std/streams/reader_from_stream_reader.ts

// Copyright 2018-2023 the Deno authors. All rights reserved. MIT license.
// This module is browser compatible.

import { Buffer } from "../io/buffer.ts";
import { writeAll } from "./write_all.ts";
import type { Reader } from "../types.d.ts";

/**
 * Create a `Reader` from a `ReadableStreamDefaultReader`.
 *
 * @example
 * ```ts
 * import { copy } from "https://deno.land/std@$STD_VERSION/streams/copy.ts";
 * import { readerFromStreamReader } from "https://deno.land/std@$STD_VERSION/streams/reader_from_stream_reader.ts";
 *
 * const res = await fetch("https://deno.land");
 * const file = await Deno.open("./deno.land.html", { create: true, write: true });
 *
 * const reader = readerFromStreamReader(res.body!.getReader());
 * await copy(reader, file);
 * file.close();
 * ```
 */
export function readerFromStreamReader(
  streamReader: ReadableStreamDefaultReader<Uint8Array>,
): Reader {
  const buffer = new Buffer();

  return {
    async read(p: Uint8Array): Promise<number | null> {
      if (buffer.empty()) {
        const res = await streamReader.read();
        if (res.done) {
          return null; // EOF
        }

        await writeAll(buffer, res.value);
      }

      return buffer.read(p);
    },
  };
}