Reading files in JavaScript using the file APIs
By Eric Bidelman
Originally published June 18, 2010
Summary
An introduction to HTML5 JavaScript file API.
Introduction
HTML5 finally provides a standard way to interact with local files, via the File API specification. As examples of its capabilities, the File API could be used to create a thumbnail preview of images as they’re sent to the server, or to allow an app to save a file reference while the user is offline. Additionally, you could use client-side logic to verify that an upload’s MIME type matches its file extension or to restrict the size of an upload.
The specification provides several interfaces for accessing files from a local filesystem:
File- an individual file; provides read-only information such as name, file size, MIME type, and a reference to the file handle.FileList- an array-like sequence ofFileobjects. (Think<input type="file" multiple>or dragging a directory of files from the desktop.)Blob- Allows for slicing a file into byte ranges.
When used in conjunction with the above data structures, the FileReader interface can be used to asynchronously read a file through familiar JavaScript event handling. Thus, it is possible to monitor the progress of a read, catch errors, and determine when a load is complete. In many ways the APIs resemble XMLHttpRequest's event model.
Selecting files
The first thing to do is check that your browser fully supports the File API:
// Check for the various File API support.
if (window.File && window.FileReader && window.FileList && window.Blob) {
// Great success! All the File APIs are supported.
} else {
alert('The File APIs are not fully supported in this browser.');
}
Of course, if your app will only use some of these APIs, modify this snippet accordingly.
Using form input for selecting
The most straightforward way to load a file is to use a standard <input type="file"> element. JavaScript returns the list of selected File objects as a FileList. Here’s an example that uses the multiple attribute to allow selecting several files at once:
<input type="file" id="files" name="files[]" multiple />
<output id="list"></output>
<script>
function handleFileSelect(evt) {
var files = evt.target.files; // FileList object
// files is a FileList of File objects. List some properties.
var output = [];
for (var i = 0, f; f = files[i]; i++) {
output.push('<li><strong>', escape(f.name), '</strong> (', f.type || 'n/a', ') - ',
f.size, ' bytes, last modified: ',
f.lastModifiedDate ? f.lastModifiedDate.toLocaleDateString() : 'n/a',
'</li>');
}
document.getElementById('list').innerHTML = '<ul>' + output.join('') + '</ul>';
}
document.getElementById('files').addEventListener('change', handleFileSelect, false);
</script>
Example: Using form input for selecting. Try it here!
Using drag and drop for selecting
Another technique for loading files is native drag and drop from the desktop to the browser. We can modify the previous example slightly to include drag and drop support.
<div id="drop_zone">Drop files here</div>
<output id="list"></output>
<script>
function handleFileSelect(evt) {
evt.stopPropagation();
evt.preventDefault();
var files = evt.dataTransfer.files; // FileList object.
// files is a FileList of File objects. List some properties.
var output = [];
for (var i = 0, f; f = files[i]; i++) {
output.push('<li><strong>', escape(f.name), '</strong> (', f.type || 'n/a', ') - ',
f.size, ' bytes, last modified: ',
f.lastModifiedDate ? f.lastModifiedDate.toLocaleDateString() : 'n/a',
'</li>');
}
document.getElementById('list').innerHTML = '<ul>' + output.join('') + '</ul>';
}
function handleDragOver(evt) {
evt.stopPropagation();
evt.preventDefault();
evt.dataTransfer.dropEffect = 'copy'; // Explicitly show this is a copy.
}
// Setup the dnd listeners.
var dropZone = document.getElementById('drop_zone');
dropZone.addEventListener('dragover', handleDragOver, false);
dropZone.addEventListener('drop', handleFileSelect, false);
</script>
Example: Using drag and drop for selecting. Try it here!
Note: Some browsers treat <input type="file"> elements as native drop targets. Try dragging files onto the input field in the previous example.
Reading files
Now comes the fun part!
After you’ve obtained a File reference, instantiate a FileReader object to read its contents into memory. When the load finishes, the reader’s onload event is fired and its result attribute can be used to access the file data.
FileReader includes four options for reading a file, asynchronously:
FileReader.readAsBinaryString(Blob|File)- Theresultproperty will contain the file/blob’s data as a binary string. Every byte is represented by an integer in the range [0…255].FileReader.readAsText(Blob|File, opt_encoding)- Theresultproperty will contain the file/blob’s data as a text string. By default the string is decoded as UTF-8. Use the optional encoding parameter can specify a different format.FileReader.readAsDataURL(Blob|File)- Theresultproperty will contain the file/blob’s data encoded as a data URL.FileReader.readAsArrayBuffer(Blob|File)- Theresultproperty will contain the file/blob’s data as an ArrayBuffer object.
Once one of these read methods is called on your FileReader object, the onloadstart, onprogress, onload, onabort, onerror, and onloadend events can be used to track its progress.
The example below filters out images from the user’s selection, calls reader.readAsDataURL() on the file, and renders a thumbnail by setting the src attribute to a data URL.
<style>
.thumb {
height: 75px;
border: 1px solid #000;
margin: 10px 5px 0 0;
}
</style>
<input type="file" id="files" name="files[]" multiple />
<output id="list"></output>
<script>
function handleFileSelect(evt) {
var files = evt.target.files; // FileList object
// Loop through the FileList and render image files as thumbnails.
for (var i = 0, f; f = files[i]; i++) {
// Only process image files.
if (!f.type.match('image.*')) {
continue;
}
var reader = new FileReader();
// Closure to capture the file information.
reader.onload = (function(theFile) {
return function(e) {
// Render thumbnail.
var span = document.createElement('span');
span.innerHTML = ['<img class="thumb" src="', e.target.result,
'" title="', escape(theFile.name), '"/>'].join('');
document.getElementById('list').insertBefore(span, null);
};
})(f);
// Read in the image file as a data URL.
reader.readAsDataURL(f);
}
}
document.getElementById('files').addEventListener('change', handleFileSelect, false);
</script>
Example: Reading files. Try it here!
