A quick overview of the node.js streams interface with basic examples.

This is based on @brycebaril’s presentation, Node.js Streams2 Demystified

Overview

Streams are a first-class construct in Node.js for handling data.

Think of them as as lazy evaluation applied to data.

There are essentially three major concepts:

• source - where the data comes from
• pipeline - where you filter or transform your data as it passes through
• sink - where your data ultimately goes

Benefits in using streams:

• lazily produce or consume data in buffered chunks
• evented and non-blocking
• low memory footprint
• automatically handle back-pressure
• buffers allow you to work around the v8 heap memory limit
• most core node.js content sources/sinks are streams already!

Five classes of streams:

• Readable - sources
• Writable - sinks
• Duplex - both source and sink
• Transform - in-flight stream operations
• Passthrough - stream spy

Below is a quick overview of Readable, Writable, and Transform streams.

See also:

• API
• Handbook
• Workshop
• 5 min guide

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Readable

Use a Readable stream when supplying data as a stream.

Think: spigot/faucet.

How to implement

1. Subclass stream.Readable.

2. Implement a _read(size) method.

Methods

_read(size)

• size is in bytes, but can be ignored (especially for objectMode streams)
• _read(size) must call this.push(chunk) to send a chunk to the consumer

Options

• highWaterMark number: maximum number of bytes to store in the internal buffer before ceasing to read (default: 16kb)

• encoding string: if set, buffers will be decoded to strings instead of passing buffers (default: null)

• objectmode boolean: instead of using buffers/strings, use javascript objects (default: false)

How to use

• readable.pipe(target)
• readable.read(size)
• readable.on("data", ... )

See also

• stream-spigot - creates readable streams from Arrays or simple functions

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Writable

Use a Writable stream when collecting data from a stream.

Think: drain/collect.

How to implement

1. Subclass stream.Writable.

2. Implement a _write(chunk, encoding, callback) method.

Methods

_write(chunk, encoding, callback)

• chunk is the content to write
• Call callback() when you’re done with this chunk

Options

• highWaterMark number: maximum number of bytes to store in the internal buffer before ceasing to read (default: 16kb)

• decodeStrings boolean: whether to decode strings to Buffers before passing them to _write() (default: true)

How to use

• source.pipe(sink)
• writable.write(chunk [,encoding] [,callback])

See also

• concat-stream - writable stream that concatenates strings or binary data and calls a callback with the result

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Transform

Use a Transform stream when you want to operate on a stream in transit. This is a special kind of Duplex stream where the input and output stream are the same stream.

Think: filter/map.

How to implement

1. Subclass stream.Transform.
2. Implement a _transform(chunk, encoding, callback) method.
3. Optionally implement a _flush(callback) method.

Methods

_transform(chunk, encoding, callback)

Call this.push(something) to forward it to the next consumer. You don’t have to push anything, this will skip a chunk. You must call callback one time per _transform call.

_flush(callback)

When the stream ends, this is your chance to do any cleanup or last-minute this.push() calls to clear any buffers or work. Call callback() when done.

Options

Superset of Readable and Writable options.

How to use

• source.pipe(transform).pipe(drain)
• transform.on("data", ... )

See also

• through2 - makes it easy to generate Transforms without all the subclassing boilerplate
• through2-map - Array.prototype.map analog for streams
• through2-filter - Array.prototype.filter analog for streams
• through2-reduce - Array.prototype.reduce analog for streams
• stream reducer demo - showing how to extend a Transform stream to create reducers/accumulators for streamed objects
• sculpt - a collection of transform stream utilities (all operating in objectMode)
• pipe-iterators - another collection of functions for iterating over object mode streams