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stream_slicing

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

Examples

import { PartialReader } from "https://deno.land/x/stream_slicing@v0.1.0/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

PartialReader

API Docs

The PartialReader class is exported from partial_reader.ts, and you instantiate it by calling PartialReader.fromStream(stream: ReadableStream). 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.
  • 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@v0.1.0/exact_bytes_transform_stream.ts";
import { PartialReader } from "https://deno.land/x/stream_slicing@v0.1.0/partial_reader.ts";

const response = fetch("...");
const stream = response.body!;
const partialReader = PartialReader.fromStream(stream);
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.