---
title: ReadableStream BYOBReader
description: Use ReadableStreamBYOBReader in Workers to read streamed data into your own buffer.
image: https://developers.cloudflare.com/dev-products-preview.png
---

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# ReadableStream BYOBReader

## Background

`BYOB` is an abbreviation of bring your own buffer. A `ReadableStreamBYOBReader` allows reading into a developer-supplied buffer, thus minimizing copies.

An instance of `ReadableStreamBYOBReader` is functionally identical to [ReadableStreamDefaultReader](https://developers.cloudflare.com/workers/runtime-apis/streams/readablestreamdefaultreader/) with the exception of the `read` method.

A `ReadableStreamBYOBReader` is not instantiated via its constructor. Rather, it is retrieved from a [ReadableStream](https://developers.cloudflare.com/workers/runtime-apis/streams/readablestream/):

JavaScript

```

const { readable, writable } = new TransformStream();

const reader = readable.getReader({ mode: 'byob' });


```

---

## Methods

* `read(bufferArrayBufferView)` : Promise<ReadableStreamBYOBReadResult>  
   * Returns a promise with the next available chunk of data read into a passed-in buffer.
* `readAtLeast(minElements, bufferArrayBufferView)` : Promise<ReadableStreamBYOBReadResult>  
   * Returns a promise with the next available chunk of data read into a passed-in buffer. The promise will not resolve until at least `minElements` elements have been read. The element size is determined by `bufferArrayBufferView`, for example 4 bytes per element for a `Uint32Array`. However, fewer than `minElements` elements may be returned if the end of the stream is reached or the underlying stream is closed. Specifically:  
         * If `minElements` or more elements are available, the promise resolves with `{ value: <buffer view sized to bytes read>, done: false }`.  
         * If the stream ends after some data has been read but fewer than `minElements` elements, the promise resolves with the partial data: `{ value: <buffer view sized to bytes actually read>, done: false }`. The next call to `read` or `readAtLeast` will then return `{ value: undefined, done: true }`.  
         * If the stream ends with zero bytes available (that is, the stream is already at EOF), the promise resolves with `{ value: <zero-length view>, done: true }`.  
         * If the stream errors, the promise rejects.  
         * `minElements` must be at least 1, and `minElements * elementSize` must not exceed the byte length of `bufferArrayBufferView`, or the promise rejects with a `TypeError`. For a `Uint8Array`, element size is 1, so `minElements` is effectively a byte count.

---

## Common issues

Warning

`read` provides no control over the minimum number of bytes that should be read into the buffer. Even if you allocate a 1 MiB buffer, the kernel is perfectly within its rights to fulfill this read with a single byte, whether or not an EOF immediately follows.

In practice, the Workers team has found that `read` typically fills only 1% of the provided buffer.

`readAtLeast` is a non-standard extension to the Streams API which allows users to specify that at least `minElements` elements must be read into the buffer before resolving the read. For a `Uint8Array` (the most common case), each element is one byte, so `minElements` is effectively a byte count. If the stream ends before `minElements` elements are available, the partial data that was read is still returned rather than throwing an error — refer to the [readAtLeast method documentation above](#methods) for the full details.

---

## Related resources

* [Streams](https://developers.cloudflare.com/workers/runtime-apis/streams/)
* [Background about BYOB readers in the Streams API WHATWG specification ↗](https://streams.spec.whatwg.org/#byob-readers)

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