FileSystemSyncAccessHandle: write() method
Baseline
Widely available
This feature is well established and works across many devices and browser versions. It’s been available across browsers since March 2023.
Secure context: This feature is available only in secure contexts (HTTPS), in some or all supporting browsers.
Note: This feature is only available in Dedicated Web Workers.
The write() method of the
FileSystemSyncAccessHandle interface writes the content of a specified buffer to the file associated with the handle, optionally at a given offset.
Files within the origin private file system are not visible to end-users, therefore are not subject to the same security checks as methods running on files within the user-visible file system. As a result, writes performed using FileSystemSyncAccessHandle.write() are much more performant. This makes them suitable for significant, large-scale file updates such as SQLite database modifications.
Syntax
write(buffer, options)
Parameters
buffer-
An
ArrayBufferorArrayBufferView(such as aDataView) representing the buffer to be written to the file. optionsOptional-
An options object containing the following properties:
at-
A number representing the offset in bytes from the start of the file that the buffer should be written at.
Return value
A number representing the number of bytes written to the file.
Exceptions
InvalidStateErrorDOMException-
Thrown if the associated access handle is already closed, or if the modification of the file's binary data completely fails.
QuotaExceededError-
Thrown if the increased data capacity exceeds the browser's storage quota.
TypeError-
Thrown if the underlying file system does not support writing the file from the specified file offset.
Examples
The following asynchronous event handler function is contained inside a Web Worker. On receiving a message from the main thread it:
- Creates a synchronous file access handle.
- Gets the size of the file and creates an
ArrayBufferto contain it. - Reads the file contents into the buffer.
- Encodes the message and writes it to the end of the file.
- Persists the changes to disk and closes the access handle.
onmessage = async (e) => {
// Retrieve message sent to work from main script
const message = e.data;
// Get handle to draft file
const root = await navigator.storage.getDirectory();
const draftHandle = await root.getFileHandle("draft.txt", { create: true });
// Get sync access handle
const accessHandle = await draftHandle.createSyncAccessHandle();
// Get size of the file.
const fileSize = accessHandle.getSize();
// Read file content to a buffer.
const buffer = new DataView(new ArrayBuffer(fileSize));
const readBuffer = accessHandle.read(buffer, { at: 0 });
// Write the message to the end of the file.
const encoder = new TextEncoder();
const encodedMessage = encoder.encode(message);
const writeBuffer = accessHandle.write(encodedMessage, { at: readBuffer });
// Persist changes to disk.
accessHandle.flush();
// Always close FileSystemSyncAccessHandle if done.
accessHandle.close();
};
Note:
In earlier versions of the spec, close(), flush(), getSize(), and truncate() were wrongly specified as asynchronous methods, and older versions of some browsers implement them in this way. However, all current browsers that support these methods implement them as synchronous methods.
Specifications
| Specification |
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| File System # api-filesystemsyncaccesshandle-write |
Browser compatibility
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Legend
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- Full support
- Full support