Node.js Streams: Everything you need to know
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What exactly are streams?
Streams are collections of data — just like arrays or strings. The difference is that streams might not be available all at once, and they don’t have to fit in memory. This makes streams really powerful when working with large amounts of data, or data that’s coming from an external source one chunk at a time.
However, streams are not only about working with big data. They also give us the power of composability in our code. Just like we can compose powerful linux commands by piping other smaller Linux commands, we can do exactly the same in Node with streams.
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Streams 101
There are four fundamental stream types in Node.js: Readable, Writable, Duplex, and Transform streams.
- A readable stream is an abstraction for a source from which data can be consumed. An example of that is the fs.createReadStream method.
- A writable stream is an abstraction for a destination to which data can be written. An example of that is the fs.createWriteStream method.
- A duplex streams is both Readable and Writable. An example of that is a TCP socket.
- Transform streams are basically a duplex stream that can be used to modify or transform the data as it is written and read.
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Stream events
The most important events on a readable stream are:
- The data event, which is emitted whenever the stream passes a chunk of data to the consumer
- The end event, which is emitted when there is no more data to be consumed from the stream.
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Paused and Flowing Modes of Readable Streams
Readable streams have two main modes that affect the way we can consume them:
- They can be either in the paused mode
- Or in the flowing mode.
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Implementing Streams
When we talk about streams in Node.js, there are two main different tasks:
- The task of implementing the streams.
- The task of consuming them.
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Implementing a Writable Stream
This write method takes three arguments.
- The chunk is usually a buffer unless we configure the stream differently.
- The encoding argument is needed in that case, but usually we can ignore it.
- To signal a failure, call the Callback with an error object.
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Implementing Duplex/Transform Streams
The readable and Writable sides of a duplex stream operate completely independently from one another. A transform stream is the more interesting duplex stream because its output is computed from its input.
For a transform stream, we don’t have to implement the read or write methods, we only need to implement a transform method, which combines both of them. It has the signature of the write method and we can use it to push data as well. Here’s an example duplex stream which
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