files is the same whether the user used the <code>input</code> element or drag and drop.</p>
<h2>Getting information about selected files</h2>
<p>The <a href="/en/DOM/FileList" title="en/DOM/FileList"><code>FileList</code></a> object provided by the DOM lists all the files selected by the user, each specified as a <a href="/en/DOM/File" title="en/DOM/File"><code>File</code></a> object. You can determine how many files the user selected by checking the value of the file list's <code>length</code> attribute:</p>
<pre>var numFiles = files.length;
</pre>
<p>Individual <a href="/en/DOM/File" title="en/DOM/File"><code>File</code></a> objects can be retrieved by simply accessing the list as an array:</p>
<pre class="brush: js">for (var i = 0; i &lt; files.length; i++) {
  var file = files[i];
  ..
}
</pre>
<p>This loop iterates over all the files in the file list.</p>
<p>There are three attributes provided by the <a href="/en/DOM/File" title="en/DOM/File"><code>File</code></a> object that contain useful information about the file.</p>
<dl> <dt><code>name</code></dt> <dd>The file's name as a read-only string. This is just the file name, and does not include any path information.</dd> <dt><code>size</code></dt> <dd>The size of the file in bytes as a read-only 64-bit integer.</dd> <dt><code>type</code></dt> <dd>The MIME type of the file as a read-only string, or "" if the type couldn't be determined.</dd>
</dl>
<h2>Example: Showing thumbnails of user-selected images</h2>
<p>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. Simply establish your input element or drop zone as discussed previously, and have them call a function such as the <code>handleFiles()</code> function below.</p>
<pre class="brush: js">function handleFiles(files) {
  for (var i = 0; i &lt; 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.onloadend = (function(aImg) { return function(e) { aImg.src = e.target.result; }; })(img);
    reader.readAsDataURL(file);
  }
}
</pre>
<p>Here our loop handling the user-selected files looks at each file's <code>type</code> 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 <code>img</code> 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.</p>
<p>Each image has the CSS class "obj" added to it, to make them easy to find in the DOM tree. We also add a <code>file</code> attribute to each image specifying the <a href="/en/DOM/File" title="en/DOM/File"><code>File</code></a> for the image; this will let us fetch the images for actually uploading later. Finally, we use <a href="/En/DOM/Node.appendChild" title="en/DOM/element.appendChild"><code>appendChild()</code></a> to add the new thumbnail to the preview area of our document.</p>
<p>Then we establish the <a href="/en/DOM/FileReader" title="en/DOM/FileReader"><code>FileReader</code></a> to handle actually asynchronously loading the image and attaching it to the <code>img</code> element. After creating the new <code>FileReader</code> object, we set up its <code>onloadend</code> function, then call <code>readAsDataURL()</code> to start the read operation in the background. When the entire contents of the image file are loaded, they are converted into a <code>data:</code> URL, which is passed to the <code>onloadend</code> callback. Our implementation of this routine simply sets the <code>img</code> element's <code>src</code> attribute to the loaded image, which results in the image appearing in the thumbnail on the user's screen.</p>
<h2>Example: Uploading a user-selected file</h2>
<p>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.</p>
<h3>Creating the upload tasks</h3>
<p>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 <a href="/en/DOM/File" title="en/DOM/File"><code>File</code></a> attached in a <code>file</code> attribute. This lets us very easily select all the images the user has chosen for uploading using <a href="/En/DOM/Document.querySelectorAll" title="En/DOM/Document.querySelectorAll"><code>querySelectorAll()</code></a>, like this:</p>
<pre class="brush: js">function sendFiles() {
  var imgs = document.querySelectorAll(".obj");
  
  for (var i = 0; i &lt; imgs.length; i++) {
    new FileUpload(imgs[i], imgs[i].file);
  }
}
</pre>
<p>Line 2 creates an array, called <code>imgs</code>, 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 <code>FileUpload</code> instance for each. Each of these handles uploading the corresponding file.</p>
<h3>Handling the upload process for a file</h3>
<p>The <code>FileUpload</code> function accepts two inputs: an image element and a file from which to read the image data.</p>
<pre class="brush: js">function FileUpload(img, file) {
  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');
  xhr.sendAsBinary(file.getAsBinary());
}
</pre>
<p>The <code>FileUpload()</code> function shown above creates a throbber, which is used to display progress information, then creates an <a href="/en/XMLHttpRequest" title="en/XMLHttpRequest"><code>XMLHttpRequest</code></a> to handle uploading the data.</p>
<p>Before actually transferring the data, several preparatory steps are taken:</p>
<ol> <li>The <code>XMLHttpRequest</code>'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.</li> <li>The <code>XMLHttpRequest</code>'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.</li> <li>The request to upload the image file is opened by calling <code>XMLHttpRequest</code>'s <code>open()</code> method to start generating a POST request.</li> <li>The MIME type for the upload is set by calling the <code>XMLHttpRequest</code> function <code>overrideMimeType()</code>. 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.</li> <li>Finally, the <code>XMLHttpRequest</code> function <code>sendAsBinary()</code> is called to upload the file's content. <em>This needs to be revised; currently using the deprecated synchronous getAsBinary() routine to pull the data from the file.</em></li>
</ol>
<h2>See also</h2>
<ul> <li><code><a href="/en/DOM/File" title="en/DOM/File">File</a><br> </code></li> <li><code><a href="/en/DOM/FileList" title="en/DOM/FileList">FileList</a><br> </code></li> <li><a href="/en/DOM/FileReader" title="en/DOM/FileReader"><code>FileReader</code></a></li> <li><a href="/En/Using_XMLHttpRequest" title="En/Using XMLHttpRequest">Using XMLHttpRequest</a></li> <li><a href="/en/XMLHttpRequest" title="en/XMLHttpRequest"><code>XMLHttpRequest</code></a></li>
</ul>
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