流

稳定性：2 - 稳定

流是 Node.js 中用于处理流式数据的抽象接口。 node:stream 模块提供了实现流接口的 API。

Node.js 提供了许多流对象。例如，对 HTTP 服务器的请求 和 process.stdout 都是流实例。

流可以是可读的、可写的，或两者皆是。所有流都是 EventEmitter 的实例。

要访问 node:stream 模块：

const stream = require('node:stream');

node:stream 模块对于创建新类型的流实例很有用。通常不需要使用 node:stream 模块来消费流。

本文档包含两个主要部分和一个备注部分。第一部分解释如何在应用程序中使用现有流。第二部分解释如何创建新类型的流。

Node.js 内有四种基本流类型：

• Writable：可以向其写入数据的流（例如，fs.createWriteStream()）。
• Readable：可以从中读取数据的流（例如，fs.createReadStream()）。
• Duplex：既是 Readable 又是 Writable 的流（例如，net.Socket）。
• Transform：可以在写入和读取数据时修改或转换数据的 Duplex 流（例如，zlib.createDeflate()）。

此外，该模块还包括实用函数 stream.duplexPair()、stream.pipeline()、stream.finished()、stream.Readable.from() 和 stream.addAbortSignal()。

stream/promises API 提供了一组替代的流异步实用函数，它们返回 Promise 对象而不是使用回调。该 API 可通过 require('node:stream/promises') 或 require('node:stream').promises 访问。

stream.pipeline(streams, options?): void

stream.pipeline(source, ...transforms?, destination, options?): void

const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');
const zlib = require('node:zlib');

async function run() {
  await pipeline(
    fs.createReadStream('archive.tar'),
    zlib.createGzip(),
    fs.createWriteStream('archive.tar.gz'),
  );
  console.log('管道成功。');
}

run().catch(console.error);

import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';
import { createGzip } from 'node:zlib';

await pipeline(
  createReadStream('archive.tar'),
  createGzip(),
  createWriteStream('archive.tar.gz'),
);
console.log('管道成功。');

要使用 AbortSignal，将其作为最后一个参数传递给选项对象。 当信号被中止时，底层管道将被调用 destroy，并带有 AbortError。

const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');
const zlib = require('node:zlib');

async function run() {
  const ac = new AbortController();
  const signal = ac.signal;

  setImmediate(() => ac.abort());
  await pipeline(
    fs.createReadStream('archive.tar'),
    zlib.createGzip(),
    fs.createWriteStream('archive.tar.gz'),
    { signal },
  );
}

run().catch(console.error); // AbortError

import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';
import { createGzip } from 'node:zlib';

const ac = new AbortController();
const { signal } = ac;
setImmediate(() => ac.abort());
try {
  await pipeline(
    createReadStream('archive.tar'),
    createGzip(),
    createWriteStream('archive.tar.gz'),
    { signal },
  );
} catch (err) {
  console.error(err); // AbortError
}

pipeline API 也支持异步生成器：

const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');

async function run() {
  await pipeline(
    fs.createReadStream('lowercase.txt'),
    async function* (source, { signal }) {
      source.setEncoding('utf8');  // 使用字符串而不是 `Buffer`。
      for await (const chunk of source) {
        yield await processChunk(chunk, { signal });
      }
    },
    fs.createWriteStream('uppercase.txt'),
  );
  console.log('管道成功。');
}

run().catch(console.error);

import { pipeline } from 'node:stream/promises';
import { createReadStream, createWriteStream } from 'node:fs';

await pipeline(
  createReadStream('lowercase.txt'),
  async function* (source, { signal }) {
    source.setEncoding('utf8');  // 使用字符串而不是 `Buffer`。
    for await (const chunk of source) {
      yield await processChunk(chunk, { signal });
    }
  },
  createWriteStream('uppercase.txt'),
);
console.log('管道成功。');

记得处理传递给异步生成器的 signal 参数。 特别是在异步生成器是管道的源（即第一个参数）的情况下，否则管道将永远不会完成。

const { pipeline } = require('node:stream/promises');
const fs = require('node:fs');

async function run() {
  await pipeline(
    async function* ({ signal }) {
      await someLongRunningfn({ signal });
      yield 'asd';
    },
    fs.createWriteStream('uppercase.txt'),
  );
  console.log('管道成功。');
}

run().catch(console.error);

import { pipeline } from 'node:stream/promises';
import fs from 'node:fs';
await pipeline(
  async function* ({ signal }) {
    await someLongRunningfn({ signal });
    yield 'asd';
  },
  fs.createWriteStream('uppercase.txt'),
);
console.log('管道成功。');

pipeline API 提供了 回调版本：

stream.finished(stream, options?): void

const { finished } = require('node:stream/promises');
const fs = require('node:fs');

const rs = fs.createReadStream('archive.tar');

async function run() {
  await finished(rs);
  console.log('流已完成读取。');
}

run().catch(console.error);
rs.resume(); // 排空流。

import { finished } from 'node:stream/promises';
import { createReadStream } from 'node:fs';

const rs = createReadStream('archive.tar');

async function run() {
  await finished(rs);
  console.log('流已完成读取。');
}

run().catch(console.error);
rs.resume(); // 排空流。

finished API 还提供了 回调版本。

stream.finished() 在返回的 promise 被履行或拒绝后，会留下悬空的事件监听器（特别是 'error'、'end'、'finish' 和 'close'）。 这样做的原因是，意外的 'error' 事件（由于不正确的流实现）不会导致意外的崩溃。 如果这是不需要的行为，则应将 options.cleanup 设置为 true：

await finished(rs, { cleanup: true });