Slicing a file
In some cases reading the entire file into memory isn’t the best option. For example, say you wanted to write an asynchronous file uploader. One possible way to speed up the upload would be to read and send the file in separate byte range chunks. The server component would then be responsible for reconstructing the file content in the correct order.
Luckily for us, the File interface supports a slice method to support this use case. The method takes a starting byte as its first argument, an ending byte as its second, and an optional content type string as a third.
var blob = file.slice(<var>startingByte</var>, <var>endindByte</var>);
reader.readAsBinaryString(blob);
The following example demonstrates reading chunks of a file. Something worth noting is that it uses the onloadend and checks the evt.target.readyState instead of using the onload event.
<style>
#byte_content {
margin: 5px 0;
max-height: 100px;
overflow-y: auto;
overflow-x: hidden;
}
#byte_range { margin-top: 5px; }
</style>
<input type="file" id="files" name="file" /> Read bytes:
<span class="readBytesButtons">
<button data-startbyte="0" data-endbyte="4">1-5</button>
<button data-startbyte="5" data-endbyte="14">6-15</button>
<button data-startbyte="6" data-endbyte="7">7-8</button>
<button>entire file</button>
</span>
<div id="byte_range"></div>
<div id="byte_content"></div>
<script>
function readBlob(opt_startByte, opt_stopByte) {
var files = document.getElementById('files').files;
if (!files.length) {
alert('Please select a file!');
return;
}
var file = files[0];
var start = parseInt(opt_startByte) || 0;
var stop = parseInt(opt_stopByte) || file.size - 1;
var reader = new FileReader();
// If we use onloadend, we need to check the readyState.
reader.onloadend = function(evt) {
if (evt.target.readyState == FileReader.DONE) { // DONE == 2
document.getElementById('byte_content').textContent = evt.target.result;
document.getElementById('byte_range').textContent =
['Read bytes: ', start + 1, ' - ', stop + 1,
' of ', file.size, ' byte file'].join('');
}
};
var blob = file.slice(start, stop + 1);
reader.readAsBinaryString(blob);
}
document.querySelector('.readBytesButtons').addEventListener('click', function(evt) {
if (evt.target.tagName.toLowerCase() == 'button') {
var startByte = evt.target.getAttribute('data-startbyte');
var endByte = evt.target.getAttribute('data-endbyte');
readBlob(startByte, endByte);
}
}, false);
</script>
Example: Slicing a file. Try it here!
Monitoring the progress of a read
One of the nice things that we get for free when using async event handling is the ability to monitor the progress of the file read; useful for large files, catching errors, and figuring out when a read is complete. The onloadstart and onprogress events can be used to monitor the progress of a read.
The example below demonstrates displaying a progress bar to monitor the status of a read. To see the progress indicator in action, try a large file or one from a remote drive.
<style>
#progress_bar {
margin: 10px 0;
padding: 3px;
border: 1px solid #000;
font-size: 14px;
clear: both;
opacity: 0;
-moz-transition: opacity 1s linear;
-o-transition: opacity 1s linear;
-webkit-transition: opacity 1s linear;
}
#progress_bar.loading {
opacity: 1.0;
}
#progress_bar .percent {
background-color: #99ccff;
height: auto;
width: 0;
}
</style>
<input type="file" id="files" name="file" />
<button onclick="abortRead();">Cancel read</button>
<div id="progress_bar"><div class="percent">0%</div></div>
<script>
var reader;
var progress = document.querySelector('.percent');
function abortRead() {
reader.abort();
}
function errorHandler(evt) {
switch(evt.target.error.code) {
case evt.target.error.NOT_FOUND_ERR:
alert('File Not Found!');
break;
case evt.target.error.NOT_READABLE_ERR:
alert('File is not readable');
break;
case evt.target.error.ABORT_ERR:
break; // noop
default:
alert('An error occurred reading this file.');
};
}
function updateProgress(evt) {
// evt is an ProgressEvent.
if (evt.lengthComputable) {
var percentLoaded = Math.round((evt.loaded / evt.total) * 100);
// Increase the progress bar length.
if (percentLoaded < 100) {
progress.style.width = percentLoaded + '%';
progress.textContent = percentLoaded + '%';
}
}
}
function handleFileSelect(evt) {
// Reset progress indicator on new file selection.
progress.style.width = '0%';
progress.textContent = '0%';
reader = new FileReader();
reader.onerror = errorHandler;
reader.onprogress = updateProgress;
reader.onabort = function(e) {
alert('File read cancelled');
};
reader.onloadstart = function(e) {
document.getElementById('progress_bar').className = 'loading';
};
reader.onload = function(e) {
// Ensure that the progress bar displays 100% at the end.
progress.style.width = '100%';
progress.textContent = '100%';
setTimeout("document.getElementById('progress_bar').className='';", 2000);
}
// Read in the image file as a binary string.
reader.readAsBinaryString(evt.target.files[0]);
}
document.getElementById('files').addEventListener('change', handleFileSelect, false);
</script>
Example: Monitoring the progress of a read. Try it here!
References
- File API specification
- FileReader interface specification
- Blob interface specification
- FileError interface specification
- ProgressEvent specification
Attributions
Portions of this content come from HTML5Rocks! article