Selecting and interacting with files on the user's local device is one of the most commonly used features of the web. It allows users to select files and upload them to a server, for example, when sharing photos or submitting tax documents. It also allows sites to read and manipulate them without ever having to transfer the data across the network. This page walks through how to use JavaScript to interact with files.
The modern File System Access API
The File System Access API provides a way to read from and write to files and directories on the user's local system. It's available in most Chromium-based browsers such as Chrome and Edge. To learn more about it, refer to The File System Access API.
Because the File System Access API isn't compatible with all browsers, we recommend using browser-fs-access, a helper library that uses the new API wherever it's available and falls back to legacy approaches when it isn't.
Work with files, the classic way
This guide shows you how to interact with files using legacy JavaScript methods.
Select files
There are two primary ways of selecting files: using the HTML input element, and using a drag-and-drop zone.
HTML input element
The easiest way for users to select files is using the
<input type="file">
element, which is supported in every major browser. When clicked, it lets a user
select a file, or multiple files if the
multiple
attribute is included, using their operating system's built-in file selection
UI. When the user finishes selecting a file or files, the element's change
event fires. You can access the list of files from event.target.files, which
is a FileList object.
Each item in the FileList is a File object.
<!-- The `multiple` attribute lets users select multiple files. -->
<input type="file" id="file-selector" multiple>
<script>
const fileSelector = document.getElementById('file-selector');
fileSelector.addEventListener('change', (event) => {
const fileList = event.target.files;
console.log(fileList);
});
</script>
The following example lets a user select multiple files using their operating system's built-in file selection UI and then logs each selected file to the console.
Limit the types of files users can select
In some cases, you might want to limit the types of files users can select. For
example, an image editing app should only accept images, not text files. To set
file type restrictions, add an
accept
attribute to the input element to specify which file types are accepted:
<input type="file" id="file-selector" accept=".jpg, .jpeg, .png">
Custom drag-and-drop
In some browsers, the <input type="file"> element is also a drop target,
allowing users to drag-and-drop files into your app. However, this drop target
is small and can be hard to use. Instead, after you provide core features using
an <input type="file"> element, you can provide a large, custom drag-and-drop
surface.
Choose your drop zone
Your drop surface depends on the design of your application. You might only want part of the window to be a drop surface, but you can use the entire window.
The image compression app Squoosh lets user drag an image anywhere into the
window, and click select an image to invoke the <input type="file">
element. Whatever you choose as your drop zone, make sure it's clear to the user
that they can drag files onto that surface.
Define the drop zone
To enable an element as a drag-and-drop zone, create listeners for
two events: dragover
and drop.
The dragover event updates the browser UI to visually indicate that the
drag-and-drop action is creating a copy of the file. The drop event fires
after the user drops the files onto the surface. As with the input element, you
can access the list of files from event.dataTransfer.files, which is a
FileList object. Each
item in the FileList is a File object.
const dropArea = document.getElementById('drop-area');
dropArea.addEventListener('dragover', (event) => {
event.stopPropagation();
event.preventDefault();
// Style the drag-and-drop as a "copy file" operation.
event.dataTransfer.dropEffect = 'copy';
});
dropArea.addEventListener('drop', (event) => {
event.stopPropagation();
event.preventDefault();
const fileList = event.dataTransfer.files;
console.log(fileList);
});
event.stopPropagation()
and event.preventDefault()
stop the browser's default behavior and let your code run instead. Without them,
the browser would otherwise navigate away from your page and open the files
the user dropped into the browser window.
For a live demonstration, refer to Custom drag-and-drop.
What about directories?
Unfortunately, there isn't a good way to access a directory using JavaScript.
The webkitdirectory
attribute on the <input type="file"> element lets the user choose a directory
or directories. It's supported in most major browsers
except for Firefox for Android and Safari on iOS.
If drag-and-drop is enabled, a user might try to drag a directory into the
drop zone. When the drop event fires, it includes a File object for the
directory, but doesn't provide access to any of the files in the directory.
Read file metadata
The File object contains metadata about the file. Most browsers
provide the filename, the size of the file, and the MIME type, though depending
on the platform, different browsers might provide different or additional
information.
function getMetadataForFileList(fileList) {
for (const file of fileList) {
// Not supported in Safari for iOS.
const name = file.name ? file.name : 'NOT SUPPORTED';
// Not supported in Firefox for Android or Opera for Android.
const type = file.type ? file.type : 'NOT SUPPORTED';
// Unknown cross-browser support.
const size = file.size ? file.size : 'NOT SUPPORTED';
console.log({file, name, type, size});
}
}
You can see this in action in the input-type-file
demo.
Read a file's content
Use FileReader to
read the content of a File object into memory. You can tell FileReader to
read a file as an array buffer,
a data URL,
or text:
function readImage(file) {
// Check if the file is an image.
if (file.type && !file.type.startsWith('image/')) {
console.log('File is not an image.', file.type, file);
return;
}
const reader = new FileReader();
reader.addEventListener('load', (event) => {
img.src = event.target.result;
});
reader.readAsDataURL(file);
}
This example reads a File provided by the user, then converts it to a data
URL, and uses that data URL to display the image in an img element.
To learn how to verify that the user has selected an image file, refer to the
read-image-file demo.
Monitor the progress of a file read
When reading large files, it can be helpful to provide some UX to tell the user
how far the read has progressed. For that, use the
progress
event provided by FileReader. The progress event has two properties:
loaded (the amount read) and total (the amount to read).
function readFile(file) {
const reader = new FileReader();
reader.addEventListener('load', (event) => {
const result = event.target.result;
// Do something with result
});
reader.addEventListener('progress', (event) => {
if (event.loaded && event.total) {
const percent = (event.loaded / event.total) * 100;
console.log(`Progress: ${Math.round(percent)}`);
}
});
reader.readAsDataURL(file);
}
Hero image by Vincent Botta from Unsplash