由 Node.js API 创建的所有流仅对字符串、Buffer、TypedArray 和 DataView 对象进行操作：

• Strings 和 Buffers 是与流一起使用的最常见类型。
• TypedArray 和 DataView 允许你使用 Int32Array 或 Uint8Array 等类型处理二进制数据。当你将 TypedArray 或 DataView 写入流时，Node.js 会处理原始字节。

然而，流实现有可能与其他类型的 JavaScript 值一起工作（null 除外，它在流中有特殊用途）。 此类流被认为是在“对象模式”下操作。

流实例在创建时使用 objectMode 选项切换到对象模式。尝试将现有流切换到对象模式是不安全的。

Writable 和 Readable 流都会将数据存储在一个内部缓冲区中。

潜在缓冲的数据量取决于传递给流构造函数的 highWaterMark 选项。对于普通流，highWaterMark 选项指定 字节总数。对于以对象模式操作的流，highWaterMark 指定对象总数。对于操作字符串（但不解码）的流，highWaterMark 指定 UTF-16 代码单元总数。

当实现调用 stream.push(chunk) 时，数据会在 Readable 流中缓冲。如果流的消费者不调用 stream.read()，数据将停留在内部队列中直到被消费。

一旦内部读取缓冲区的总大小达到 highWaterMark 指定的阈值，流将暂时停止从底层资源读取数据，直到当前缓冲的数据可以被消费（即，流将停止调用用于填充读取缓冲区的内部 readable._read() 方法）。

当反复调用 writable.write(chunk) 方法时，数据会在 Writable 流中缓冲。当内部写入缓冲区的总大小低于 highWaterMark 设置的阈值时，对 writable.write() 的调用将返回 true。一旦内部缓冲区的大小达到或超过 highWaterMark，将返回 false。

stream API 的一个关键目标，特别是 stream.pipe() 方法，是将数据缓冲限制在可接受的水平，以便不同速度的源和目标不会压倒可用内存。

highWaterMark 选项是一个阈值，而不是限制：它规定了流在停止请求更多数据之前缓冲的数据量。它通常不强制执行严格的内存限制。特定的流实现可以选择执行更严格的限制，但这是可选的。

因为 Duplex 和 Transform 流既是 Readable 又是 Writable，所以它们各自维护 两个 独立的内部缓冲区用于读取和写入，允许每一侧独立操作，同时保持适当且高效的数据流。例如，net.Socket 实例是 Duplex 流，其 Readable 侧允许消费 从 套接字接收的数据，而其 Writable 侧允许写入数据 到 套接字。因为写入套接字的数据速率可能比接收数据的速率快或慢，所以每一侧都应独立操作（和缓冲）。

内部缓冲的机制是内部实现细节，可能随时更改。但是，对于某些高级实现，可以使用 writable.writableBuffer 或 readable.readableBuffer 检索内部缓冲区。不鼓励使用这些未记录的属性。

几乎所有的 Node.js 应用程序，无论多么简单，都会以某种方式使用流。以下是在实现 HTTP 服务器的 Node.js 应用程序中使用流的示例：

const http = require('node:http');

const server = http.createServer((req, res) => {
  // `req` 是一个 http.IncomingMessage，它是一个可读流。
  // `res` 是一个 http.ServerResponse，它是一个可写流。

  let body = '';
  // 将数据获取为 utf8 字符串。
  // 如果未设置编码，将接收到 Buffer 对象。
  req.setEncoding('utf8');

  // 可读流一旦添加了监听器就会发出 'data' 事件。
  req.on('data', (chunk) => {
    body += chunk;
  });

  // 'end' 事件表示整个 body 已接收完毕。
  req.on('end', () => {
    try {
      const data = JSON.parse(body);
      // 向用户写回一些有趣的内容：
      res.write(typeof data);
      res.end();
    } catch (er) {
      //  哎呀！JSON 错误！
      res.statusCode = 400;
      return res.end(`error: ${er.message}`);
    }
  });
});

server.listen(1337);

// $ curl localhost:1337 -d "{}"
// object
// $ curl localhost:1337 -d "\"foo\""
// string
// $ curl localhost:1337 -d "not json"
// error: Unexpected token 'o', "not json" is not valid JSON

