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# peek-readable
A promise based asynchronous stream reader, which makes reading from a stream easy.
Allows to read and peek from a [Readable Stream](https://nodejs.org/api/stream.html#stream_readable_streams)
This module is used by [strtok3](https://github.com/Borewit/strtok3)
The `peek-readable` contains one class: `StreamReader`, which reads from a [stream.Readable](https://nodejs.org/api/stream.html#stream_class_stream_readable).
- Class `StreamReader` is used to read from Node.js [stream.Readable](https://nodejs.org/api/stream.html#stream_class_stream_readable).
- Class `WebStreamByobReader` is used to read from [ReadableStream<Uint8Array>](https://developer.mozilla.org/docs/Web/API/ReadableStream)
## Compatibility
Module: version 5 migrated from [CommonJS](https://en.wikipedia.org/wiki/CommonJS) to [pure ECMAScript Module (ESM)](https://gist.github.com/sindresorhus/a39789f98801d908bbc7ff3ecc99d99c).
JavaScript is compliant with [ECMAScript 2019 (ES10)](https://en.wikipedia.org/wiki/ECMAScript#10th_Edition_%E2%80%93_ECMAScript_2019).
Requires a modern browser or Node.js ≥ 18 engine.
For TypeScript CommonJs backward compatibility, you can use [load-esm](https://github.com/Borewit/load-esm).
## Usage
### Installation
```shell script
npm install --save peek-readable
```
## API Documentation
Both `StreamReader` and `WebStreamByobReader` implement the [IStreamReader interface](#istreamreader-interface).
### `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.
#### Methods
##### `peek` function
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.
```ts
peek(buffer: Uint8Array, offset: number, length: number): Promise<number>
```
Parameters:
- `buffer`: `Uint8Array`: The buffer into which the data will be peeked.
This is where the peeked data will be stored.
- `maybeBeLess`: If true, the buffer maybe partially filled.
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` function
```ts
read(buffer: Uint8Array, offset: number, length: number): Promise<number>
```
Parameters:
- `buffer`: `Uint8Array`: The buffer into which the data will be read.
This is where the read data will be stored.
- `maybeBeLess`: If true, the buffer maybe partially filled.
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` function
Abort active asynchronous operation (`read` or `peak`) before it has completed.
```ts
abort(): Promise<void>
```
## Examples
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](test/examples.ts).
```js
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);;
// buffer contains 16 bytes, if the end-of-stream has not been reached
})();
```
End-of-stream detection:
```js
(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);
// 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:
```js
import fs from 'node:fs';
import { StreamReader } from 'peek-readable';
function closeNodeStream(stream: ReadStream): Promise<void> {
return new Promise<void>((resolve, reject) => {
stream.close(err => {
if(err)
reject(err);
else
resolve();
});
})
}
(async () => {
const fileReadStream = fs.createReadStream('JPEG_example_JPG_RIP_001.jpg');
try {
const streamReader = new StreamReader(fileReadStream);
try {
const buffer = Buffer.alloc(20);
let bytesRead = await streamReader.peek(buffer.subarray(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); // Read JPEG header
if (bytesRead === 20) {
console.log('Got the JPEG header');
} else {
throw Error('Failed to read JPEG header');
}
} finally {
await streamReader.close(); // Release fileReadStream
}
} finally {
await closeNodeStream(fileReadStream);
}
})();
```
## Licence
This project is licensed under the [MIT License](LICENSE.txt). Feel free to use, modify, and distribute as needed.