stream_slicing

This is a Deno and Node.js library containing some utilities for working with web standard streams (ReadableStream). This library is useful for incrementally reading data from streams.

This library is available for Deno at https://deno.land/x/stream_slicing and is available for Node.js on npm as stream-slicing.

Examples

import { PartialReader } from "https://deno.land/x/stream_slicing/partial_reader.ts";

const response = fetch("...");
const stream = response.body!;
const partialReader = PartialReader.fromStream(stream);
const header = await partialReader.readAmountStrict(30);
// `header` will be a 30 byte long Uint8Array
const nextMegabyteStream = partialReader.streamAmount(1024 * 1024).stream;
// `nextMegabyteStream` will be a ReadableStream of the next
// megabyte of data read from `stream`.

The Deno library streaming_zip's read.ts shows some real-world examples of this library being used for parsing zip file data from a stream.

API

partialReaderFromDenoFsFile

API Docs

Helper function for creating a PartialReader from a Deno.FsFile with seeking support configured.

import { partialReaderFromDenoFsFile } from "https://deno.land/x/stream_slicing/deno_helpers.ts";

const partialReader = partialReaderFromDenoFsFile(await Deno.open("foo.zip"));

PartialReader

API Docs

The PartialReader class is exported from partial_reader.ts, and you instantiate it by calling PartialReader.fromDenoFsFile(file: Deno.FsFile) or PartialReader.fromStream(stream: ReadableStream, options?: PartialReaderOptions). The class contains the following methods:

• limitedRead(maxSize: number) Like calling read() on a reader of the stream, but returns no more than maxSize bytes at a time.
• readAmount(size: number) Reads and returns size bytes from the stream, or fewer if the stream ends while reading.
• readAmountStrict(size: number) Reads and returns size bytes from the stream. Throws an error if the stream ends while reading.
• skipAmount(size: number) Skips over size bytes from the stream. If you are working with a local file, make sure to use partialReaderFromDenoFsFile() or provide the seek option to PartialReader.fromStream() so that efficient seeking can be done, otherwise seeking will be done by reading and then ignoring data.
• streamAmount(size: number) Returns an object with a stream property containing a ReadableStream that forwards the next size bytes from the PartialReader's stream.

There is no streamAmountStrict(size: number) method that returns a stream that errors if PartialReader's stream ends early. You must use streamAmount() and ExactBytesTransformStream together if you want to accomplish this:

import { ExactBytesTransformStream } from "https://deno.land/x/stream_slicing/exact_bytes_transform_stream.ts";
import { PartialReader } from "https://deno.land/x/stream_slicing/partial_reader.ts";

const response = await fetch("...");
if (!response.ok) throw new Error("Bad response: " + response.status);
const partialReader = PartialReader.fromStream(response.body!);
const nextMegabyteStream = partialReader
  .streamAmount(1024 * 1024).stream
  .pipeThrough(new ExactBytesTransformStream(1024 * 1024));

ExactBytesTransformStream

API Docs

The ExactBytesTransformStream class is exported from exact_bytes_transform_stream.ts, and you instantiate it by calling new ExactBytesTransformStream(size: number). The class is a TransformStream that will emit an error if a number of bytes unequal to size are piped through it.