[可读流][] 流（例如示例中的 [Readable][]）暴露了 [push][] 和 [unshift][] 等方法，用于将数据写入流。

[流][] 流使用 [EventEmitter][] API 在数据可供从流中读取时通知应用程序代码。可以通过多种方式从流中读取可用数据。

[可读流][] 和 [可写流][] 流都以各种方式使用 [EventEmitter][] API 来通信流的当前状态。

[双工流][] 和 [转换流][] 流既是 [可读流][] 也是 [可写流][]。

向流写入数据或从流消费数据的应用程序不需要直接实现流接口，并且通常没有理由调用 _write()。

希望实现新类型流的开发者应参考 流实现者 API 部分。

可写流是对数据写入_目的地_的抽象。

[可写流][] 流的示例包括：

• [HTTP 请求，在客户端][]
• [HTTP 响应，在服务器端][]
• [fs 写入流][]
• zlib 流
• crypto 流
• TCP 套接字
• 子进程 stdin
• [文件系统][fs]，[文件描述符][]

其中一些示例实际上是实现了 [stream.Writable][] 接口的 [流][] 流。

所有 [Writable][] 流都实现了 [stream.Writable][] 类定义的接口。

虽然 [Writable][] 流的具体实例可能在各方面有所不同，但所有 [Writable][] 流都遵循与以下示例中说明相同的基本使用模式：

const myStream = getWritableStreamSomehow();
myStream.write('some data');
myStream.write('some more data');
myStream.end('done writing data');

当流及其任何底层资源（例如文件描述符）已关闭时，会发出 'close' 事件。该事件表示将不再发出更多事件，也不会发生进一步的计算。

如果使用 emitClose 选项创建 Writable 流，它将始终发出 'close' 事件。

如果对 stream.write(chunk) 的调用返回 false，则当适合恢复向流写入数据时，将发出 'drain' 事件。

// 向提供的可写流写入数据一百万次。
// 注意背压。
function writeOneMillionTimes(writer, data, encoding, callback) {
  let i = 1000000;
  write();
  function write() {
    let ok = true;
    do {
      i--;
      if (i === 0) {
        // 最后一次！
        writer.write(data, encoding, callback);
      } else {
        // 看看我们是应该继续，还是等待。
        // 不要传递回调，因为我们还没完成。
        ok = writer.write(data, encoding);
      }
    } while (i > 0 && ok);
    if (i > 0) {
      // 不得不提前停止！
      // 一旦排空，再写入一些。
      writer.once('drain', write);
    }
  }
}

• 类型：Error

如果在写入或通过管道传输数据时发生错误，则会发出 'error' 事件。调用监听器回调时会传递单个 Error 参数。

除非在创建流时将 autoDestroy 选项设置为 false，否则在发出 'error' 事件时流会关闭。

在 'error' 之后，应该 不再发出除 'close' 之外的其他事件（包括 'error' 事件）。

在调用 stream.end() 方法且所有数据已刷新到底层系统后，会发出 'finish' 事件。

const writer = getWritableStreamSomehow();
for (let i = 0; i < 100; i++) {
  writer.write(`hello, #${i}!\n`);
}
writer.on('finish', () => {
  console.log('所有写入现已完成。');
});
writer.end('This is the end\n');

当在可读流上调用 stream.pipe() 方法并将此可写流添加到其目的地集合时，会发出 'pipe' 事件。

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('pipe', (src) => {
  console.log('Something is piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);

当在 Readable 流上调用 stream.unpipe() 方法并将此 Writable 从其目的地集合中移除时，会发出 'unpipe' 事件。

如果此 Writable 流在有 Readable 流管道传输到它时发出错误，也会发出此事件。

const writer = getWritableStreamSomehow();
const reader = getReadableStreamSomehow();
writer.on('unpipe', (src) => {
  console.log('Something has stopped piping into the writer.');
  assert.equal(src, reader);
});
reader.pipe(writer);
reader.unpipe(writer);

writable.cork(): void

writable.cork() 方法强制将所有写入的数据缓冲在内存中。当调用 stream.uncork() 或 stream.end() 方法时，缓冲的数据将被刷新。

writable.cork() 的主要目的是适应这种情况：多个小块数据连续快速地写入流。writable.cork() 不会立即将它们转发到底层目的地，而是缓冲所有块，直到调用 writable.uncork()，如果存在，这将把它们全部传递给 writable._writev()。这防止了头阻塞情况，即数据在等待第一个小块被处理时被缓冲。但是，如果不实现 writable._writev() 而使用 writable.cork() 可能会对吞吐量产生不利影响。

另见：writable.uncork()，writable._writev()。

writable.destroy(error?): void

销毁流。可选地发出 'error' 事件，并发出 'close' 事件（除非 emitClose 设置为 false）。在此调用之后，可写流已结束，后续调用 write() 或 end() 将导致 ERR_STREAM_DESTROYED 错误。 这是一种破坏性的且立即销毁流的方式。之前的 write() 调用可能尚未排空，并可能触发 ERR_STREAM_DESTROYED 错误。如果数据应在关闭前刷新，请使用 end() 而不是 destroy，或者在销毁流之前等待 'drain' 事件。

const { Writable } = require('node:stream');

const myStream = new Writable();

const fooErr = new Error('foo error');
myStream.destroy(fooErr);
myStream.on('error', (fooErr) => console.error(fooErr.message)); // foo error

const { Writable } = require('node:stream');

const myStream = new Writable();

myStream.destroy();
myStream.on('error', function wontHappen() {});

const { Writable } = require('node:stream');

const myStream = new Writable();
myStream.destroy();

myStream.write('foo', (error) => console.error(error.code));
// ERR_STREAM_DESTROYED

