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 with the exception of the read method.

A ReadableStreamBYOBReader is not instantiated via its constructor. Rather, it is retrieved from a ReadableStream:

JavaScript

const { readable, writable } = new TransformStream();
const reader = readable.getReader({ mode: 'byob' });

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Methods

• read(bufferArrayBufferView) : Promise<ReadableStreamBYOBReadResult>

  • Returns a promise with the next available chunk of data read into a passed-in buffer.

• readAtLeast(minBytes, 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 minBytes have been read.

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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 minBytes bytes must be read into the buffer before resolving the read.

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Related resources

• Streams
• Background about BYOB readers in the Streams API WHATWG specification ↗