---
description: Use WritableStreamDefaultWriter in Workers to write data directly to a WritableStream.
title: WritableStream DefaultWriter
image: https://developers.cloudflare.com/og-docs.png
---

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# WritableStream DefaultWriter

Last updated Aug 25, 2026|Copy as Markdown|[View as Markdown](https://66f4ea29.previews.developers.cloudflare.com/workers/runtime-apis/streams/writablestreamdefaultwriter/index.md)|[Agent setup](https://66f4ea29.previews.developers.cloudflare.com/agent-setup/)

## Background

A writer is used when you want to write directly to a [WritableStream](https://66f4ea29.previews.developers.cloudflare.com/workers/runtime-apis/streams/writablestream/), rather than piping data to it from a [ReadableStream](https://66f4ea29.previews.developers.cloudflare.com/workers/runtime-apis/streams/readablestream/). For example:

```js
function writeArrayToStream(array, writableStream) {
  const writer = writableStream.getWriter();
  array.forEach(chunk => writer.write(chunk).catch(() => {}));

  return writer.close();
}

writeArrayToStream([1, 2, 3, 4, 5], writableStream)
  .then(() => console.log('All done!'))
  .catch(e => console.error('Error with the stream: ' + e));
```

## Properties

* `writer.desiredSize` int

  * The size needed to fill the stream’s internal queue, as an integer. Always returns 1, 0 (if the stream is closed), or `null` (if the stream has errors).
* `writer.closed` Promise<void>

  * A promise that indicates if the writer is closed. The promise is fulfilled when the writer stream is closed and rejected if there is an error in the stream.

## Methods

* `abort(reasonstringoptional)` : Promise<void>

  * Aborts the stream. This method returns a promise that fulfills with a response `undefined`. `reason` is an optional human-readable string indicating the reason for cancellation. `reason` will be passed to the underlying sink’s abort algorithm. If this writable stream is one side of a [TransformStream](https://66f4ea29.previews.developers.cloudflare.com/workers/runtime-apis/streams/transformstream/), then its abort algorithm causes the transform’s readable side to become errored with `reason`.  
Warning  
Any data not yet written is lost upon abort.
* `close()` : Promise<void>

  * Attempts to close the writer. Remaining writes finish processing before the writer is closed. This method returns a promise fulfilled with `undefined` if the writer successfully closes and processes the remaining writes, or rejected on any error.
* `releaseLock()` : void

  * Releases the writer’s lock on the stream. Once released, the writer is no longer active. You can call this method before all pending `write(chunk)` calls are resolved. This allows you to queue a `write` operation, release the lock, and begin piping into the writable stream from another source, as shown in the example below.

```js
let writer = writable.getWriter();
// Write a preamble.
writer.write(new TextEncoder().encode('foo bar'));
// While that’s still writing, pipe the rest of the body from somewhere else.
writer.releaseLock();
await someResponse.body.pipeTo(writable);
```

* `write(chunkany)` : Promise<void>

  * Writes a chunk of data to the writer and returns a promise that resolves if the operation succeeds.
  * The underlying stream may accept fewer kinds of type than `any`, it will throw an exception when encountering an unexpected type.

---

## Related resources

* [Streams](https://66f4ea29.previews.developers.cloudflare.com/workers/runtime-apis/streams/)
* [Writable streams in the WHATWG Streams API specification ↗](https://streams.spec.whatwg.org/#ws-model)

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