一旦调用了 destroy()，任何进一步的调用都将是无操作，并且除了来自 _destroy() 的错误外，不会再作为 'error' 发出其他错误。

实现者不应覆盖此方法，而应实现 writable._destroy()。

• 类型：boolean

在发出 'close' 事件后为 true。

• 类型：boolean

在调用 writable.destroy() 后为 true。

const { Writable } = require('node:stream');

const myStream = new Writable();

console.log(myStream.destroyed); // false
myStream.destroy();
console.log(myStream.destroyed); // true

writable.end(chunk?, encoding?, callback?): void

调用 writable.end() 方法表示不再有数据写入 Writable。可选的 chunk 和 encoding 参数允许在关闭流之前立即写入最后一个额外的数据块。

在调用 stream.end() 后调用 stream.write() 方法将引发错误。

// 写入 'hello, ' 然后以 'world!' 结束。
const fs = require('node:fs');
const file = fs.createWriteStream('example.txt');
file.write('hello, ');
file.end('world!');
// 现在不允许再写入！

writable.setDefaultEncoding(encoding): void

writable.setDefaultEncoding() 方法为 Writable 流设置默认 encoding。

writable.uncork(): void

writable.uncork() 方法刷新自调用 stream.cork() 以来缓冲的所有数据。

当使用 writable.cork() 和 writable.uncork() 管理流写入的缓冲时，请使用 process.nextTick() 延迟调用 writable.uncork()。这样做允许批处理在给定 Node.js 事件循环阶段内发生的所有 writable.write() 调用。

stream.cork();
stream.write('some ');
stream.write('data ');
process.nextTick(() => stream.uncork());

如果在流上多次调用 writable.cork() 方法，则必须调用相同次数的 writable.uncork() 来刷新缓冲的数据。

stream.cork();
stream.write('some ');
stream.cork();
stream.write('data ');
process.nextTick(() => {
  stream.uncork();
  // 直到第二次调用 uncork() 数据才会被刷新。
  stream.uncork();
});

另见：writable.cork()。

• 类型：boolean

如果安全调用 writable.write() 则为 true，这意味着流未被销毁、出错或结束。

• 类型：boolean

返回流是否在发出 'finish' 之前被销毁或出错。

• 类型：boolean

在调用 writable.end() 后为 true。此属性不指示数据是否已刷新，为此请使用 writable.writableFinished。

• 类型：integer

需要调用 writable.uncork() 的次数才能完全打开流。

• 类型：Error

如果流已因错误被销毁，则返回错误。

• 类型：boolean

在 'finish' 事件发出之前立即设置为 true。

• 类型：number

返回创建此 Writable 时传递的 highWaterMark 值。

• 类型：number

此属性包含队列中准备写入的字节数（或对象数）。该值提供有关 highWaterMark 状态的内省数据。

• 类型：boolean

如果流的缓冲区已满且流将发出 'drain'，则为 true。

• 类型：boolean

给定 Writable 流的 objectMode 属性的 getter。

writable[Symbol.asyncDispose](): void

Calls writable.destroy() with an AbortError and returns a promise that fulfills when the stream is finished.

writable.write(chunk, encoding?, callback?): boolean

The writable.write() method writes some data to the stream, and calls the supplied callback once the data has been fully handled. If an error occurs, the callback will be called with the error as its first argument. The callback is called asynchronously and before 'error' is emitted.

The return value is true if the internal buffer is less than the highWaterMark configured when the stream was created after admitting chunk. If false is returned, further attempts to write data to the stream should stop until the 'drain' event is emitted.

While a stream is not draining, calls to write() will buffer chunk, and return false. Once all currently buffered chunks are drained (accepted for delivery by the operating system), the 'drain' event will be emitted. Once write() returns false, do not write more chunks until the 'drain' event is emitted. While calling write() on a stream that is not draining is allowed, Node.js will buffer all written chunks until maximum memory usage occurs, at which point it will abort unconditionally. Even before it aborts, high memory usage will cause poor garbage collector performance and high RSS (which is not typically released back to the system, even after the memory is no longer required). Since TCP sockets may never drain if the remote peer does not read the data, writing a socket that is not draining may lead to a remotely exploitable vulnerability.

Writing data while the stream is not draining is particularly problematic for a Transform, because the Transform streams are paused by default until they are piped or a 'data' or 'readable' event handler is added.

If the data to be written can be generated or fetched on demand, it is recommended to encapsulate the logic into a Readable and use stream.pipe(). However, if calling write() is preferred, it is possible to respect backpressure and avoid memory issues using the 'drain' event:

function write(data, cb) {
  if (!stream.write(data)) {
    stream.once('drain', cb);
  } else {
    process.nextTick(cb);
  }
}

// Wait for cb to be called before doing any other write.
write('hello', () => {
  console.log('Write completed, do more writes now.');
});

A Writable stream in object mode will always ignore the encoding argument.

Readable streams are an abstraction for a source from which data is consumed.

Examples of Readable streams include:

• HTTP responses, on the client
• HTTP requests, on the server
• [fs read streams][]
• zlib streams
• crypto streams
• [TCP sockets][]
• [child process stdout and stderr][]
• process.stdin

All Readable streams implement the interface defined by the stream.Readable class.

