# Streams in Node.js

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Published 2022-12-03

This tutorial explains to you what is and how you can use a Streams in Node.js.

Streams in Node.js (or in general) are objects that let you read data from a source, transform the data it contains or write data to a destination in continuous fashion.

There are essentially 3 major concepts:

source - where the data comes from
pipe - pipe's job is to get the output of one program and pass it as input to another one
sink - where your data ultimately goes

The 5 classes of streams are:

Readable Streams : sources
Writable Streams : sinks
Duplex Streams : both source and sink
Transform Streams : in-flight stream operations
Passthrough Streams : a trivial implementation of a Transform stream that simply passes the input bytes across to the output

Here it is a Readable Stream example:

const { Readable } = require('stream'); 
 
const inStream = new Readable({
    read(size) {
      console.log('size='+size);
      console.log('');
    }
  }).on('data', function(chunk) {
    console.log('chunk='+chunk);
});
 
console.log('');
 
inStream.push('123456');
inStream.push('AB');
inStream.push('3');
inStream.push(null);
 
inStream.on('end',function(){
    console.log('STREAM ended.');
});
  
inStream.on('error', function(err){
    console.log(err.stack);
});

And here it is the output of that script into the Console:

Remarks:

On Readable Streams in Node.js you can put data (with push method)
On Readable Streams in Node.js you can define events "when the stream receives data", "when the stream has an error" or "when the stream ends"
push(null) ends the stream;

Here it is another Readable Stream example:

const { Readable } = require('stream'); 
 
const inStream = new Readable({
    read(size) {
      console.log('size='+size);
      console.log('');
    }
  }).on('data', function(chunk) {
    console.log('chunk='+chunk);
});
 
console.log('');
 
inStream.push('123456');
inStream.push('AB');
x = inStream.read();
console.log('x='+x);
 
y = inStream.read();
console.log('y='+y);
 
inStream.push('3');
inStream.push(null);
 
inStream.on('end',function(){
    console.log('STREAM ended.');
});
  
inStream.on('error', function(err){
    console.log(err.stack);
});

And here it is the output of that script into the Console:

Remarks:

On Readable Streams in Node.js you can read data (with read method)
On Readable Streams in Node.js you can read data as it has been arrived.
You can add a value as parameter into the read method call in order to get more/ less data from the stream.

Here it is another Readable Stream example:

const { Readable } = require('stream'); 
 
const inStream = new Readable({
    read(size) {
      console.log('size='+size);
      console.log('');
    }
  }).on('data', function(chunk) {
    console.log('chunk='+chunk);
});
 
inStream.addListener('data', chunk1 => {
    console.log('fromNewListener='+chunk1);
 });
 
console.log('');
 
inStream.push('123456');
inStream.push('AB');
 
inStream.push('3');
inStream.push(null);
 
inStream.on('end',function(){
    console.log('STREAM ended.');
});
  
inStream.on('error', function(err){
    console.log(err.stack);
});

And here it is the output of that script into the Console:

Remark:

On Readable Streams in Node.js you can add/ remove more listeners, during the stream creation or after.

Piping is a mechanism where we provide the output of one stream as the input to another stream. Here it is an example:

const { Readable } = require('stream'); 
 
const inStream = new Readable({
 
  }).on('data', function(chunk) {
      // Do something if you want ...
});
 
console.log('');
 
inStream.push('123456');
inStream.push('AB');
 
inStream.pipe(process.stdout);
 
inStream.push('3');
inStream.push(null);
 
inStream.on('end',function(){
    console.log('');
    console.log('STREAM ended.');
});
  
inStream.on('error', function(err){
    console.log(err.stack);
});

And here it is the output of that script into the Console:

Remarks:

the Console is a writable stream as it take an input and put it down on the Console.
the data from the readable stream is put into the console using a pipe.

Here it is a writable stream in Node.js which receive data from a readable stream (the writable stream put the data into a file at the OS level):

const { Readable } = require('stream'); 
var fs = require("fs");
 
// Create a writable stream
var writerStream = fs.createWriteStream('output.txt');
 
//Create a Readable Stream
const readableStream = new Readable({
    read(size) {
    }
  }).on('data', function(chunk) {
      // Do something if you want ...
 
});
 
console.log('');
 
readableStream.push('123456');
readableStream.push('AB');
 
readableStream.pipe(writerStream);
 
readableStream.push('3');
readableStream.push(null);
 
readableStream.on('end',function(){
    console.log('');
    console.log('STREAM ended.');
    // Mark the end of file
    writerStream.end();
});
  
readableStream.on('error', function(err){
    console.log(err.stack);
});

If you look into the output.txt file you will see: 123456AB3.