A promise based asynchronous stream reader, which makes reading from a stream easy.

Allows to read and peek from a Readable Stream

This module is used by strtok3

The peek-readable contains one class: StreamReader, which reads from a stream.Readable.

• Class StreamReader is used to read from Node.js stream.Readable.
• Class WebStreamReader is used to read from ReadableStream

Module: version 5 migrated from CommonJS to pure ECMAScript Module (ESM). JavaScript is compliant with ECMAScript 2019 (ES10). Requires Node.js ≥ 14.16 engine.

npm install --save peek-readable

Both StreamReader and WebStreamReader implement the IStreamReader interface.

The IStreamReader interface defines the contract for a stream reader, which provides methods to read and peek data from a stream into a Uint8Array buffer. The methods are asynchronous and return a promise that resolves with the number of bytes read.

This method allows you to inspect data from the stream without advancing the read pointer. It reads data into the provided Uint8Array at a specified offset but does not modify the stream's internal position, allowing you to look ahead in the stream.

peek(uint8Array: Uint8Array, offset: number, length: number): Promise<number>

Parameters:

• uint8Array: Uint8Array: The buffer into which the data will be peeked. This is where the peeked data will be stored.
• offset: number: The offset in the Uint8Array where the peeked data should start being written.
• length: number: The number of bytes to peek from the stream.

Returns Promise<number>: A promise that resolves with the number of bytes actually peeked into the buffer. This number may be less than the requested length if the end of the stream is reached.

read(buffer: Uint8Array, offset: number, length: number): Promise<number>

Parameters:

• uint8Array: Uint8Array: The buffer into which the data will be read. This is where the read data will be stored.
• offset: number: The offset in the Uint8Array where the read data should start being written.
• length: number: The number of bytes to read from the stream.

Returns Promise<number>: A promise that resolves with the number of bytes actually read into the buffer. This number may be less than the requested length if the end of the stream is reached.

Abort active asynchronous operation (read or peak) before it has completed.

abort(): Promise<void>

In the following example we read the first 16 bytes from a stream and store them in our buffer. Source code of examples can be found here.

import fs from 'node:fs';
import { StreamReader } from 'peek-readable';

(async () => {
  const readable = fs.createReadStream('JPEG_example_JPG_RIP_001.jpg');
  const streamReader = new StreamReader(readable);
  const uint8Array = new Uint8Array(16);
  const bytesRead = await streamReader.read(uint8Array, 0, 16);;
  // buffer contains 16 bytes, if the end-of-stream has not been reached
})();

End-of-stream detection:

(async () => {

  const fileReadStream = fs.createReadStream('JPEG_example_JPG_RIP_001.jpg');
  const streamReader = new StreamReader(fileReadStream);
  const buffer = Buffer.alloc(16); // or use: new Uint8Array(16);

  try {
    await streamReader.read(buffer, 0, 16);
    // buffer contains 16 bytes, if the end-of-stream has not been reached
  } catch(error) {
    if (error instanceof EndOfStreamError) {
      console.log('End-of-stream reached');
    }
  }
})();

With peek you can read ahead:

import fs from 'node:fs';
import { StreamReader } from 'peek-readable';

const fileReadStream = fs.createReadStream('JPEG_example_JPG_RIP_001.jpg');
const streamReader = new StreamReader(fileReadStream);
const buffer = Buffer.alloc(20);

(async () => {
  let bytesRead = await streamReader.peek(buffer, 0, 3);
  if (bytesRead === 3 && buffer[0] === 0xFF && buffer[1] === 0xD8 && buffer[2] === 0xFF) {
    console.log('This is a JPEG file');
  } else {
    throw Error('Expected a JPEG file');
  }

  bytesRead = await streamReader.read(buffer, 0, 20); // Read JPEG header
  if (bytesRead === 20) {
    console.log('Got the JPEG header');
  } else {
    throw Error('Failed to read JPEG header');
  }
})();