Readable streams effectively operate in one of two modes: flowing and paused. These modes are separate from object mode. A Readable stream can be in object mode or not, regardless of whether it is in flowing mode or paused mode.

• In flowing mode, data is read from the underlying system automatically and provided to an application as quickly as possible using events via the EventEmitter interface.

• In paused mode, the stream.read() method must be called explicitly to read chunks of data from the stream.

All Readable streams begin in paused mode but can be switched to flowing mode in one of the following ways:

• Adding a 'data' event handler.
• Calling the stream.resume() method.
• Calling the stream.pipe() method to send the data to a Writable.

The Readable can switch back to paused mode using one of the following:

• If there are no pipe destinations, by calling the stream.pause() method.
• If there are pipe destinations, by removing all pipe destinations. Multiple pipe destinations may be removed by calling the stream.unpipe() method.

The important concept to remember is that a Readable will not generate data until a mechanism for either consuming or ignoring that data is provided. If the consuming mechanism is disabled or taken away, the Readable will attempt to stop generating the data.

For backward compatibility reasons, removing 'data' event handlers will not automatically pause the stream. Also, if there are piped destinations, then calling stream.pause() will not guarantee that the stream will remain paused once those destinations drain and ask for more data.

If a Readable is switched into flowing mode and there are no consumers available to handle the data, that data will be lost. This can occur, for instance, when the readable.resume() method is called without a listener attached to the 'data' event, or when a 'data' event handler is removed from the stream.

Adding a 'readable' event handler automatically makes the stream stop flowing, and the data has to be consumed via readable.read(). If the 'readable' event handler is removed, then the stream will start flowing again if there is a 'data' event handler.

The "two modes" of operation for a Readable stream are a simplified abstraction for the more complicated internal state management that is happening within the Readable stream implementation.

Specifically, at any given point in time, every Readable is in one of three possible states:

• readable.readableFlowing === null
• readable.readableFlowing === false
• readable.readableFlowing === true

When readable.readableFlowing is null, no mechanism for consuming the stream's data is provided. Therefore, the stream will not generate data. While in this state, attaching a listener for the 'data' event, calling the readable.pipe() method, or calling the readable.resume() method will switch readable.readableFlowing to true, causing the Readable to begin actively emitting events as data is generated.

Calling readable.pause(), readable.unpipe(), or receiving backpressure will cause the readable.readableFlowing to be set as false, temporarily halting the flowing of events but not halting the generation of data. While in this state, attaching a listener for the 'data' event will not switch readable.readableFlowing to true.

const { PassThrough, Writable } = require('node:stream');
const pass = new PassThrough();
const writable = new Writable();

pass.pipe(writable);
pass.unpipe(writable);
// readableFlowing is now false.

pass.on('data', (chunk) => { console.log(chunk.toString()); });
// readableFlowing is still false.
pass.write('ok');  // Will not emit 'data'.
pass.resume();     // Must be called to make stream emit 'data'.
// readableFlowing is now true.

While readable.readableFlowing is false, data may be accumulating within the stream's internal buffer.

The Readable stream API evolved across multiple Node.js versions and provides multiple methods of consuming stream data. In general, developers should choose one of the methods of consuming data and should never use multiple methods to consume data from a single stream. Specifically, using a combination of on('data'), on('readable'), pipe(), or async iterators could lead to unintuitive behavior.

The 'close' event is emitted when the stream and any of its underlying resources (a file descriptor, for example) have been closed. The event indicates that no more events will be emitted, and no further computation will occur.

A Readable stream will always emit the 'close' event if it is created with the emitClose option.

The 'data' event is emitted whenever the stream is relinquishing ownership of a chunk of data to a consumer. This may occur whenever the stream is switched in flowing mode by calling readable.pipe(), readable.resume(), or by attaching a listener callback to the 'data' event. The 'data' event will also be emitted whenever the readable.read() method is called and a chunk of data is available to be returned.

Attaching a 'data' event listener to a stream that has not been explicitly paused will switch the stream into flowing mode. Data will then be passed as soon as it is available.

The listener callback will be passed the chunk of data as a string if a default encoding has been specified for the stream using the readable.setEncoding() method; otherwise the data will be passed as a Buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});

The 'end' event is emitted when there is no more data to be consumed from the stream.

The 'end' event will not be emitted unless the data is completely consumed. This can be accomplished by switching the stream into flowing mode, or by calling stream.read() repeatedly until all data has been consumed.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
});
readable.on('end', () => {
  console.log('There will be no more data.');
});

The 'error' event may be emitted by a Readable implementation at any time. Typically, this may occur if the underlying stream is unable to generate data due to an underlying internal failure, or when a stream implementation attempts to push an invalid chunk of data.

The listener callback will be passed a single Error object.

The 'pause' event is emitted when stream.pause() is called and readableFlowing is not false.

