--- description: Use ReadableStreamBYOBReader in Workers to read streamed data into your own buffer. title: ReadableStream BYOBReader image: https://developers.cloudflare.com/og-docs.png --- [Skip to content](#main-content) > Documentation Index > Fetch the complete documentation index at: https://developers.cloudflare.com/workers/llms.txt > Use this file to discover all available pages before exploring further. # ReadableStream BYOBReader Last updated Aug 24, 2026|Copy as Markdown|[View as Markdown](https://mchen-ai-models-sync.previews.developers.cloudflare.com/workers/runtime-apis/streams/readablestreambyobreader/index.md)|[Agent setup](https://mchen-ai-models-sync.previews.developers.cloudflare.com/agent-setup/) ## 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://mchen-ai-models-sync.previews.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://mchen-ai-models-sync.previews.developers.cloudflare.com/workers/runtime-apis/streams/readablestream/): ```js const { readable, writable } = new TransformStream(); const reader = readable.getReader({ mode: 'byob' }); ``` --- ## Methods * `read(bufferArrayBufferView)` : Promise * Returns a promise with the next available chunk of data read into a passed-in buffer. * `readAtLeast(minElements, bufferArrayBufferView)` : Promise * 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: , 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: , 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: , 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://mchen-ai-models-sync.previews.developers.cloudflare.com/workers/runtime-apis/streams/) * [Background about BYOB readers in the Streams API WHATWG specification ↗](https://streams.spec.whatwg.org/#byob-readers) Was this helpful? YesNo ## On this page [![](https://mchen-ai-models-sync.previews.developers.cloudflare.com/_astro/logo.te5VL_aD.svg)Docs](https://mchen-ai-models-sync.previews.developers.cloudflare.com/) ```json {"@context":"https://schema.org","@type":"TechArticle","@id":"https://developers.cloudflare.com/workers/runtime-apis/streams/readablestreambyobreader/#page","headline":"ReadableStreamBYOBReader · Cloudflare Workers docs","description":"Use ReadableStreamBYOBReader in Workers to read streamed data into your own buffer.","url":"https://developers.cloudflare.com/workers/runtime-apis/streams/readablestreambyobreader/","inLanguage":"en","image":"https://developers.cloudflare.com/og-docs.png","dateModified":"2026-08-24","publisher":{"@type":"Organization","name":"Cloudflare","description":"One platform for your apps, agents, and workforce. 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