The 'readable' event is emitted when there is data available to be read from the stream, up to the configured high water mark (state.highWaterMark). Effectively, it indicates that the stream has new information within the buffer. If data is available within this buffer, stream.read() can be called to retrieve that data. Additionally, the 'readable' event may also be emitted when the end of the stream has been reached.

const readable = getReadableStreamSomehow();
readable.on('readable', function() {
  // There is some data to read now.
  let data;

  while ((data = this.read()) !== null) {
    console.log(data);
  }
});

If the end of the stream has been reached, calling stream.read() will return null and trigger the 'end' event. This is also true if there never was any data to be read. For instance, in the following example, foo.txt is an empty file:

const fs = require('node:fs');
const rr = fs.createReadStream('foo.txt');
rr.on('readable', () => {
  console.log(`readable: ${rr.read()}`);
});
rr.on('end', () => {
  console.log('end');
});

The output of running this script is:

$ node test.js
readable: null
end

In some cases, attaching a listener for the 'readable' event will cause some amount of data to be read into an internal buffer.

In general, the readable.pipe() and 'data' event mechanisms are easier to understand than the 'readable' event. However, handling 'readable' might result in increased throughput.

If both 'readable' and 'data' are used at the same time, 'readable' takes precedence in controlling the flow, i.e. 'data' will be emitted only when stream.read() is called. The readableFlowing property would become false. If there are 'data' listeners when 'readable' is removed, the stream will start flowing, i.e. 'data' events will be emitted without calling .resume().

The 'resume' event is emitted when stream.resume() is called and readableFlowing is not true.

readable.destroy(error?): this

Destroy the stream. Optionally emit an 'error' event, and emit a 'close' event (unless emitClose is set to false). After this call, the readable stream will release any internal resources and subsequent calls to push() will be ignored.

Once destroy() has been called any further calls will be a no-op and no further errors except from _destroy() may be emitted as 'error'.

Implementors should not override this method, but instead implement readable._destroy().

Is true after 'close' has been emitted.

Is true after readable.destroy() has been called.

readable.isPaused(): boolean

The readable.isPaused() method returns the current operating state of the Readable. This is used primarily by the mechanism that underlies the readable.pipe() method. In most typical cases, there will be no reason to use this method directly.

const readable = new stream.Readable();

readable.isPaused(); // === false
readable.pause();
readable.isPaused(); // === true
readable.resume();
readable.isPaused(); // === false

readable.pause(): this

The readable.pause() method will cause a stream in flowing mode to stop emitting 'data' events, switching out of flowing mode. Any data that becomes available will remain in the internal buffer.

const readable = getReadableStreamSomehow();
readable.on('data', (chunk) => {
  console.log(`Received ${chunk.length} bytes of data.`);
  readable.pause();
  console.log('There will be no additional data for 1 second.');
  setTimeout(() => {
    console.log('Now data will start flowing again.');
    readable.resume();
  }, 1000);
});

The readable.pause() method has no effect if there is a 'readable' event listener.

readable.pipe(destination, options?): stream.Writable

The readable.pipe() method attaches a Writable stream to the readable, causing it to switch automatically into flowing mode and push all of its data to the attached Writable. The flow of data will be automatically managed so that the destination Writable stream is not overwhelmed by a faster Readable stream.

The following example pipes all of the data from the readable into a file named file.txt:

const fs = require('node:fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt'.
readable.pipe(writable);

It is possible to attach multiple Writable streams to a single Readable stream.

The readable.pipe() method returns a reference to the destination stream making it possible to set up chains of piped streams:

const fs = require('node:fs');
const zlib = require('node:zlib');
const r = fs.createReadStream('file.txt');
const z = zlib.createGzip();
const w = fs.createWriteStream('file.txt.gz');
r.pipe(z).pipe(w);

By default, stream.end() is called on the destination Writable stream when the source Readable stream emits 'end', so that the destination is no longer writable. To disable this default behavior, the end option can be passed as false, causing the destination stream to remain open:

reader.pipe(writer, { end: false });
reader.on('end', () => {
  writer.end('Goodbye\n');
});

One important caveat is that if the Readable stream emits an error during processing, the Writable destination is not closed automatically. If an error occurs, it will be necessary to manually close each stream in order to prevent memory leaks.

The process.stderr and process.stdout Writable streams are never closed until the Node.js process exits, regardless of the specified options.

read(size?): string | Buffer | null | any

The readable.read() method reads data out of the internal buffer and returns it. If no data is available to be read, null is returned. By default, the data is returned as a Buffer object unless an encoding has been specified using the readable.setEncoding() method or the stream is operating in object mode.

The optional size argument specifies a specific number of bytes to read. If size bytes are not available to be read, null will be returned unless the stream has ended, in which case all of the data remaining in the internal buffer will be returned.

If the size argument is not specified, all of the data contained in the internal buffer will be returned.

The size argument must be less than or equal to 1 GiB.

The readable.read() method should only be called on Readable streams operating in paused mode. In flowing mode, readable.read() is called automatically until the internal buffer is fully drained.

const readable = getReadableStreamSomehow();

// 'readable' may be triggered multiple times as data is buffered in
readable.on('readable', () => {
  let chunk;
  console.log('Stream is readable (new data received in buffer)');
  // Use a loop to make sure we read all currently available data
  while (null !== (chunk = readable.read())) {
    console.log(`Read ${chunk.length} bytes of data...`);
  }
});

// 'end' will be triggered once when there is no more data available
readable.on('end', () => {
  console.log('Reached end of stream.');
});

Each call to readable.read() returns a chunk of data or null, signifying that there's no more data to read at that moment. These chunks aren't automatically concatenated. Because a single read() call does not return all the data, using a while loop may be necessary to continuously read chunks until all data is retrieved. When reading a large file, .read() might return null temporarily, indicating that it has consumed all buffered content but there may be more data yet to be buffered. In such cases, a new 'readable' event is emitted once there's more data in the buffer, and the 'end' event signifies the end of data transmission.

Therefore to read a file's whole contents from a readable, it is necessary to collect chunks across multiple 'readable' events:

const chunks = [];

readable.on('readable', () => {
  let chunk;
  while (null !== (chunk = readable.read())) {
    chunks.push(chunk);
  }
});

readable.on('end', () => {
  const content = chunks.join('');
});

A Readable stream in object mode will always return a single item from a call to readable.read(size), regardless of the value of the size argument.

If the readable.read() method returns a chunk of data, a 'data' event will also be emitted.

Calling stream.read([size]) after the 'end' event has been emitted will return null. No runtime error will be raised.

Is true if it is safe to call readable.read(), which means the stream has not been destroyed or emitted 'error' or 'end'.

Returns whether the stream was destroyed or errored before emitting 'end'.

Returns whether 'data' has been emitted.

Getter for the property encoding of a given Readable stream. The encoding property can be set using the readable.setEncoding() method.

Becomes true when 'end' event is emitted.

Returns error if the stream has been destroyed with an error.

This property reflects the current state of a Readable stream as described in the [Three states][] section.

Returns the value of highWaterMark passed when creating this Readable.

This property contains the number of bytes (or objects) in the queue ready to be read. The value provides introspection data regarding the status of the highWaterMark.

Getter for the property objectMode of a given Readable stream.

readable.resume(): this

The readable.resume() method causes an explicitly paused Readable stream to resume emitting 'data' events, switching the stream into flowing mode.

The readable.resume() method can be used to fully consume the data from a stream without actually processing any of that data:

getReadableStreamSomehow()
  .resume()
  .on('end', () => {
    console.log('Reached the end, but did not read anything.');
  });

The readable.resume() method has no effect if there is a 'readable' event listener.

readable.setEncoding(encoding): this

The readable.setEncoding() method sets the character encoding for data read from the Readable stream.

By default, no encoding is assigned and stream data will be returned as Buffer objects. Setting an encoding causes the stream data to be returned as strings of the specified encoding rather than as Buffer objects. For instance, calling readable.setEncoding('utf8') will cause the output data to be interpreted as UTF-8 data, and passed as strings. Calling readable.setEncoding('hex') will cause the data to be encoded in hexadecimal string format.

The Readable stream will properly handle multi-byte characters delivered through the stream that would otherwise become improperly decoded if simply pulled from the stream as Buffer objects.

const readable = getReadableStreamSomehow();
readable.setEncoding('utf8');
readable.on('data', (chunk) => {
  assert.equal(typeof chunk, 'string');
  console.log('Got %d characters of string data:', chunk.length);
});

readable.unpipe(destination?): this

The readable.unpipe() method detaches a Writable stream previously attached using the stream.pipe() method.

If the destination is not specified, then all pipes are detached.

If the destination is specified, but no pipe is set up for it, then the method does nothing.

const fs = require('node:fs');
const readable = getReadableStreamSomehow();
const writable = fs.createWriteStream('file.txt');
// All the data from readable goes into 'file.txt',
// but only for the first second.
readable.pipe(writable);
setTimeout(() => {
  console.log('Stop writing to file.txt.');
  readable.unpipe(writable);
  console.log('Manually close the file stream.');
  writable.end();
}, 1000);

readable.unshift(chunk, encoding?): void

Passing chunk as null signals the end of the stream (EOF) and behaves the same as readable.push(null), after which no more data can be written. The EOF signal is put at the end of the buffer and any buffered data will still be flushed.

The readable.unshift() method pushes a chunk of data back into the internal buffer. This is useful in certain situations where a stream is being consumed by code that needs to "un-consume" some amount of data that it has optimistically pulled out of the source, so that the data can be passed on to some other party.

The stream.unshift(chunk) method cannot be called after the 'end' event has been emitted or a runtime error will be thrown.

Developers using stream.unshift() often should consider switching to use of a Transform stream instead. See the [API for stream implementers][] section for more information.

// Pull off a header delimited by \n\n.
// Use unshift() if we get too much.
// Call the callback with (error, header, stream).
const { StringDecoder } = require('node:string_decoder');
function parseHeader(stream, callback) {
  stream.on('error', callback);
  stream.on('readable', onReadable);
  const decoder = new StringDecoder('utf8');
  let header = '';
  function onReadable() {
    let chunk;
    while (null !== (chunk = stream.read())) {
      const str = decoder.write(chunk);
      if (str.includes('\n\n')) {
        // Found the header boundary.
        const split = str.split(/\n\n/);
        header += split.shift();
        const remaining = split.join('\n\n');
        const buf = Buffer.from(remaining, 'utf8');
        stream.removeListener('error', callback);
        // Remove the 'readable' listener before unshifting.
        stream.removeListener('readable', onReadable);
        if (buf.length)
          stream.unshift(buf);
        // Now the body of the message can be read from the stream.
        callback(null, header, stream);
        return;
      }
      // Still reading the header.
      header += str;
    }
  }
}

Unlike stream.push(chunk), stream.unshift(chunk) will not end the reading process by resetting the internal reading state of the stream. This can cause unexpected results if readable.unshift() is called during a read (i.e. from within a stream._read() implementation on a custom stream). Following the call to readable.unshift() with an immediate stream.push('') will reset the reading state appropriately, however it is best to simply avoid calling readable.unshift() while in the process of performing a read.

readable.wrap(stream): this

Prior to Node.js 0.10, streams did not implement the entire node:stream module API as it is currently defined. (See [Compatibility][] for more information.)

When using an older Node.js library that emits 'data' events and has a stream.pause() method that is advisory only, the readable.wrap() method can be used to create a Readable stream that uses the old stream as its data source.

It will rarely be necessary to use readable.wrap() but the method has been provided as a convenience for interacting with older Node.js applications and libraries.

const { OldReader } = require('./old-api-module.js');
const { Readable } = require('node:stream');
const oreader = new OldReader();
const myReader = new Readable().wrap(oreader);

myReader.on('readable', () => {
  myReader.read(); // etc.
});

readable[Symbol.asyncIterator](): AsyncIterator

const fs = require('node:fs');

async function print(readable) {
  readable.setEncoding('utf8');
  let data = '';
  for await (const chunk of readable) {
    data += chunk;
  }
  console.log(data);
}

print(fs.createReadStream('file')).catch(console.error);

If the loop terminates with a break, return, or a throw, the stream will be destroyed. In other terms, iterating over a stream will consume the stream fully. The stream will be read in chunks of size equal to the highWaterMark option. In the code example above, data will be in a single chunk if the file has less than 64 KiB of data because no highWaterMark option is provided to fs.createReadStream().

readable[Symbol.for('Stream.toAsyncStreamable')](): AsyncIterable

When the --experimental-stream-iter flag is enabled, Readable streams implement the Stream.toAsyncStreamable protocol, enabling efficient consumption by the stream/iter API.

This provides a batched async iterator that drains the stream's internal buffer into Uint8Array[] batches, amortizing the per-chunk Promise overhead of the standard Symbol.asyncIterator path. For byte-mode streams, chunks are yielded directly as Buffer instances (which are Uint8Array subclasses). For object-mode or encoded streams, each chunk is normalized to Uint8Array before batching.

The returned iterator is tagged as a validated source, so from() passes it through without additional normalization.

import { Readable } from 'node:stream';
import { text, from } from 'node:stream/iter';

const readable = new Readable({
  read() { this.push('hello'); this.push(null); },
});

// Readable is automatically consumed via toAsyncStreamable
console.log(await text(from(readable))); // 'hello'

const { Readable } = require('node:stream');
const { text, from } = require('node:stream/iter');

async function run() {
  const readable = new Readable({
    read() { this.push('hello'); this.push(null); },
  });

  console.log(await text(from(readable))); // 'hello'
}

run().catch(console.error);

Without the --experimental-stream-iter flag, calling this method throws ERR_STREAM_ITER_MISSING_FLAG.

readable[Symbol.asyncDispose](): void

Calls readable.destroy() with an AbortError and returns a promise that fulfills when the stream is finished.

readable.compose(stream, options?): Duplex

import { Readable } from 'node:stream';

async function* splitToWords(source) {
  for await (const chunk of source) {
    const words = String(chunk).split(' ');

    for (const word of words) {
      yield word;
    }
  }
}

const wordsStream = Readable.from(['text passed through', 'composed stream']).compose(splitToWords);
const words = await wordsStream.toArray();

console.log(words); // prints ['text', 'passed', 'through', 'composed', 'stream']

readable.compose(s) is equivalent to stream.compose(readable, s).

This method also allows for an AbortSignal to be provided, which will destroy the composed stream when aborted.

See stream.compose(...streams) for more information.

readable.iterator(options?): AsyncIterator

The iterator created by this method gives users the option to cancel the destruction of the stream if the for await...of loop is exited by return, break, or throw, or if the iterator should destroy the stream if the stream emitted an error during iteration.

const { Readable } = require('node:stream');

async function printIterator(readable) {
  for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
    console.log(chunk); // 1
    break;
  }

  console.log(readable.destroyed); // false

  for await (const chunk of readable.iterator({ destroyOnReturn: false })) {
    console.log(chunk); // Will print 2 and then 3
  }

  console.log(readable.destroyed); // True, stream was totally consumed
}

async function printSymbolAsyncIterator(readable) {
  for await (const chunk of readable) {
    console.log(chunk); // 1
    break;
  }

  console.log(readable.destroyed); // true
}

async function showBoth() {
  await printIterator(Readable.from([1, 2, 3]));
  await printSymbolAsyncIterator(Readable.from([1, 2, 3]));
}

showBoth();